Voltammetry for level 800 students 2021

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Unit three: The fundamentals of voltammery and polarography
Unit outline
Session 1: Basic concepts voltammetry
Session 2: Voltammetric instrumentation
Session 3: Hydrodynamic voltammetry and voltammograms
Session 4: Polarography
Session 5: Cyclic voltammometry
Session 6: Quantitative aspects of voltammetry and polarography
Unit three discusses one of the four methods of the electroanalytical techniques that are widely
used in qualitative and quantitative analytical chemistry. The unit discusses the fundamental
ideas of voltammetry that include the instrumentation and some types of voltammetry.
Unit objectives
By the end of the unit, you should be able to;
1. Explain the basic principle of voltammetry
2. State some types of voltammetry
3. Discuss the quantitative applications of voltammetry
Session 1: Basic concepts voltammetry
This session introduces you to how electrochemistry is applied as an analytical tool in the
detection and quantification of analytes. The measurement of current as a function of applied
voltage will be the central technique discussed in this session and all other sessions in this unit.

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Objectives
By the end of this session, you should be able to:
1. Name all electro-analytical methods
2. State the working principle of voltammetry and polarography
3. Describe the various excitation signals and their corresponding voltametric
techniques
4. Identify the various electrode used in voltammetry
5. Describe a typical voltametric cell
6. Identify the features of a voltammogram
3.1.1: Introduction to electro-analytical methods
You might have realized that in quantitative electrochemistry, measurements of current,
charge and voltage can be made. In units one and two you learned how the
concentration or the amount of an analyte is related to voltage and charge. Various
techniques have been developed to involve the measurement of current, charge and
voltage and relate these quantities to the amount of analyte.
There are four main types of electro-analytical methods, namely voltammetry,
potentiometry, coulometry and conductimetry. Potentiometry and conductimetry
measurements do not require electrolysis of the sample solution. That is no current flow
and the sample is recovered and is not altered by the analysis. You will learn more
about these in later units. Voltammetry and coulometry involve electrolysis of the

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sample solution. That is current flows and the sample cannot be recovered. You will
learn more about voltammetry in this unit and the next. Coulometry will be discussed
in a later unit.
3.1.2: Voltammetric methods
The term voltammetry refers to a group of electroanalytical methods in which you
acquire information about the analyte by measuring current in an electrochemical cell
as a function of applied potential. You obtain this information under conditions that
promote polarization of a small indicator, or working electrode. There are various types
of voltammetry which you will learn shortly. For instance, when current is proportional
to analyte concentration when monitored at a fixed potential, the technique is called
amperometry. To enhance polarization, working electrodes in voltammetry and
amperometry have a surface area of a few square millimeters at the most and in some
applications, a few square micrometers or less. Voltammetry is widely used by
inorganic, physical and biological chemists for fundamental studies of oxidation and
reduction process in various media, absorption processes on surfaces, and electron
transfer mechanisms at chemically modified electrode surfaces.
In voltammetry, the current that develops in an electrochemical cell is measured under
condition of complete concentration polarization. A polarized electrode is one to which
you have applied a voltage in excess of that predicted by the Nernst equation to cause
oxidation or reduction to occur. You can recall from your diploma programme that

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potentiometric measurements are made at currents that approach zero and where
polarization is absent.
Though voltammetry and coulometry both involve electrolysis of sample solution, you
should note certain differences between them. In coulometry, measures are taken to
minimize or compensate for the effects of concentration polarization. Also, in
voltammetry, there is minimal consumption of analyte, while in coulometry essentially
all of the analyte is converted to another state.
Historically, the field of voltammetry developed from polarography, which is a
particular type of voltammetry that was invented by the Czechoslovakian chemist
Jaroslav Heyrovsky in the early 1920s. Polarography differs from the other types of
voltammetry in that the working electrode is the unique dropping mercury electrode.
You will learn more about polarography in later sessions of this unit. At one time,
polarography was an important tool used by chemists for the determination of
inorganic ions and certain organic species in aqueous solutions. In recent years, the
number of applications of polarm12ography in the analytical laboratory has declined
dramatically. This decline has been largely as a result of concerns about the use of
mercury in the laboratory and possible contamination of the environment. Also, the
somewhat cumbersome nature of the apparatus, and the broad availability of faster and
more convenient, mainly spectroscopic methods have lessen the use of polarography.
Nonetheless, you will be introduced briefly to the technique since both working and
teaching laboratories still perform polarographic experiments.

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While polarography has declined in importance, voltammetry and amperometry at
working electrodes other than the dropping mercury electrode have grown at an
astonishing pace. Furthermore, voltammetry and amperometry coupled with liquid
chromatography have become powerful tools for the analysis of complex mixtures.
Modern voltammetry also continues to be an excellent tool in diverse areas of
chemistry, biochemistry, materials science and engineering, and the environmental
sciences for studying oxidation, reduction, and absorption processes.
3.1.3: Excitation signals in voltammetry
In voltammetry, a variable potential excitation signal is impressed on a working
electrode in an electrochemical cell. This excitation signal produces a characteristic
current response, which is the measureable quantity. The waveforms of three of the
most common excitation signals used in voltammetry are shown in figure 3.1
Figure 3.1: Some excitation signals used in voltammetry
The classical voltammetric excitation signal is the linear scan shown in figure 3.1a in
which the voltage applied to the cell increases linearity (usually over a 2- to -3-V range)
as a function of time. The current in the cell is then recorded as a function of time and

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thus a function of the applied voltage. In amperometry, current is recorded at a fixed
applied voltage.
Two pulse excitation signals are shown in figures 3.1b and 3.1c. Currents are measured
at various times during the lifetime of these pulses. Triangular excitation signal will be
discussed when you get to the session on cyclic voltammetry. Each excitation signal
corresponds to a particular type of voltammetric technique. You will learn more about
these techniques in later sessions of this unit or unit four.
Self Assessment Questions
1. Why are voltammetric and coulometric methods of chemical analyses described
as destructive?
Session 2: Voltammetric instrumentation
In this session, you will learn about the design and requirements that will enable you perform
voltammetric measurements. You will learn about the types of electrodes used in voltammetry
just as you learned about the types of potentiometric electrodes during your diploma programme.
Objectives
1. Identify various voltammetric electrodes
2. Explain why voltammetric electrodes are preferably microelectrodes
3. Describe the construction of a voltammetric cell

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4. Explain the role of counter or auxiliary electrode in a voltammetric cell
3.2.1: The voltammetric cell
The voltammetric cell is usually made up of three electrodes immersed in a solution
containing the analyte and also an excess of nonreactive electrolyte called a supporting
electrolyte. As you can identify in figure 3.2, one of these three electrodes in the
working electrode (WE) whose potential versus a reference electrode is varied linearity
with time.
Figure 3.2: Typical electrochemical for use in voltammetry
The dimensions of the working electrode are kept small to enhance its tendency to
become polarized. The reference electrode (RE) has a potential that remains constant
throughout the experiment. The third electrode is a counter electrode (CE) which is
often a coil of platinum wire or a pool of mercury. The counter electrode is also called

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auxiliary electrode. The current in the cell passes between the working electrode and
the counter electrode.
In a very simplified design, the signal source is a variable direct current (dc) voltage
source consisting of a battery in series with a variable resistor R. The desired excitation
r5potential is selected by moving a contact to the proper position on the resistor. For
you to measure the voltage, a digital voltmeter with a high electrical resistance is
connected in parallel, such that there is essentially no current in the circuit containing
the meter and the reference electrode. Thus, virtually all the current from the source
passes between the counter electrode and the working electrode. You can vary the
voltage by moving the contact positions on the resistor and recording the resulting
current as a function of the potential between the working electrode and the reference
electrode.
In principle, you can use a manual potentiostat to generate a linear-sweep
voltammogram. In such an experiment, you will move the contact on the resistor at a
constant rate from one end to another to produce the excitation signal similar to linear
scan as described in figure 3.1a above. The current and voltage are then recorded at
consecutive equal time intervals during the voltage (or time) scan.
In modern voltammetric instruments, however the various excitation signals such as
those that you have just learned and others are generated electronically. These
instruments vary the potential in a systematic way with respect to the reference
electrode and record the resulting current. The independent variable in this experiment

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is the potential of the working electrode versus the reference electrode and not the
potential between the working electrode and the counter electrode.
3.2.2: Types of voltammetric working electrodes
The working electrodes used in voltammetry take a variety of shapes and forms. Often,
they are small flat disks of a conductor that are press fitted into a rod of an inert
material, such as Teflon or kel-F that has imbedded in its wire contact, figure 3.3
Figure 3.3: Schematic diagram of a solid electrode
The conductor may be a noble metal such as platinum, gold, carbon paste, carbon fiber,
pyrolytic graphite, glassy carbon, diamond, or carbon nanotubes. A semiconductor,
such as tin or indium oxide; or a metal coat with a film of mercury can also be used.
You should note that the range of potentials that can be used with these electrodes in
aqueous solutions varies and depends not only on electrodes material but also on the

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composition of the solution in which it is immersed. Generally, the positive potential
limitations are caused by the large currents that develop due to oxidation of the water
to give molecular oxygen. The negative limits arise from the reduction of water to
produce hydrogen. Note that relatively large negative potentials can be tolerated with
mercury electrodes because of the high overvoltage of hydrogen on this metal. Suppose
you still remember overvoltage in unit two.
Mercury working electrodes have been widely used in voltammetry for several reasons.
One is the relatively large negative potential range that you just read about. An
additional advantage of mercury electrodes is that many metal ions are reversibly
reduced to amalgams at the surface of a mercury electrode, simplifying the chemistry.
Mercury electrodes take several forms. The simplest is a mercury film electrode formed
by electro-deposition of the metal onto a disk electrode. Figure 3.4(a) hanging mercury
drop electrode (HMDE); 3.4(b) dropping mercury electrode; 3.4(c) static mercury drop
electrode shows some mercury electrodes.

