This document covers the principles and techniques of voltammetry, focusing on polarography and amperometric titration, particularly in the context of pharmaceutical applications. It describes the function and structure of voltammetric cells, types of electrodes, and various voltammetric methods, including classic polarography and different pulse techniques. Additionally, it details how to determine ascorbic acid in citrus juice using polarographic methods, highlighting the importance of mass transport and the electrochemical principles involved.

1. Polarography and Amperometric Titration
Sania Ashrafi
Lecturer
Department of Pharmaceutical Chemistry
Faculty of Pharmacy
University of Dhaka

2. Voltammetry
❑ Voltammetry refers to electrochemical methods in which a specific voltage is applied to a working electrode as a
function of time and the current produced by the system is measured. This is commonly done with an instrument
called a potentiostat, which is capable of applying variable potentials to the working electrode relative to a
reference electrode (like Ag/AgCl) while measuring the current that flows as a result of the electrode reaction.
❑ Voltammetry is concerned with the study of the voltage-current-time relationship during electrolysis in a cell.
❑ We call the resulting plot of current versus applied potential a voltammogram
❑ Depending on the particular method, it is possible to apply reducing and/or oxidizing potentials. When reduction
occurs, the current is called a cathodic current. When oxidation occurs, the current is called an anodic current.

3. ➢ Although early voltammetric methods used only two electrodes, a modern voltammeter makes use
of a three-electrode—
• Working electrode/indicator electrode: At which electrolysis in a cell takes place (Dropping Mercury
Electrode- DME).
• Reference electrode: used to measure the working electrode’s potential and which has a stable and
well-known electrode potential (Saturated Calomel Electrode-SCE or a Ag/AgCl electrode).
Importance: Potential is not something that can be directly measured. Rather, the measurement of
applied potential requires that a reference point first be established, and individual potentials be
measured relative to that reference point.
• Auxiliary electrodes: Which is used with the indicator electrode to carry the electrolysis current.
Generally a platinum wire.
Importance: The purpose of the auxiliary electrode is to provide a pathway for current to flow in the
electrochemical cell without passing a significant current through the reference electrode.
Voltammetric Electrodes

4. Typical electrochemical cell for voltammetry

5. For the working electrode, we can choose from several different materials, including
➢ mercury,
➢ platinum,
➢ gold,
➢ silver, and
➢ carbon.
Voltammetric Electrodes
Because mercury is a liquid, the working electrode is often a drop suspended from the end of a capillary tube.
In the hanging mercury drop electrode, or HMDE, we extrude the drop of Hg by rotating a micrometer screw
that pushes the mercury from a reservoir through a narrow capillary tube.
In the dropping mercury electrode or DME, mercury drops form at the end of the capillary tube as a result of
gravity. Unlike the HMDE, the mercury drop of a DME grows continuously—as mercury flows from the reservoir
under the influence of gravity—and has a finite lifetime of several seconds. At the end of its lifetime, the
mercury drop is dislodged, either manually or on its own, and replaced by a new drop.
Mercury

6. Three examples of mercury electrodes: (a) hanging mercury drop electrode, or HMDE; (b) dropping mercury
electrode, or DME; and (c) static mercury drop electrode, or SMDE.
Voltammetric Electrodes
The static mercury drop electrode, or SMDE, uses
a solenoid-driven plunger to control the flow of
mercury. Activation of the solenoid momentarily
lifts the plunger, allowing mercury to flow
through the capillary and forming a single,
hanging Hg drop. Repeatedly activating the
solenoid produces a series of Hg drops. In this
way, the SMDE may be used as either an HMDE
or a DME.

7. Current in Voltammetry
Faradaic Current
The current from redox reactions at the working electrode and the auxiliary electrodes is called a faradaic current.
Because the reaction of interest occurs at the working electrode, we describe the faradaic current using this reaction.
• A faradaic current due to the analyte’s reduction is a cathodic current, and its sign is positive.
• An anodic current is due to an oxidation reaction at the working electrode, and its sign is negative.
Residual Current
Even in the absence of an analyte, a small, measurable current flows through an electrochemical cell. This
residual current has two components: a faradaic current due to the oxidation or reduction of trace impurities
and the charging current (the current flows due to continuous charging of new Hg drops to the applied
potential).