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Figure 3.4: Mercury electrodes: (a) hanging mercury drop electrode (b) dropping
mercury electrode (c) static mercury drop electrode.
The hanging mercury drop electrode is available from commercial sources and consists
of a very fine capillary tube connected to a mercury–containing reservoir. The metal is
forced out of the capillary by a piston arrangement driven by a micrometer screw. The
micrometer permits formation of drops having surface areas that are quite reproducible.
Figure 3.4b shows typical dropping mercury (DME), which was used in nearly all early
polargraphic experiments. It consists of roughly 10 cm of a fine capillary tubing (inside
diameter = 0.05 mm) through which mercury is forced by a mercury head of perhaps
50cm. the diameter of the capillary is such that a new drop forms and breaks every 2 to
6 s. the diameter of the drop is 0.5 to 1 mm and is highly reproducible. In some
applications the drop time is controlled by a mechanical knocker that dislodges the
drop at a fixed time after it begins to form. Furthermore, a fresh metallic surface is

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formed by simply producing a new drop. The fresh reproducible surface is important
because the currents measured in voltammetry are quite sensitive to cleanliness and
freedom from irregularities.
Apart from these mercury electrodes, you are going to encounter other commercial
microelectrodes. Some of such electrodes consist of small diameter metal wires or fibres
(5 to 100 µm) sealed within tempered glass bodies. The flattened end of the
microelectrodes is polished to a mirror finish, which can be maintained using alumina
and/ or diamond polish. The electrical connection is a 0.060” gold plated pin.
Microelectrodes are available in variety of materials including carbon fibre, platinum,
gold, and silver. Other materials can be incorporated into microelectrodes if they are
available as a wire or a fibre and form a good seal with epoxy
There are other commercially available, sandwich types of working electrodes for
voltammetry (or amperometry) in flowing streams. The block is made of
polyetheretherketone (PEEK) and is available in several formats with different size
electrodes and various arrays. The working electrodes can be made of glassy carbon,
carbon paste, gold, copper, nickel, platinum, or other suitable custom materials.
1. Explain briefly why in the voltammetric cell, current is not allowed to flow
between the working electrode and the reference electrode but between the
working electrode and the auxiliary electrode.

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Session 3: Hydrodynamic voltammetry and voltammograms
We are now going to turn our attention to the outcome of a voltammetric measurement. You still
remember in your diploma programme that in spectroscopy information about analytes are
displayed as spectra. You are going to learn about similar graphical display of information about
analytes in voltammetry.
Objectives
1. State the conditions under which hydrodynamic voltammetry occurs
2. Identify the various types of voltammograms
3. Identify cathodic and anodic currents
4. Identify basic features of voltammograms that are useful in qualitative and
quantitative analysis
3.3.1: Shapes of voltammograms
A plot of current as a function of applied potential is called a voltammogram and is the
electrochemical equivalent of a spectrum in spectroscopy. You can obtain both
qualitative and quantitative information about the species involved in the oxidation or
reduction reaction. The shape of a voltammogram is determined by several
experimental factors, the most important of which are how the current is measured and
whether convection is included as a means of mass transport. Figure 3.5 gives the
general shape of a linear scan voltammogram.

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Figure 3.5: General shape of a linear scan voltammogram
You are aware that there are different voltammetric techniques and you can guess that
each will have a characteristic voltammogram. You will be introduced to three common
shapes of voltammograms in this unit.
3.3.2: linear scan voltammograms
The voltammogram in Figures 3.5 and 3.6a is characterized by a current that increases
from the background residual current to a limiting current at potentials at which the
analyte is oxidized or reduced. When you obtain such a limiting current, it implies that
the thickness of the diffusion layer around the electrode remains constant.

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Figure 3.6: Three common shapes of voltammograms
The simplest method that you can use to obtain a limiting current is to stir the solution
possibly using a magnetic stirring bar, or by rotating the electrode.
Voltammetric techniques that include convection by stirring are called hydrodynamic
voltammetry. When convection is absent, the thickness of the diffusion layer increases
with time. In this case you will obtain a peak current in place of a limiting current
(Figure 3.6b).
In the voltammograms in both figures 3.6a and 3.6b, the current is monitored as a
function of the applied potential. Alternatively, the change in current following a
change in potential may be measured. The resulting voltammogram, which is shown in
figure 3.6c, also is characterized by a peak current.
Linear-scan voltammograms generally have a sigmoidal shape and are called
voltammetric waves. The constant current beyond the steep rise is called the limiting
current, i1im, because the rate at which the reactant can be brought to the surface of the
electrode by mass-transport processes limits the current. Limiting currents are usually
directly proportional to reactant concentration. You will learn more about this
quantitative relation in a later session
The potential at which the current is equal to one half the limiting current is called the
half-wave potential and given the symbol E1/2 (figure 3.5). The half-wave potential is
closely related to the standard potential for the half reaction but is usually not identical
to it. Half-wave potentials are sometimes useful for identification of the component of a
solution.

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You can obtain reproducible limiting currents rapidly when either the analyte solution
or the working electrode is in continuous and reproducible motion. Linear-scan
voltammetry in which the solution or the electrode is in constant motion is called
hydrodynamic voltammetry. You will soon learn how to perform hydrodynamic
voltammetry.
3.3.3: Performing hydrodynamic voltammetry
Hydrodynamic voltammetry is performed in several ways. In one method the solution
is stirred vigorously while it is in contact with a fixed working electrode in a cell. In this
cell, stirring is accomplished with an ordinary magnetic stirrer. Another approach is to
rotate the working electrode at a constant high speed in the solution to provide the
stirring action.
Still another way of performing hydrodynamic voltammetry is to pass an analyte
solution through a tube fitted with a working electrode. The last technique is
widely used for detecting oxidizable or reducible analytes as they exit from
liquid chromatographic column.
3.3.4: Application of hydrodynamic voltammetry
The most important uses of hydrodynamic voltammetry include;
1. Detection and determination of chemical species as they exit from
chromatographic columns or flow-injection apparatus

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2. Routine determination of oxygen and certain species of biochemical interest such
as glucose, lactose, and sucrose
3. Detection of end points in coulometric and volumetric titrations
4. Fundamental studies of electrochemical processes.
3.3.5: Voltammograms for mixtures of reactants
One advantage of voltammetry as a quantitative method of analysis is its capability for
analyzing two or more analytes in a single sample. As long as the components behave
independently, the resulting voltammogram for a multicomponent mixture is a
summation of their respective individual voltammograms. If the separation between the
half-wave potentials or peak potentials is sufficient, each component can be determined
independently as if it were the only component in the sample. Figure 3.7 shows the
voltammograms for a pair of two-component mixtures.
Figure 3.7: Voltammogram showing the independent analysis of two components.

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The minimum separation between the half-wave potentials or peak potentials for the
independent analysis of two components depends on several factors, including the type
of electrode and the potential-excitation signal.
1. What is the advantage in using voltammetry in analyzing a multi-component
sample?
Session 4: Polarography
You learned earlier in this unit that when the hanging dropping mercury voltammetric
electrode used in analysis it gives a different type of voltammetry. In this session, you will learn
more about this technique called polarography.
Objectives
1. Explain why polarography is a widely used voltammetric technique
2. Explain the features of a polarogram
3.4.1: About polarography
You remember in earlier sessions that linear-scan polarography was said to be the first
type of voltammetry to be discovered and used. It differs from hydrodynamic
voltammetry in two significant ways. First, there is essentially no convection or
migration, and second, a dropping mercury electrode (DME), such as that shown in
figure 3.4 in session 2, is used as the working electrode. Once there is no convection,

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you should expect diffusion alone to control polargraphic limiting currents. Compared
with hydrodynamic voltammetry, however, polargraphic limiting currents are an order
of magnitude or more smaller since convection is absent in polarography.
3.4.2: Polarographic currents
The current in the cell containing a dropping mercury electrode undergoes periodic
fluctuation corresponding in frequency to the drop rate. As a drop dislodges from the
capillary, you will expect the current falls towards zero, as shown in figure 3.8.
Figure 3.8: Voltammogram for normal polarography (Polarogram)
As the surface area of a new drop increases, so does the current. The diffusion current is
usually taken at the maximum of the current fluctuations. In the older literature, the
average current was measured because instruments responded slowly and damped the
oscillations as shown by the straight lines of figure 3.8. Some modern polargrams have
electronic filtering that allows either the maximum or the average current to be
determined if the drop rate is reproducible. You will notice that the irregular drops,

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probably caused by vibrations of the apparatus, have an effect in the upper part of the
curve.
3.4.3: POLAROGRAMS
Consider the polarogram in figure 3.8, as a polarogram for a solution that is 1.0M in
KC1 and 3 x 10-4M in lead ion. Can you guess the role of the 1.0 M KCl solution?
Certainly you will say it is the supporting electrolyte. You can assume that the
polarographic wave arises from the reduction of Pb2+ to Pb according the reaction Pb2+
+2e- + Hg Pb (Hg),
Pb (Hg), represents elemental lead dissolved in mercury to form an amalgam. You
recall you learned earlier that mercury easily forms amalgam with other metals. If you
examine the polarogram to the left of the wave you will find that there is a small
current, called the residual current, even when lead ions are not being reduced. The
sharp rise in current is then due to the reduction of the Pb2+. The wavy nature of the
polarogram is due to the repeated gradual formation of the mercury drop, detaching
from the electrode and reformation of a similar drop.
As in hydrodynamic voltammetry, limiting currents are observed when the magnitude
of the current is limited by the rate at which analyte can be brought up to the electrode
surface. In polarography however, the only mechanism of mass transport is diffusion.
For this reason, polarographic limiting currents are usually termed diffusion currents
and given the symbol id . As shown in figure 3.8, the diffusion current is the difference
between the maximum (or average) limiting current and the residual current. The

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diffusion current is directly proportional to analyte concentration in the bulk of
solution. You will learn about this relation in the later sessions
1. State any two advantages in using the hanging droping mercury electrode
(HDME) in voltammetry
Session 5: Cyclic voltammometry
The triangular excitation wave signal that you heard of in session 1 of this unit is the signal for
cyclic voltammetry. In this section you will learn more about this type of Voltammetry.
Objectives
1. Describe the nature of the excitation signal in cyclic voltammetry.
2. Identify the various features of a cyclic voltammogram
3. Identify cathodic and anodic currents
3.5.1: Signals in cyclic voltammetry
The excitation signal in cyclic voltammetry is called the triangular waveform and is
shown in figure 3.9

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Figure 3.9: Triangular excitation signals
You will realize that the potential is cycled between two values, first increasing linearity
to a maximum (figure 3.9a) and then decreasing linearity with the same slope to its
original value. Alternatively, you can first decrease the potential linearly to a minimum
(figure 3.9b) and then increase linearly with the same slope to the original value.
Whichever way you choose the process of scan may be repeated numerous times as the
current is recorded as a function of time. A complete cycle may take 100 or more
seconds or be completed in less than one second.
3.5.2: Description of a typical cyclic voltammogram
Figure 3.10 shows the current response when a solution of a hypothetical analyte A that
is 6mM in A and 1 M in KNO3 is subjected to the cycle excitation signal shown in
figures 3.8b

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Figure 3.10: Cyclic voltammogram of a hypothetical analyte A
You may assume that the working electrode is carefully polished stationary platinum
electrode, and the reference electrode was a saturated calomel electrode. At the initial
potential of +1.1 V, a tiny anodic current is observed, which immediately decreases to
zero as the scan is continued. No current is observed between the potential range of
+1.0 and +0.9 V because no reducible species is present in this potential range. When
the potential becomes less positive than approximately + 0.8 V, a cathodic current
(negative current) begins to develop. You can attribute this to the reduction of the the
analyte A. The reaction at the cathode is then
A + ne ⇌ P
P is the hypothetical product.

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A rapid increase in the current occurs and ending at a peak. The peak current is made
up of two components. One is the initial current surge required to adjust the surface
concentration of the reactant to its equilibrium concentration is given by the Nernst
equation. The second is the normal diffusion-controlled current. The first current then
decays rapidly as the diffusion layer is extended farther and farther away from the
electrode surface. At potential +0.3 V, the scan direction is switched. The current,
however, continues to be cathodic (negative current) even though the scan is toward
more positive potentials because the potentials are still negative enough to cause
reduction of A. As the potential sweeps in the positive direction, eventually reduction
of A no longer occurs, and the current goes to zero and then becomes anodic. The
anodic current (positive current) results from the re-oxidation of P that has accumulated
near the surface during the forward scan. This anodic current peaks and then decreases
as the accumulated P is used up by the anodic reaction.
Note that by convention cathodic currents are always taken to be positive whereas
anodic currents are given a negative sign.
Important variables in a cyclic voltammogram that you should note are the cathodic
peak potential Epc , the anodic peak potential Epa, the cathodic peak current ipc, and the
anodic peak current ipa. The definition and measurement of these parameters are
illustrated in figure 3.10. For a reversible electrode reaction, anodic and cathodic peak
currents are approximately equal to absolute value but opposite in sign. For a reversible
electrode reaction at 25ºC, the difference in peak potentials, ∆Ep is expected to be

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∆Ep = │ Epa - Epc│ = 0.0592/n
Where n is the number of electrons involved in the half-reaction. Irreversibility because
of slow electron transfer kinetics results in ∆Ep exceeding the expected value. While an
electron transfer reaction may appear reversible at a slow sweep rate, increasing the
sweep rate may lead to increasing values ∆Ep , a sure sign of irreversibility . Hence, to
detect slow electron transfer kinetics and to obtain rate constants, ∆Ep is measured for
different sweep rates.
Quantitative information is obtained from the Randles-Sevcik equation, which at 25ºC is
ip = 2.686 x 105n3/2 AcD1/2v1/2
where ip is the peak current in amperes, A is the electrode area in cm2, D is the diffusion
coefficient in cm2/s, c is the concentration in mol/cm3, and v is the scan rate in V/s.
Cyclic voltammetry offers a way of determining diffusion coefficients if the
concentration electrode area and the scan rate are known.
1. How can a cyclic voltammogram help you determine whether the electrode
reaction is reversible or not? For a reversible electrode reaction, anodic and
cathodic peak currents are approximately equal to absolute value but opposite in
sign.
Session 6: Quantitative aspects of voltammetry and polarography

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You have just learned the features of voltammograms and the various quantities that can be
identified from it. In this session you will learn how the various quantities that can be obtained
from a voltammogram are quantitatively related to the concentration of the analyte.
Objectives
1. State that quantitative relation between concentration of analyte and current in
linear scan voltammetry
2. State that quantitative relation between concentration of analyte, current and
other electrode properties in polarography
3. Explain two experimental procedures in quantitative voltammetry.
3.6.1: Quantitative aspects of linear-scan voltammogram
Consider a hypothetical experiment involving an electrolytic reduction of an analyte
species A to give a product P in linear scan voltammetry. In this hypothetical
experiment, assume that the solution is about 10-4M in A, 0.0M in P, and 0.1 M in KCl,
which serves as the supporting electrolyte. The half- reaction at the working electrode is
the reversible reaction.
A+ ne- ⇌ P E0 = - 0.26 V
For convenience, you have to neglect the charges on A and P and also have assumed
that the standard potential for the half reaction is -0.26 V.

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This we may write
i1 = kcA
Where cA is the analyte concentration and k is a constant.
This is the quantitative linear–scan voltammetry relationship that you will always rely
on.
3.6.2: Relationship between the diffusion current at the dropping mercury electrode
and the concentration of analyte
To derive an equation for polarographic diffusion currents, you must take into account
the rate of growth of the spherical electrode, which is related to the drop time in
seconds t and the rate of flow of mercury through the capillary m, in mg/s and the
diffusion coefficient of the analyte D in cm 2/s. These variables are taken into accounts
in the Ilkovic equation:
(id )max = 706nD1/2m2/3t1/6c
Where (id )max is the maximum diffusion current in µA and c is the analyte concentration
in mM.
If you want to obtain an expression for the average current rather than the maximum,
the constant in the foregoing equation becomes 607 rather than 706. That
is;
(id)ave = 607nD1/2m2/3t1/6c

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You should note that either the average or the maximum current can be used in
quantitative polarography.
The product m2/3t1/6 in the Ilkovic equation is called the capillary constant and describes
the influence of dropping electrode characteristics upon the diffusion current. Both m
and t are readily evaluated experimentally. This makes comparison of diffusion
currents from different capillaries possible.
3.6.3: Quantitative experimental measurements
Experimental measurements in voltammetry can be carried out similar to those in
potentiometry which you learned during your diploma programme. You need to
refresh your memory on these. Do you still remember them? They are discussed again
here.
Direct voltammetry measurement
This is a convenient and fast method of determining the concentration of analytes in
solution. Two measurements are generally usually involved;
(a) Measurement of the voltammetric current that flows when a solution of known
concentration of the analyte is placed in the voltammetric cell.
(b) Measurement of the voltammetric current that flows when a solution of the
unknown concentration of the analyte is placed in the cell.

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Then, depending on the type of voltammetry involved then you will use the
appropriate quantitative relation to establish the concentration of the unknown
solution.
Standard addition method
In this method, you will also require two measurements after which the appropriate
quantitative relation is used.
(a) Measurement of the voltammetric current that flows when a known volume of
unknown concentration of the analyte (sample solution) is placed in the
voltammetric cell.
(b) Measurement of the voltammetric current that flows after a solution of known
volume and known concentration is added to the solution in (a) and placed in
the voltammetric cell.
There is however a third method in voltammetry called pilot-ion method. Read more
about this on your own.
1. An organic substance is reduced polarographically. At a concentration of 2.0 Χ
10-4 M, it gives a wave with maximum diffusion current of 20.4 µA when a
capillary with flow rate of 3.4 mg/s and a drop time of 2.7 s is used. If the
diffusion coefficient of the compound in the supporting electrolyte has been

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determined by other means to be 9 Χ 10-6 cm2/s. What is the value of n, number
of electrons transferred for the polarographic reduction of the compound?

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Unit Four: Applied voltammetric techneques
Unit outline
Session 1: Detectors and sensors in voltammetry
Session 2: Amperometry and amperometric titrations
Session 3: Pulse voltammetry
Session 4: Square-wave voltammetry
Session 5: Stripping methods
Session 6: Applications of voltammetry in analytical chemistry
In this unit, you will learn about som specialised types of voltammetry and how they are applied
in analytical chemistry
Unit objectives
By the end of the unit, you should be able to;
1. Describe amperometry and its applications
2. Describe pulse and square wave voltammetry
3. Identify the various stripping methods in voltammetry
Session 1: Detectors and sensors in voltammetry

32
In this session you will learn about modified voltammetric electrodes that uses molecular
recognition phenomenon in the detection of analytes as against the ordinary metallic electrodes
that you learned in unit three.
Objectives
1. State working principle of membrane based voltammetric electrodes
2. Describe the enzyme-based glucose sensor
3. Develop a modified voltammetric electrode for a particular analyte base on a
molecular recognition process
4. Illustrate how a modified voltammetric electrode can be couple with a separation
technique as a detector.
4.1.1: Voltammetric sensors
You will still recall from your diploma programme about potentiometric pH glass
electrode. The glass membrane responds specifically to hydrogen ions in solution. You
were told that such specificity of potentiometric electrodes could be enhanced by
applying molecular recognition layers to the electrode surfaces. You will learn more
about such electrodes under ion-selective electrodes in the next unit.
Nonetheless, there has been much research in recent years to apply the same concepts
to voltammetric electrodes. A number of voltammetric systems are available