8. The rate at which material gets from the bulk of the solution to the electrode, is known as mass transport. There
are three modes of mass transport that affect the rate at which reactants and products move toward or away from
the electrode surface:
❑ Diffusion
❑ Convection
❑ Migration
Influence of Mass Transport on the Faradaic Current
Schematic showing the transport of Fe(CN)6
4– away
from the electrode’s surface and the transport of
Fe(CN)6
3– toward the electrode’s surface following
the reduction of Fe(CN)6
3– to Fe(CN)6
4–

9. ➢ Diffusion occurs whenever the concentration of an ion or molecule at the surface of the electrode is
different from that in bulk solution. If we apply a potential sufficient to completely reduce Fe(CN)6
3– at the
electrode surface, the result is a concentration gradient. The region of solution over which diffusion occurs is
the diffusion layer. In the absence of other modes of mass transport, the width of the diffusion layer, δ,
increases with time as the Fe(CN)6
3– must diffuse from increasingly greater distances.
➢ Convection occurs when we mechanically mix the solution, carrying reactants toward the electrode and
removing products from the electrode. The most common form of convection is stirring the solution with a
stir bar. Other methods that have been used include rotating the electrode and incorporating the electrode
into a flow cell.
➢ Migration occurs when a charged particle in solution is attracted to or repelled from an electrode that
carries a surface charge. If the electrode carries a positive charge, for example, an anion will move toward
the electrode and a cation will move toward the bulk solution. Unlike diffusion and convection, migration
only affects the mass transport of charged particles.

10. The movement of material to and from the electrode surface is a complex function of all three modes of mass
transport. In the limit where diffusion is the only significant form of mass transport, the current in a voltammetric
cell is equal to-
where n the number of electrons in the redox reaction,
• F is Faraday’s constant (96,485 C/mol),
• A is the area of the electrode,
• D is the diffusion coefficient for the species reacting at the
electrode,
• Cbulk and Cx=0 are its concentrations in bulk solution and at the
electrode surface, and
• δ is the thickness of the diffusion layer.
Influence of Mass Transport on the Faradaic Current
Concentration gradient for Fe(CN)6
3– when stirring
the solution. Diffusion is the only significant form of
mass transport close to the electrode’s surface. At
distances greater than δ, convection is the only
significant form of mass transport, maintaining a
homogeneous solution in which the concentration of
Fe(CN)6
3– at the electrode is the same as its
concentration in bulk solution.

11. In the absence of convection the diffusion
layer increases with time, and the resulting
voltammogram has a peak current instead of a
limiting current (b).
The current increases from a background residual current to a limiting current, il.
Because the faradaic current is inversely proportional to the thickness of the diffusion layer, δ, a limiting current
occurs only if the thickness of the diffusion layer remains constant because we are stirring the solution.
We also can monitor the change in current, Δi,
following a change in potential. As shown in
(c), the resulting voltammogram also exhibits a
peak current.
Voltammetric Measurements
a b
c
d
o

12. Types of Voltammetric Techniques
Polarography- The first important voltammetric technique to be developed—polarography
Amperometry
The final voltammetric technique we will consider is amperometry, in which we apply a constant potential to
the working electrode and measure current as a function of time.
Cyclic Voltammetry
We scan the potential in one direction, either to more positive potentials or to more negative potentials. In
cyclic voltammetry we complete a scan in both directions.
Stripping Voltammetry
Stripping voltammetry is an electroanalytical technique that involves the preconcentration of an analyte
on an electrode, followed by a potential sweep to selectively oxidize or reduce the analyte, with the
current generated proportional to the amount of analyte present on the electrode.
Hydrodynamic Voltammetry
The analyte solution flows relative to a working electrode. In polarography If we replace the DME with a
solid electrode we can still obtain a limiting current if we mechanically stir the solution either using a stir
bar or by rotating the electrode. We call this approach hydrodynamic voltammetry.