33
commercially for the determination of specific species in industrial, biomedical,
environmental, and research applications. These devices are sometimes called
electrodes of detectors but are in fact, complete voltammetric cells and are better
referred to as sensors. You will learn about enzyme-based sensors in this session in later
session in this unit, you also learn about the oxygen sensor. These sensors are available
commercially.
4.1.2: Enzyme-based sensors
A number of enzyme-based voltammetric sensors are available commercially. One such
sensor is the glucose sensor that is widely used in clinical laboratories for the routine
determination of glucose in the blood serums. The membrane in this sensor consists of
three layers. The outer layer is a polycarbonate film that is permeable to glucose but
impermeable to protein and other constituents of blood. The middle layer is an
immobilised enzyme, glucose oxidase. This serves as your molecular recognition layer.
The inner layer is a cellulose acetate membrane, which is permeable to small molecules
such as hydrogen peroxide.
When you immerse this device in a solution containing glucose, the glucose diffuses
through the outer membrane into the immobilized enzyme. The following catalytic
reaction occurs;
Glucose + O2 glucose oxidase H2O2 + gluconic acid

34
The hydrogen peroxide then diffuses through the inner layer of the membrane and to
the electrode surface, where it is oxidized to oxygen according to the equation;
H2O2 + OH- O2 + H2O + 2e
The resulting current is directly proportional to the glucose concentration of the
solution.
NB: Most of the home glucose monitors widely used by patients are this type of sensor.
4.1.3: Voltammetric detectors in chromatography and flow- injection analysis
Hydrodynamic voltammetry is widely used for detection and determination of
oxidizable of reducible compounds or ions that have been separated by liquid
chromatography of that are produced by flow-injection methods. A thin-layer cell is
used in these applications. The working electrode in these cells is usually imbedded in
the wall of an insulating block that is separated from a counter electrode by a thin
spacer as shown. The volume of such cell is typically 0.1 to 1 µ L. A voltage
corresponding to the limiting current region for analyte is applied between the working
electrode and a silver /silver chloride reference electrode that is located downstream
from the detector. You can have five different configurations of working electrode.
These configurations help you to optimize the detection and sensitivity under a variety
of experimental conditions. Voltammetric sensors have been applied and detection
limits as low as 10-10 M has been achieved.

35
1. Name any analyte and the corresponding membrane material that be used in the
voltammetric detection of the analyte.
Session 2 Amperometry and amperometric titrations
You were told in unit three that there are various types of voltammetry. In this session you are
going to meet yet another type which unlike most of the others does not produce a
voltammogram.
Objectives
1. Explain the principle of amperometry
2. Explain the process of amperometric titration
3. Plot amperometric titration curves
4. Determine the end point of amperometric titration by extrapolation from the
titration curve.
5. Name some amperometric biosensors
4.2.1: General principle
Amperometry is a voltammetric technique in which a constant potential is applied to
the working electrode, and current is measured as a function of time. You will notice at

36
once that plot of current versus applied voltage cannot be obtained in this case. So since
the potential is not scanned, amperometry does not lead to a voltammogram.
One important application of amperometry that you will meet is in the construction of
chemical sensors. One of the first amperometric sensors to be developed was for
dissolved O2 in blood. The sensor was developed in 1956 by L. C. Clark.
The design of the amperometric sensor is similar to potentiometric membrane
electrodes. Do you still remember membrane electrodes in you diploma programme? A
gas-permeable membrane is stretched across the end of the sensor and is separated
from the working and counter electrodes by a thin solution of KCl. The working
electrode is a platinum disk cathode, and a silver ring anode is the counter electrode.
Although several gases can diffuse across the membrane, including oxygen, nitrogen
and carbon dioxide, only oxygen is reduced at the cathode.
O2(aq) + 4H3O+(aq) + 4e ⇌ 6H2O(l)
4.2.2: Amperometric titrations
In principle you can use hydrodynamic voltammetry to estimate the equivalence point
of titrations if at least one of the participants or products of the reaction involved is
oxidized or reduced at a working electrode. In this case the current at some fixed
potential in the limiting current region is measured as a function of the reagent volume
or of time. If you plot the data on either sides of the equivalence point you will get
straight lines with different slopes. You can then establish the end point is by

37
extrapolation to the intersection of the lines. This is basically what is referred to as
amperometric titration.
Amperometric titration curves typically take one of the forms shown in figure 4.1.
Figure 4.1: Typical amperometric titrationcurves
Figure 4.1a represents a titration in which the analyte reacts at the working electrode
while the reagent does not. You will observe from the plot, that the current decreases as
the electroactive analyte decreases in amount as the reaction progresses. What can you
say about figure 4.1b? In this typical titration, the reagent reacts at the working
electrode and the analyte does not. Finally in figure 4.1c corresponds to a titration in
which both the analyte and the titrant react at the working electrode.

38
There are two types of Amperometric electrode systems. One uses a single polarizable
electrode coupled to a reference, while the other uses a pair of identical solid-state
electrodes immersed in stirred solution. For the first, the working electrode is often a
rotating platinum electrode constructed by sealing a platinum wire into the side of a
glass tube that is connected to a stirring motor.
Amperometric titrations with one indicator electrode have, with one notable exception,
been confined to titrations in which a precipitate or a stable complex is the product.
Precipitating reagent include silver nitrate for halide ions, lead nitrate for sulfate ion,
and several organic reagents, such as 8-hydroxyquinoline, dimethylglyoxime, and
cupferron, for various metallic ions that are reducible at working electrodes. Several
metal ions have also been determined by titration with standard solutions of EDTA.
The exception just noted involves titrations of organic compounds, such as certain
phenols, aromatic amines, and olefins; hydrazine; and arsenic (III) and antimony (III)
with bromine. The bromine is often generated coulometrically. It has also been formed
by adding a standard solution of potassium brominates to an acidic solution of the
analyte that also contains an excess of potassium bromide. Bromine is formed in the
acidic medium by the reaction
BrO3- + 5Br - +6H+ 3Br2 + 3H2O
This type of titration has been carried out with a rotating platinum electrode or twin
platinum electrodes. There is no current prior to the equivalence point. After the

39
equivalence point, there is a rapid increase in current because of the electrochemical
reduction of the excess bromine.
There are two advantages in using a pair of identical metallic electrodes to establish the
equivalence point in amperometric titrations. One has to do with the simplicity of
equipment and not having to purchase or prepare and maintain designed for routine
automatic determination of a single species. An instrument of this type is often used for
the automatic determination of chloride in samples of serum, sweat, tissues extracts,
pesticides, and food products.
The reagent in this system is silver ion generated from a silver anode. A voltage of
about 0.1V is applied between a pair of twin silver electrodes that serve as the indicator
system. Short of the equivalence point in the titration of chloride ion, there is essentially
no current because no electroactive species is present in the solution. You will therefore
expect no electron transfer at the cathode, and the electrode is completely polarized.
You should note that the anode is not polarized because the reaction
Ag ⇌ Ag+ + e- occurs in the presence of a suitable cathodic reactant or depolarizer.
When you pass the equivalence point, then the cathode becomes depolarized because
silver ions are present. These ions react to give silver:
Ag+ + e- ⇌ Ag
This half-reaction and the corresponding oxidation of silver at the anode produce a
current whose magnitude is, as in other amperometric methods, directly proportional to

40
the concentration of the excess reagent. Thus, the titration curve is similar to that shown
in figure 4.1b.
The most common end-point detection method for the Karl Fisher titration for
determining water is the amperometric method with dual polarized electrodes. Several
manufacturers offer fully automated instruments for use in performing these titrations.
A closely related end-point detection method for Karl Fisher titration measures the
potential difference between two identical electrodes through which a small constant
current is passed.
4.2.3: Amperometric biosensors
Earlier you learned that there are membrane sensors that can be applied as
voltammotric electrodes. In amperometry, several biosensors have developed to the
detection of analytes just as you saw with the case of glucose. Table 4.1 shows some
other biosensors used in amperometry and the analyte as well as the redox species
involved in the electrode reaction.
Table 4.1: representative examples of amperometric biosensors
Analyte Enzyme Species Detected
Choline Choline oxidase H2O2

41
Ethanol Alcohol oxidase H2O2
Formaldehyde Formaldehyde dehydrogenase NADHa
Glucose Glucose oxidase H2O2
Glutamine Glutaminase , glutamate oxidase H2O2
Glycerol Glycerol dehydrogenase NADH, O2
Lactate Lactate oxidase H2O2
Phenol Polyphenol oxidase Quinine
Inorganic P Nucleoside phosphorylase O2
1. Distinguish between voltammetry and amperometry
Ans there’s no voltammogram in amperometry because the potential is fixed
Session 3: Pulse voltammetry
Objectives
1. Explain how measurements are made in pulse voltammetry
2. Identify the types of pulse voltammetry
3. Solve quantitative problem involving differential pulse voltammetry.
In this session, you will about the voltammetry that is associated with pulse excitation signals
that you learned earlier

42
4.3.1: Background
You read in unit three after it discovery, polarography was supplanted by voltammetry.
Also, linear-scan voltammetry, by the 1960s, ceased to be an important analytical tool in
most laboratories. The reason for the decline in use of this once popular technique was
not only the appearance of several more convenient spectroscopic methods but also the
inherent disadvantages of the method including slowness, inconvenient apparatus , and
particularly , poor detection limits. Many of these limitations were overcome by the
development of pulse methods.
Figure 4.2 shows the excitation signal for normal pulse voltammetry with the
corresponding voltammogram.
Figure 4.2: Excitation signal for normal pulse voltammetry (left) and the corresponding
voltammogram (right)
You will learn about the two most important pulse techniques; differential-pulse
voltammetry (in this session) and square-wave voltammetry (next seesion). The idea
behind all pulse-voltammetric methods is to measure the current at a time when the

43
difference between the desired faradaic curve and the interfering charging current is
large.
4.3.2: Differential-pulse voltammetry
The excitation signal and the corresponding voltammogram for differential pulse
voltammetry are shown on figure 4.3.
Figure 4.3: Excitation signal for differential pulse voltammetry (left) and the
corresponding voltammogram (right)
The waveform in figure 4.3 is typically used in digital instruments and is the sum of the
pulse and a staircase signal. There is yet another excitation signal, which is usually used
in analog instruments and is obtained by superimposing a periodic pulse on a linear
scan.
In either case, a small pulse, typically 50 mV is applied during the last 50 ms of the
lifetime of the period of the excitation signal.
In figure 4.3, the difference in current per pulse (∆i) is recorded as a function of the
linearly increasing excitation voltage. You can then plot the differential curve. The plot
consists of a peak (voltammogram on the right of figure 4.3), the height of which is

44
directly proportional to concentration. For a reversible reaction, the peak potential is
approximately equal to the standard potential for the half-reaction.
One advantage of the derivative-type voltammogram is that individual peak maxima
can be observed for substances with half-wave potentials deferring by as little as 0.04 to
0.05 V. In contrast, classical and normal-pulse voltammetry require a potential
difference of about 0.2 V for resolving waves. More important, however, differential-
pulse voltammetry increases the sensitivity of voltammetry. Typically you will observe
well defined peaks in differential-pulse voltammetry at a concentration levels that are
2x 10-3 time that for the classic voltammetric wave. Note also that the current scale for ∆i
is in nanoamperes.
The greater sensitivity of differential-pulse voltammetry can be attributed to two
sources.
1. An enhancement of the faradaic current
2. Decrease in the nonfaradaic charging current.
Reliable instruments for differential-pulse voltammetry are now available commercially
at reasonable cost. The method has thus become one of the most widely used analytical
voltammetric procedure and is especially useful for determining trace concentrations of
heavy metal ions.
4.3.3: worked example
The concentration of As(III) in water can be determined by differential pulse
polarography in 1 M HCl. The initial potential is set to –0.1 V versus the SCE, and is

45
scanned toward more negative potentials at a rate of 5 mV/s. Reduction of As(III) to
As(0) occurs at a potential of approximately -0.44 V versus the SCE. The peak currents,
corrected for the residual current, for a set of standard solutions are shown in the
following table.
[As(III)], M ip, μA
1.00 x 10-6 0.298
3.00 x 10-6 0.947
6.00 x 10-6 1.83
9.00 x 10-6 2.72
What is the concentration of As(III) ina sample of water if the peak current under the
same conditions is 1.37 μA?
Solution
Linear regression gives the equation for the calibration curve as;
ip(μA) = 0.0176 + 3.01 x 105[As(III)]
substituting the peak current into the regression equation, gives the concentration of
As(III) as 4.49 x 10-6M
1. The differential pulse polarographic analysis of mixtures of indium and cadmium in
0.1 M HCl is complicated by the overlap of their respective voltammograms. The peak
potential for indium is at –0.557 V, and that for cadmium occurs at a potential of –0.597
V. When a 0.800-ppm indium standard is analyzed, the peak current (in arbitrary units)

46
is found to be 200.5 at –0.557 V and 87.5 at –0.597 V. A standard solution of 0.793-ppm
cadmium gives peak currents of 58.5 at –0.557 V and 128.5 at 0.597 V. What is the
concentration of indium and cadmium in a sample if the peak current is 167.0 at a
potential of –0.557 V and 99.5 at a potential of –0.597 V?
Session 4 Square-wave voltammetry
This session discusses the second pulse voltammetry. You will be introduce to its excitation
signal and the features of the resulting voltammogram.
Objectives
1. Describe the nature of the excitation signal
2. Identify the features of square wave voltammogram
4.4.1: Nature of the excitation signal in square wave voltammetry
Square-wave voltammetry is a type of pulse voltammetry that offers the advantage of
great speed and high sensitivity. An entire voltammogram is obtained in less than 10
ms. Square-wave voltammetry has been used with hanging mercury drop electrodes
and with other electrodes and sensors.
Figure 4.4(left) shows the excitation signal in Square-wave voltammetry. This is
obtained by superimposing the pulse train shown onto a staircase signal.

47
Figure 4.4: excitation signal and corresponding square wave voltammogram
The length of each step of the staircase and the period T of the pulses are identical and
usually about 5 ms. The potential step of the staircase ∆Es is typically 10mV. For a
reversible reduction reaction, the size of a pulse is great enough so that oxidation of the
product formed on the forward pulse occurs during the reverse pulse. Thus if your
forward pulse produces a cathodic current iv and then the reverse pulse gives an anodic
current i2. Usually the difference in these currents, ∆i, is plotted to give voltammograms
(figure 4.4 right). This difference is directly proportional to concentration of your
analyte. One thing you need to also note is that the potential of the peak corresponds to
the voltammetric half-wave potential. Detection limits for Square-wave voltammetry
are reported to be 10-7 to 10-8 M.
Commercial instruments for Square-wave voltammetry are available from several
manufacturers and as a consequence, this technique is being used routinely for
determining inorganic and organic species. Square-wave voltammetry is also being
used in detectors for liquid chromatography.

48
1. distinguish between differential pulse voltammetry and square-wave
voltammetry
Square-wave voltammetry is a type of pulse voltammetry that offers the
advantage of great speed and high sensitivity
Session 5: Stripping methods
You are going to learn probably the most important quantitative voltammetric technique in this
session. It is usually a two-step technique called stripping analysis.
Objectives
1. Identify the three types of stripping analysis
2. Explain the processes in striping analysis
3. Identify typical analytes and the particular stripping method used.
4.5.1: Process of stripping analysis
Stripping methods encompass a variety of electrochemical procedures having a
common characteristic initial step. Stripping voltammetry is composed of three related
techniques namely anodic, cathodic, and adsorptive stripping voltammetry. You later
notice that anodic stripping voltammetry has the widest application of the three.
In all of these procedures, the analyte is first deposited on a working electrode, usually
from a stirred solution. After an accurately measured period, the electrolysis is

49
discontinued, the stirring is stopped and the deposited analyte is determined by one
voltammetric procedures that have been described in the unit three.
During the second step in the analysis, the analyte is dissolved or stripped from the
working electrode; hence the name attached to these methods.
You can consider the deposition step as an electrochemical pre-concentration of the
analyte. That is, the concentration of the analyte in the surface of the working electrode
is far greater than it is in the bulk solution. As a result of the pre-concentration step,
stripping methods yield the lowest detection limits of all voltammetric procedures. For
example anodic stripping with pulse voltammetry can reach nanomolar detection limits
for environmentally important species, such as Pb2+, Ca2+ and T1+.
4.5.2: Anodic stripping method
In anodic stripping method, the working electrode behaves as a cathode during the
deposition step and as an anode during the stripping step, with the analyte being
oxidized back to its original form. That is the first step is a controlled potential
electrolysis in which the working electrode, usually a hanging mercury drop or
mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the
electrode.
You will use the case of the stripping analysis of copper as an example. The two steps
are illustrated in figure 4.5

50
Figure 4.5: Potential-excitation signal and voltammogram for anodic stripping
voltammetry at a hanging mercury drop electrode.
You can illustrate the deposition reaction as;
Cu2+(aq) + 2e ⇌ Cu(Hg)
The product Cu(Hg) indicates that the copper is amalgamated with the mercury. As
you noted earlier, this step essentially serves as a means of pre-concentrating the copper
from the larger volume of the solution to the smaller volume of the electrode. The
solution is stirred during electrolysis to increase the rate of deposition. Near the end of
the deposition time stirring is stopped. This is for you eliminate convection as a mode of
mass transport. You may do the deposition for 1–30 min. However you will have to use
longer times if the analytes at lower concentrations.
In the second step, the potential is scanned anodically toward more positive potentials
as shown in figure 4.5. When the potential of the working electrode is sufficiently

51
positive the deposited metal copper is stripped from the electrode, returning to solution
as its oxidized form.
Cu(Hg) ⇌ Cu2+(aq) + 2e–
Here you monitor the current during the stripping step and this is a function of the
applied potential. Finally, you will obtain a peak-shaped voltammogram similar to that
shown in figure 4.5. The important quantitative information you need to know is that
the peak current is proportional to the concentration of the analyte in the solution.
You should note that anodic stripping voltammetry is very sensitive to experimental
conditions. Thus you must carefully control them if you results are to be accurate and
precise. They include;
1. The area of the mercury film electrode or the size of the Hg drop when you are
using a hanging mercury drop electrode
2. The deposition time
3. The rest time
4. The rate of stirring
5. The scan rate during the stripping step.
Anodic stripping voltammetry is best used for metals that form amalgams with
mercury. You some of these metals in table 4.2
Table 4.2: Representative Examples of Analytes Determined by Stripping
Voltammetry
Anodic stripping
voltammetry
Cathodic stripping
voltammetry
Absorptive stripping
voltammetry
Bismuth Bromide Bilirubin

52
Cadmium Iodide Codeine
Copper Chloride Cocaine
Gallium Mercaptans (RSH) Digitoxin
Indium Sulphide Dopamine
Lead Thiocyanate Heme
Thallium Monesin
Tin testosterone
Zinc
4.5.3: Cathode stripping method
You will definitely expect the opposite to happen to the working electrode in a cathode
stripping method in the second step. The working electrode behaves as an anode during
the deposition step and as a cathode during stripping.
You can use the anodic stripping method for determining cadmium and copper in an
aqueous solution of these ions as a case study. A linear-scan method is often used to
complete the analysis. Initially, a constant cathode potential of about -1 V is applied to
the working electrode, causing both cadmium and copper ions to be reduced and
deposited as metals. The electrode is maintained at this potential for several minutes
until a significant amount of the two metals has accumulated at the electrode. The
stirring is then stopped for 30 s or so while the electrode is maintained at -1 V. the
potential of the electrode is then decreased linearly to less negative values while the
current in the cell is recorded as a function of time or potential. At a potential somewhat
more negative than -0.6 V, cadmium starts to be oxidized, causing a sharp increase in
the current. As the deposit cadmium is consumed, the current peaks and then decreases
to its original level. A second peak for oxidation of the cooper is then observed when
the potential has decreased to approximately -0.1 V. the heights of the two peaks are

53
proportional to the weights of the deposited metals. Stripping methods are important in
trace work because the preconcentratation step permits the determination of minute
amounts of an analyte with reasonable accuracy. Thus, the analysis of solution in the 10-
6 to 10-9 M range becomes feasible by methods that re both simple and rapid.
4.5.4: Adsorptive stripping voltammetry
In this type of stripping voltammetry, you will certainly expect the deposition step to
occur without electrolysis. Instead, your analyte will adsorb onto the surface of the
electrode. During deposition the electrode is maintained at a potential that enhances
adsorption.
For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode
is held at –0.4 V versus the SCE, a potential at which the surface charge of mercury is
approximately zero. When deposition is complete the potential is scanned in either
anodic or cathodic direction depending on whether you wish to oxidize or reduce the
analyte. Similarly, examples of compounds that have been analyzed by absorptive
stripping voltammetry also are listed in table 5.1
4.5.5: Worked example on stripping voltammetry
Example 1
The concentration of copper in a sample of sea water is determined by anodic stripping
voltammetryusing the method of standard additions. When a 50.00 mL sample was
analysed, the peak current was 0.886 μA. A 5.00 mL spike of 10.00 ppm Cu2+ was
added, a peak current of 2.52 μA was obtained. Calculate the parts per million of copper
in the sample of sea water.