13. Advantages
• Potent analytical tool and sensitivity is very high
• Less time consuming
• Several analytes can be determined simultaneously
• Both aqueous and non-aqueous solvents can be used
• Theory of voltammetry is well established
• Potent and effective analytical tool

14. Polarography
❑ The polarographic method of analysis was developed by Jaroslav Heyrovsky in 1922 which is the
earliest voltammetric technique introduced.
❑ He received Nobel Prize in 1952 for developing this technique using dropping mercury electrode as the
working electrode.
❑ He called the recorded current-potential curves as polarograms and introduced the term polarography.
➢ Polarography is an electroanalytical technique. It is restricted to the voltametric method in which the
analyte is dissolved in a suitable medium and is placed in an electrolysis cell where the electrolysis is
controlled by a variable known potential applied to the dropping-mercury-electrode (DME).
➢ DME is polarized relative to a non-polarized electrode, usually a saturated calomel electrode (SCA).

15. The pharmaceutical applications of polarography:-
(1) Trace metals and metal-containing drugs.
(2) Antiseptics and insecticides.
(3) Vitamins.
(4) Hormones.
(5) Antibiotics.
(6) Alkaloids.
(7) Miscellaneous pharmaceutical substances.
(8) Blood serum and cancer diagnosis.
(9) Dissolved oxygen and peroxides.
Pharmaceutical Application

16. Polarography (Basic Principle)
➢ In polarographic method, the sample is taken in a cell where two electrodes are immersed. The
sample solution also contains an auxiliary electrode.
➢ Two electrodes are----
• Working electrode/indicator/microelectrode: At which electrolysis in a cell takes place (DME)
• Reference electrode: Changing its potential relative to the fixed potential (SCE or a Ag/AgCl electrode)
➢ A gradually increased potential is applied to electrodes and the corresponding current is measured.
The current Vs potential is plotted graphically to get a S shaped current-voltage curve (generally) is
called polarogram.
➢ The analyte is oxidised or reduced at the working electrode. On the other hand, the potential of the
reference electrode remains constant throughout the experiment.

17. il= limiting current
ir= residual current
A Typical Polarogram
the half-wave potential, E1/2
• The gradual increase of current over the portion of AB is called residual current
• At the point B (decomposition potential), electrolysis occurs and the discharged ions begin to deposit on
the electrode—
a b
c d
o
• The half-wave potential, E1/2 , provides qualitative information about the redox reaction.
Polarography (Basic Principle)

18. When the applied potential is equal to the decomposition potential (B) of the electroactive substance (e.g.
Zn2+) at the cathode the current starts increasing due to the following cathodic reaction.
The resulting zinc will form amalgam with mercury on the surface of
the mercury electrode.
At the point O, corresponding potential is called half-wave potential,
and the concentration of oxidized and reduced forms are equal at the
electrode surface—
i.e., [Zn2+]= [Zn]
From C to D the current remains constant which is determined by the
rate of diffusion of Zn2+ ions from the bulk of the solution to the
electrode surface.
The number of Zn2+ ions diffusing from the bulk of the solution to the
electrode surface is equal to the number that is deposited when the
steady state is reached or the rate of diffusion is equal to the rate of
reduction.
il= limiting current
ir= residual current
the half-wave potential, E1/2
a b
c d
o
Polarography (Basic Principle)

19. ❑ The diffusion current can be characterized by the Ilkovic equation:
Where,
id = maximum diffusion current during the life of the drop(µA).
D= Diffusion coefficient of the analyte in the medium (cm2/s).
n = Number of electrons transferred per mole of analyte.
m = mass flow rate of Hg through the capillary (mg/sec).
t = drop lifetime(s).
c = analyte concentration (mol/cm3).
Polarography (Basic Principle)

20. Classification of Polarography
Based on variable potential excitation signal which produces a characteristic current response—
1. Classic or linear scan polarography
2. Current sampled polarography
3. Pulse polarography
a. Normal pulse polarography
b. Differential pulse polarography
c. Staircase polarography
d. Square-wave pulse polarography
Classic or linear scan polarography
❑ In this type of polarography the applied potential is increased linearly at the rate of 5 mv/sec
❑ Very simple but slow method
❑ Poor detection limit
❑ Large current variation

21. Classification of Polarography
Current sampled polarography
The current is sampled at the 5-20 ms before termination of
each drop/cycle.
This method substantially determines the current fluctuations
due to the continuous growth and fall of drops.