54
Solution
Peak currents in anodic stripping voltammetry are linear function of concentration.
Thus you write;
ip = k(ppm Cu2+), k is a constant.
You can write in this case as;
0.886 = k(ppmCu2+)
And for the standard addition;
a. = k[
0.0500 L
0.0500L+5.00 x 10−6
𝑝𝑝m𝐶𝑢2+
+
5.00 x 10−6L
0.0500L + 5.00 x 10−6L
(10.0 ppm)]
You should first solve for k, using the first equation. You will then substitute it into
the second equation and simplify.
2.52 = 0.8859 +
(8.859 x10−5)(10.0 ppm)
(ppmCu2+)
You now finally solve for the concentration of Cu2+ (ppmCu2+). This will give you
5.42 x 10-4 ppm = 0.542 ppb
1. What is the purpose of the electrodeposition step instripping analysis?
Session 6 Applications of voltammetry in analytical chemistry
In this last session of the unit four, you will learn the wide range of analytes that can determined
using voltammetry.
Objectives

55
1. Identify the conditions under which inorganic cations and anions can be
determined by voltammetry
2. Identify the organic functional groups that can be determined by voltammetry
3. Identify various solvents that can be used for voltammetry
4.6.1: Broad applications of voltammetry in analytical chemistry
In the past, linear-scan voltammetry was used for the quantitative determination of a
wide variety of inorganic species, including molecules of biological and biochemical
interest. Pulse methods have largely replaced classical voltammetry because of their
greater sensitivity, convenience, and selectivity. Generally, quantitative applications are
based on calibration curves which in peak heights are plotted as a function of analyte
concentration. In some instances the standard- addition method is used in lieu of
calibration curves. In either case, it is essential that the compositions of standard
resemble as closely as possible the composition of the sample, both as to electrolyte
concentration and pH. When this is matching is done, you can achieve relative
precisions and accuracies in the 1 to 3% range.
4.6.2: Inorganic applications
Voltammetry is applicable to the analysis of many inorganic substances. Most metallic
cations, for example, are reduced at common working electrodes. Even the alkali and
alkaline-earth metals are reducible, provided the supporting electrolyte does not react
at the high potentials required. You will find that the tetraalkyl ammonium halides are
useful electrolyte because of their high reduction potentials.

56
The successful voltammetric determination of cations frequently depends on the
supporting electrolyte that is used. Tabular compilations of half-wave potential data are
usually available that always help in your choice of an electrolyte. The judicious choice
of anion often enhances the selectivity of the method. For example, with potassium
chloride as a supporting electrolyte, the wave for iron (III) and copper (II) interfere with
one another. In a fluoride medium, however, the half-wave potential of iron (III) is
shifted by about -0.5 V, while that for cooper (II) is altered by only a few hundredths of
a volt. The presence of fluoride thus results in the appearance of well-separated waves
for the two ions.
Voltammetry is also applicable to the analysis of such inorganic anions as bromate,
iodate, dichromate, vanadate, selenite, and nitrite.
In general, voltammograms for substance are affected by the pH of the solution
because the hydrogen ion is a participant in their reduction. As a consequence, strong
buffering to some fixed pH is necessary to obtain reproducible data.
4.6.3: Organic voltammetric analysis
Almost from its inception, voltammetry has been used for the study and determination
of organic compounds with many papers being devoted to this subject. Several organic
functional groups are reduced at common working electrodes, thus making possible the
determination of a wide variety of organic compounds. Oxidizable organic functional
groups can be studied voltammetrically with platinum, gold, carbon, or various
modified electrodes. The number of functional groups that can be oxidized at mercury

57
electrodes is relatively limited because mercury is oxidized at anodic potentials greater
than +0.4 V (versus SCE).
4.6.4: Solvents for organic voltammetry
Solubility considerations frequently dictate the use of solvents other than pure water for
organic voltammetry. Aqueous mixtures containing varying amounts of such miscible
solvents as glycols, dioxane, acetonitrile, alcohols, cellulose, or acetic acid have been
used. Anhydrous media such as acetic acid, formamide, diethylamine, and ethylene
glycol have also been investigated. Supporting electrolytes are often lithium or
tetraalkyl ammonium salts.
1. Why is it possible to characterise an organic compound using voltammetry

58
Unit Five: Coulometric and electrogravimetric methods of chemical analysis
Unit outline
Session 1: Basic principles of electrogravimetry
Session 2: Basic principles of coulometry
Session 3: Controlled-Potential Coulometry
Session 4: Controlled-Current Coulometry
Session 5: Characterization and Quantitative Applications Coulometry
Session 6: Sample Quantitative Calculations
In this unit you are going to see how the principles of electrolysis that you have learned in unit
two are applied in quantitative chemical analysis. You learned that electrolysis is widely used for
commercial purposes such as gold plating to give attractive surfaces. The amount of gold
deposited on a surface can be determined by weighing the object before and after the final
electrolysis step. This technique is called electrogravimetry and will be one of the two
electroanalytical techniques that you will learn in this unit. Alternatively, the current during
the electroplating process could be integrated to find the total charge required for electroplating.
The number of moles of electrons needed could then be used to calculate the mass of gold
deposited. The technique is called coulometry and will form the second aspect of this unit.
Unit Objectives
By the end of this unit, you should be able to:
1. Explain the principle of electrogravimetry
2. Explain the principle of coulometry

59
3. Distinguish clearly between the two types of coulometry
4. Solve quantitative problems electrogravimetry and coulometry
Session 1: Basic principles of electrogravimetry
You learned about gravimetry during you diploma programme where a complex is formed
with an analyte with a suitable complexing agent. Electrogravimetry runs almost in the same
principle. The electrolytic deposition has been used for over a century for the gravimetric
determination of metals.
Objectives
By the end of the session, students should be able to:
1. Explain the basic principle of electrogravimetry
2. State the best physical requirement of a precipitate
3. State the conditions under which electrogravimetric analysis is most reliable
5.1.1: Basic principles of electrogravimetry
In electrogravimetry, your ultimate goal should be to determine the amount of analyte
present by converting it to a product that is weighed as a deposit on one of the
electrodes in an electrolytic cell. Just like the gravimetric techniques you learned during
your diploma program, electrogravimetry does not require preliminary calibration

60
against any chemical standard because the functional relationship between the quantity
measured and the analyte concentration can be derived from theory and atomic mass
data. Electrogravimetry is mostly applied in macroanalysis.
You will later observe that, in most applications of electrogravimetry, a metal is
deposited on a weighed platinum cathode and the increase in mass is determined.
There are also a number of cases that you will meet where anodic deposition is used.
For instance, in the determination of lead as lead dioxide on platinum as well as
determination of silver as silver chloride on silver anodic deposition is used.
5.1.2: Physical properties of precipitates in electrogravimetry
You have already learned during your diploma program that for any gravimetric
analysis to be reliable, the precipitate should meet certain requirements. Do you still
remember them? In the same vein there are a number of physical properties that a
precipitate must have in order to make a electrogravimetric analysis of an analyte
reliable.
Ideally, an electrolytically deposited metal should be strongly adherent, dense and
smooth so that it can be washed, dried and weighed without mechanical loss or reaction
with the atmosphere. Good metallic deposits are fine grained and have a metallic lustre.
Spongy, powdery or flaky precipitates are usually less pure and less adherent than fine
grained deposits.

61
The principal factors that influence the physical characteristics of deposits are current
density, temperature and the presence of complexing agents. The best deposits are
usually formed at low current densities, typically of less than 0.1Acm-2. Gentle stirring
usually improves the quality of the deposit. You cannot however determine the effect of
temperature since it is unpredictable and you must determine the effect empirically.
One other thing that you will realise is that when metals are deposited from solution of
metal complexes, they form smoother and more adherent films than when deposited
from the simple ions. In this regard, cyanide and ammonia complexes often provide the
best deposits.
Self-Assessment Questions
Exercise 5.1
1. Explain how the Volta Aluminium Company (VALCo) can obtain fine and
quality deposits of aluminium during their operations.
Session 2: Basic principles of coulometry
Coulometry is related to electrogravimetry which you have just learned in the last sessions of
this unit. Both methods entails electrolysis of a sample for a very long enough time to ensure
complete oxidation or reduction of the analyte to a product of known composition.
Objectives
1. Explain coulometry

62
2. State the need for current efficiency
3. Solve sample problems in coulometry
5.2.1: Basic principles of coulometry
Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte.
By exhaustive electrolysis what you are expected to do is that your analyte is
quantitatively oxidized or reduced at the working electrode or reacts quantitatively
with a reagent generated at the working electrode.
You will generally always meet two forms of coulometry. They are controlled-potential
coulometry, in which a constant potential is applied to the electrochemical cell, and the
second is controlled-current coulometry, in which a constant current is passed through
the electrochemical cell.
The total charge, Q, in coulombs, passed during electrolysis is related to the absolute
amount of analyte by Faraday’s law that you learned in unit two.
Q = nFN
In the equation above, n is the number of electrons transferred per mole of analyte, F is
Faraday’s constant (96487 C mol–1), and N is the moles of analyte. A coulomb is also
equivalent to an Ampere second (As). Thus, if you maintain a constant current, i, for
electrolysis time te,
Then you also express the charge as; Q = ite

63
Note again that te is the electrolysis time. If current varies with time, as you will later
learn in controlled potential coulometry, then the total charge is given by
Q = ∫ i(t)dt
te
0
In coulometry, current and time are measured from which you can then calculate the
quantity of charge, Q. You then use the above relation to determine the moles, N, of
analyte. To obtain an accurate value for N, therefore, all the current must result in the
analyte’s oxidation or reduction. In other words, coulometry requires 100% current
efficiency. The current efficiency is an important factor that must be considered in
designing a coulometric method of analysis.
Example
A constant current of 0.800 A is used to deposit copper at the cathode and oxygen at the
anode of an electrolytic cell. calculate the amount in grams of each product in 15.2 min,
assuming no other redox reaction occurs.
Solution
The two half reactions are;
Cu2+ + 2e Cu(s)
2H2O 4e + O2(g) + 4H+

64
Thus, 1 mol of copper is equivalent to 2 mol of electrons, and 1 mol of oxygen
corresponds to 4 mol of electrons.
Substituting into Q = ite,
You will get; Q = 0.800 A x 15.2min x 60s/min = 729.6 A.s = 729.6 C
You can find the number of moles of Cu and O2 from the relation;
N =
𝑄
𝑛𝐹
Then NCu =
729.6 C
2
mol
mol
Cu x 96,485
C
mole
e
= 3.781 x 10-3 = mol Cu
NO2 =
729.6 C
4
mol
mol
O2 x 96,485
C
mole
e
= 1.890 x 10-3 mol O2
You can obtain their masses, knowing the relative atomic masses
Mass Cu = 3.781 x 10-3 mol x
63.35 g Cu
mol
= 0.240 g Cu
Mass O2 = 1.890 x 10-3 mol x
32.00 g O2
mol
= 0.0605 g O2
5.2.2: Current efficiency requirements for coulometry
As you just learned a short while ago, current efficiency is very vital in coulometric
methods. Ideally you must obtain 100% current efficiency. That is to say, each faraday
of electricity must bring about a chemical change in the analyte equivalent to one mole
of electrons. One thing you should note is that this 100% current efficiency must be

65
achieved without direct participation in electron transfer at the electrode. For instance,
if a chloride ion can be determined with silver ions at a silver electrode, the silver ion
then reacts with chloride ion to form a precipitate. The quantity of electricity required to
complete the silver chloride formation serves as the analytical variable. In this instance,
100% current efficiency is realised because the number of moles of electrons is equal to
the number of moles of chloride ion in the sample despite the fact that the ions do not
react directly at the electrodes.
1. Briefly define current efficiency
Session 3: Controlled-Potential Coulometry
In this session, you will turn your attention to one of the two coulometric methods, controlled-
potential coulometry. You will learn its basic principles and applications.
Objectives
By the end of the session, you should be able to:
1. Explain the basic principle of controlled-potential coulometry
2. State factors the affect the choice of a potential
5.3.1: Basic Principle
Controlled-Potential Coulometry offers you the easiest method for ensuring 100%
current efficiency. The method enables you to maintain the working electrode at a

66
constant potential. This allows for the quantitative oxidation or reduction of the analyte
without simultaneously oxidizing or reducing any interfering species.
The current flowing through an electrochemical cell under a constant potential is
proportional to the concentration of the analyte. As electrolysis progresses the
concentration of the analyte decreases, as does the current. The resulting current-
versus-time profile for controlled-potential coulometry is shown in figure 5.1.
Figure 5.1: Current-time curve for controlled-potential coulometry
To get the total charge, you need to Integrate the area under the curve, from t = 0 until t
= te. You need to consider the experimental parameters and instrumentation needed to
develop a controlled-potential coulometric method of analysis.
5.3.2: Selecting a Constant Potential
In controlled-potential coulometry, you select the potential so that the desired oxidation
or reduction reaction goes to completion without interference from redox reactions
involving other components of the sample matrix.

67
To see how an appropriate potential for the working electrode is selected, consider a
constant-potential coulometric method developed for Cu2+ based on its reduction to
copper metal at a Pt cathode working electrode.
Cu2+(aq) + 2 e Cu(s)
You can develop a ladder diagram for a solution of Cu2+ as in figure 5.2 to provide a
useful means for evaluating the solution’s redox properties.
Figure 5.2: Ladder diagram for aqueous solution of Cu2+
From the ladder diagram you can deduce that the reduction of Cu2+ is favoured when
the potential of the working electrode is more negative than +0.342 V versus the SHE
(+0.093 V versus the SCE). To maintain a 100% current efficiency, however, the potential
must be selected so that the reduction of H3O+ to H2 does not contribute significantly to
the total charge passed at the electrode. The potential needed for a quantitative
reduction of Cu2+ can be calculated using the Nernst equation.

68
5.3.3: Minimizing Electrolysis Time
The current-time curve for controlled-potential coulometry, figure 5.1 shows that the
current decreases continuously throughout electrolysis. An exhaustive electrolysis,
therefore, may require a long time. Since time is an important consideration in choosing
and designing analytical methods, the factors that determine the analysis time need to
be considered. For this reason controlled-potential coulometry is carried out in small-
volume electrochemical cells, using electrodes with large surface areas and with high
stirring rates. A quantitative electrolysis typically requires approximately 30–60 min,
although shorter or longer times are possible.
5.3.4: Instrumentation
The potential in controlled-potential coulometry is set using a three-electrode
potentiostat. Two types of working electrodes are commonly used. They are, a Pt
electrode manufactured from platinum-gauze and fashioned into a cylindrical tube, and
an Hg pool electrode. The large overpotential for reducing H3O+ at mercury makes it
the electrode of choice for analytes requiring negative potentials. For example,
potentials more negative than –1 V versus the SCE are feasible at an Hg electrode but
not at a Pt electrode, even in very acidic solutions. The ease, with which mercury is
oxidized, however, prevents its use at potentials that are positive with respect to the
SHE. Platinum working electrodes are used when positive potentials are required. The
auxiliary electrode, which is often a Pt wire, is separated by a salt bridge from the
solution containing the analyte. This is necessary to prevent electrolysis products

69
generated at the auxiliary electrode from reacting with the analyte and interfering in the
analysis.
1. State any best conditions necessary for control potential coulometry
Session 4: Controlled-Current Coulometry
In this session, you will turn your attention to the second coulometric methods mentioned
earlier, controlled-current coulometry. You will learn its basic principles and applications.
Objectives
By the end of the session, you should be able to:
1. explain the working principle of controlled-current coulometry
2. state some advantages in using controlled-current coulometry
3. determine when a reaction in controlled-current coulometry ends
5.4.1: Basic principle of controlled current coulometry
Controlled-current coulometry, a second approach to coulometry uses a constant
current in place of a constant potential Figure 5.3.

70
Figure 5.3: Current-time curve for controlled-current coulometry
It may interest you to know that controlled-current coulometry is called amperostatic
coulometry or coulometric titrimetry. It has two advantages over controlled-potential
coulometry.
First, if you are using a constant current, this makes analysis fast since the current does
not decrease over time. Thus, a typical analysis time for controlled current coulometry
is less than 10 min, as opposed to approximately 30–60 min for controlled-potential
coulometry.
Second, it is easier for you to evaluate total charge. This is simply the product of current
and time. You therefore do not need a method for integrating the current–time curve.
However there are two important experimental problems that you must solve in order
to obtain accurate results.
First, as the electrolysis occurs, the concentration of the analyte and for that matter, the
current due to its oxidation or reduction steadily decreases. In order for you to maintain

71
the constant current, you must vary the cell potential until another oxidation or
reduction reaction can occur at the working electrode. Unless the system is carefully
designed, these secondary reactions will produce a current efficiency of less than 100%.
The second problem is the need for a method of determining when the analyte has been
exhaustively electrolyzed. In the case of controlled-potential coulometry, this is
signalled by a decrease in the current to a constant background or residual current. In
controlled-current coulometry, however, a constant current continues to flow even
when the analyte has been completely oxidized or reduced. You will therefore need a
suitable means of determining the time, te, when the reaction ends.
5.4.2: End Point Determination
You can add a mediator which solves the problem of maintaining 100% current
efficiency, and also solves the problem of determining when the electrolysis of the
analyte is complete. Thus, the same end points that are used in redox titrimetry such as
visual indicators, and potentiometric and conductometric measurements, may be used
to signal the end point of a controlled-current coulometric analysis. For example, ferroin
may be used to provide a visual end point for the Ce3+-mediated coulometric analysis
for Fe2+.
Using the same example, once all the Fe2+ has been oxidized current continues to flow
as a result of the oxidation of Ce3+ and, eventually, the oxidation of H2O. What is
needed is a means of indicating when the oxidation of Fe2+ is complete. In this respect it

72
is convenient to treat a controlled current coulometric analysis as if electrolysis of the
analyte occurs only as a result of its reaction with the mediator. A reaction between an
analyte and a mediator is identical to that encountered in a redox titration.
Instrumental Controlled-current coulometry normally is carried out using a galvanostat
and an electrochemical cell consisting of a working electrode and a counter electrode.
The working electrode, which often is constructed from Pt, is also called the generator
electrode since it is where the mediator reacts to generate the species reacting with the
analyte. The counter electrode is isolated from the analytical solution by a salt bridge or
porous frit to prevent its electrolysis products from reacting with the analyte.
Alternatively, oxidizing or reducing the mediator can be carried out externally, and the
appropriate products flushed into the analytical solution.
Figure 5.4: Method for the external generation of oxidizing and reducing agents in
coulometric titrations.

73
Figure 5.4 shows one simple method by which oxidizing and reducing agents can be
generated externally. A solution containing the mediator flows under the influence of
gravity into a small-volume electrochemical cell. The products generated at the anode
and cathode pass through separate tubes, and the appropriate oxidizing or reducing
reagent can be selectively delivered to the analytical solution. For example, external
generation of Ce4+ can be obtained using an aqueous solution of Ce3+ and the products
generated at the anode. The other necessary instrumental component for controlled-
current coulometry is an accurate clock for measuring the electrolysis time, te, and a
switch for starting and stopping the electrolysis.
5.4.3: Coulometric Titrations
Controlled-current coulometric methods commonly are called coulometric titrations
because of their similarity to conventional titrations. You have already noted, in
discussing the controlled-current coulometric determination of Fe2+, that the oxidation
of Fe2+ by Ce4+ is identical to the reaction used in a redox titration.
The titrant in a conventional titration is replaced in a coulometric titration by a
constant-current source whose current is analogous to the molarity of the titrant. The
time needed for an exhaustive electrolysis takes the place of the volume of titrant, and
the switch for starting and stopping the electrolysis serves the same function as a
stopcock of the burette.

74
1. What is the role of a mediator in constant current coulemetry
Session 5: Characterization and Quantitative Applications Coulometry
You will now turn your attention to some quantitative applications of coulometry. It may
interest you to note that quantitative analysis of both inorganic and organic compounds are
involved. You will learn some examples of controlled-potential and controlled-current
coulometric methods in this session.
Objectives
1. state some applications of control-potential coulometry
2. state some applications of control-current coulometry
3. state the advantages of control-current coulemetry over conventional
titrimetry
5.5.1Application of controlled-potential coulometry in the determination of inorganic
ions
The majority of controlled-potential coulometric analyses that you will meet involve the
determination of inorganic cations and anions, including trace metals and halides. Table
5.1 provides you with a summary of several of these methods.

75
Table 5.1: Representative Examples for the Controlled- Potential Coulometric Analysis
of Inorganic Ions
Analyte Electrolytic reaction Electrode
Antimony Sb(III) + 3e ⇌ Sb Pt
Arsenic As(III) + ⇌ As Pt
Cadmium Cd(II) + 2e ⇌ Cd Pt or Hg
Cobalt Co(II) + 2e ⇌ Co Pt or Hg
Copper Cu(II) ⇌ Cu Pt or Hg
Halides Ag + X- ⇌ AgX + e Ag
Iron Fe(II) ⇌ Fe(III) + e Pt
Lead Pb(II) + 2e ⇌ Pb Pt or Hg
Nickel Ni(II) + 2e ⇌ Ni Pt or Hg
Plutonium Pu(III) ⇌ Pu(IV) + e Pt
Silver Ag (I) + e ⇌ Ag Pt
Tin Sn(II) + 2e ⇌ Sn Pt
Uranium U(VI) + 2e ⇌ U(IV) Pt or Hg
Zinc Zn(II) + 2e ⇌ Zn Pt or Hg
The ability to control selectivity by carefully selecting the potential of the working
electrode, makes controlled-potential coulometry particularly useful for the analysis of
alloys. For example, you can determine the composition of an alloy containing Ag, Bi,
Cd, and Sb by dissolving the sample and placing it in a matrix of 0.2 M H2SO4. A
platinum working electrode is immersed in the solution and held at a constant potential
of +0.40 V versus the SCE. At this potential Ag(I) deposits on the Pt electrode as Ag, and
the other metal ions remain in solution. When electrolysis is complete, you can use the
total charge to determine the amount of silver in the alloy. The potential of the platinum
electrode is then shifted to –0.08 V versus the SCE, depositing Bi on the working
electrode. When the coulometric analysis for bismuth is complete, antimony is
determined by shifting the potential of the working electrode to –0.33 V versus the SCE,

76
depositing Sb. Finally, cadmium is determined following its electrodeposition on the Pt
electrode at a potential of –0.80 V versus the SCE.
Another area where controlled-potential coulometry has found application is in nuclear
chemistry, in which elements such as uranium and polonium can be determined at trace
levels. For example, microgram quantities of uranium in a medium of H2SO4 can be
determined by reducing U(VI) to U(IV) at a mercury working electrode. Controlled-
potential coulometry also can be applied to the quantitative analysis of organic
compounds, although the number of applications is significantly less than that for
inorganic analytes. One example is the six-electron reduction of a nitro group, –NO2, to
a primary amine, –NH2, at a mercury electrode. Solutions of picric acid, for instance, can
be analyzed by reducing to triaminophenol.
5.5.2: Application of Controlled-Current Coulometry in quantitative analysis
The use of a mediator makes controlled-current coulometry a more versatile analytical
method than controlled-potential coulometry. For example, the direct oxidation or
reduction of a protein at the working electrode in controlled-potential coulometry is
difficult if the redox active site of the protein lies deep within its structure. The
controlled-current coulometric analysis of the protein is made possible, however, by
coupling its oxidation or reduction to a mediator that is reduced or oxidized at the
working electrode. Controlled-current coulometric methods have been developed for
many analytes that may be determined by conventional redox titrimetry. These
methods are also called coulometric redox titrations. Coupling the mediator’s oxidation

77
or reduction to an acid–base, precipitation, or complexation reaction involving the
analyte allows for the coulometric titration of analytes that are not easily oxidized or
reduced. For example, when using H2O as a mediator, oxidation at the anode produces
H3O+ while reduction at the cathode produces OH–. If the oxidation or reduction of H2O
is carried out externally using the generator cell then H3O+ or OH– can be dispensed
selectively into a solution containing a basic or acidic analyte. The resulting reaction is
identical to that in an acid–base titration. Coulometric acid–base titrations have been
used for the analysis of strong and weak acids and bases, in both aqueous and
nonaqueous matrices. There are several examples of coulometric titrations involving
acid–base, complexation, and precipitation reactions. In comparison with conventional
titrimetry, there are several advantages to the coulometric titrations. One advantage is
that the electrochemical generation of a “titrant” that reacts immediately with the
analyte allows the use of reagents whose instability prevents their preparation and
storage as a standard solution. Thus, highly reactive reagents such as Ag2+ and Mn3+
can be used in coulometric titrations. Because it is relatively easy to measure small
quantities of charge, coulometric titrations can be used to determine small quantities of
analyte that cannot be measured accurately by a conventional titration.
1. State one advantages of control-current coulometry over conventional titrimetry
Session 6: Sample Quantitative Calculations

78
In this session you will learn to solve quantitative problems in coulometric analysis based on
Faraday’s law.
Objectives
1. Apply Faraday’s law in quantitative calculations
5.6.1: Quantitative Calculations
You can determine the absolute amount of analyte in a coulometric analysis by
applying Faraday’s law with the total charge during the electrolysis. You follow the
example given to do calculations for a typical coulometric analysis.
Example 1
The purity of a sample of Na2S2O3 was determined by a coulometric redox titration
using I– as a mediator, and I3– as the “titrant.” A sample weighing 0.1342 g is transferred
to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL
portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a
pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at
a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point.
Determine the purity of the sample.
Solution

79
The equation for the coulometric titration of S2O32– with I3– is
2S2O32–(aq) + I3–(aq) ⇌ S4O62–(aq) + 3I–(aq)
Oxidizing S2O32– to S4O62– requires one electron per S2O32– (n = 1).
The number of moles of Na2S2O3 is given as;
𝑛𝐹(𝑔𝑁𝑎2 𝑆2 𝑂3)
𝐹𝑊𝑁𝑎2 𝑆2 𝑂3
= ite
Solving for gram of Na2S2O3 gives
g Na2S2O3 =
𝑖𝑡𝑒𝐹𝑊𝑁𝑎2𝑆2 𝑂3
𝑛𝐹
=
(0.03645 𝐴)(221.8 𝑠)(158.1
𝑔
𝑚𝑜𝑙
)
(1 𝑚𝑜𝑙 𝑒)(
96487𝐶
𝑚𝑜𝑙
𝑒)
= 0.01325 g Na2S2O3
This represents the amount of Na2S2O3 in a 10.0 mL portion of a 100 mL sample. Thus
0.1325 g of Na2S2O3 is present in the original sample. The purity of the sample is
therefore;
0.1325 𝑔𝑁𝑎2 𝑆2 𝑂3
0.1342 𝑔 𝑠𝑎𝑚𝑝𝑙𝑒
x 100% = 98.73%
You should note that the calculation is worked as if S2O32– is oxidized directly at the
working electrode instead of in solution.
Example 2
The Fe(III) in 0.8202 g sample was determined by coulometric reduction to Fe(II) at a
platinum cathode. Calculate the percentage of Fe2(SO4)3 (M = 399.88 g/mol) in the
sample if 103.2775 C were required for the reduction.
Solution

80
You will realise that 1 mol of Fe2(SO4)3 consumes 2 mol of electrons. So the number of
moles of Fe2(SO4)3 is given as;
Mol(Fe2(SO4)3) =
103 .2775 𝐶
2 𝑚𝑜𝑙
𝑒
𝑚𝑜𝑙Fe2 (SO4)3
x
96485C
mol
e
= 5.3520 x 10-4 mol Fe2(SO4)3
Mass(Fe2(SO4)3 = 5.3520 x 10-4 mol Fe2(SO4)3 x
399.88 𝑔Fe2(SO4)3
𝑚𝑜𝑙 Fe2(SO4)3
= 0.21401 g Fe2(SO4)3
Percentage Fe2(SO4)3 =
0.21401 𝑔 Fe2(SO4)3
0.8202 𝑔 𝑠𝑎𝑚𝑝𝑙𝑒
x 100% = 26.09%
1. A constant current of 0.800 A is used to deposit copper at the cathode and
oxygen at the anode of an electrolytic cell. calculate the number of grams of
each product formed in 15.2 min, assuming no other redox reaction occurs

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