22. • Normal pulse polarography (Figure a), for example, uses a series of potential pulses characterized by a
cycle of time of τ, a pulse-time of tp , a pulse potential of ΔEp , and a change in potential per cycle of ΔEs .
• The current is sampled at the end of each potential pulse for approximately 17 ms before returning the potential to its
initial value.
• We apply the potential for only a small portion of the drop’s lifetime, there is less time for the analyte to undergo
oxidation or reduction and a smaller diffusion layer. As a result, the faradaic current in normal pulse polarography is
greater than in the polarography, resulting in better sensitivity and smaller detection limits.
Normal pulse polarography
Potential-excitation signals and voltammograms for (a) normal pulse polarography
Classification of Polarography
τ ≈ 1 s, tp≈ 50 ms, and ΔEp ≈ 2 mV.
The initial value of ΔEp is ≈ 2 mV,
and it increases by ≈ 2 mV with each pulse.

23. ❑ In differential pulse polarography (Figure b) the current is measured twice per cycle: for approximately 17 ms
before applying the pulse and for approximately 17 ms at the end of the cycle.
❑ The difference in the two currents gives rise to the peak-shaped voltammogram.
❑ Typical experimental conditions for differential pulse polarography are τ ≈ 1 s, tp ≈ 50 ms, ΔEp ≈
50 mV, and ΔEs ≈ 2 mV.
(b) differential pulse polarography
Classification of Polarography
Differential pulse polarography

24. Other forms of pulse polarography include staircase polarography (Figure c) and square-wave polarography
(Figure d).
One advantage of square-wave polarography is that we can make τ very small—perhaps as small as 5 ms,
compared to 1 s for other pulse polarographies—which can significantly decrease analysis time. For example,
suppose we need to scan a potential range of 400 mV. If we use normal pulse polarography with a ΔEs of 2
mV/cycle and a τ of 1 s/cycle, then we need
200 s to complete the scan. If we use square-wave polarography with a ΔEs of 2 mV/cycle and a τ of 5 ms/cycle,
we can complete the scan in 1 s.
At this rate, we can acquire a complete voltammogram using a single drop of Hg!
(c) staircase polarography, and (d) square-wave polarography.
Classification of Polarography

25. The current is sampled at the time intervals shown by the black rectangles. When measuring a change in current, Δi,
the current at point 1 is subtracted from the current at point 2. The symbols in the diagrams are as follows: τ is the
cycle time; ΔEp is a fixed or variable pulse potential; ΔEs is the fixed change in potential per cycle, and t is the pulse
time.

26. Determination of Ascorbic acid (Vitamin C) in the Citrus Juice by the Standard Addition and
Calibration Curve Methods:
Principle: Ascorbic acid gives a well defined polarographic oxidation wave.
Use freshly prepared diluted juice for the determination of ascorbic acid.
Calibration Curve Method:
1.Prepare a fresh stock solution of 50 cm3 of 0.2% ascorbic acid.
2.Prepare 5 standard solutions of ascorbic acid in volumetric flasks of 25 cm3.
3.To each volumetric flask add 0.5 cm3 of 0.5M acetate buffer and different volumes of 0.2% ascorbic acid,
0, 200, 400, 600 and 800 μdm3.
4.Dilute to the mark with distilled water.
5.For each solution record polarograms over the potential range -150 to + 200 mV vs Ag/AgCl/1M KCl
reference electrode.
6.Plot 'id' vs 'c' of ascorbic acid (calibration curve).

27. Determination of Ascorbic acid (Vitamin C) in the Citrus Juice by the Standard Addition and
Calibration Curve Methods:
Standard Addition Methods:
1.Squeeze an orange, grape fruit or lemon until about 10 cm3 of juice is obtained.
2.Filter the juice through a porous funnel (pore size about 1 mm).
3.Prepare four 25 cm3 volumetric flasks.
4.Add to each 0.5 cm3 of 0.5M acetate buffer, 2.0 cm3 of the juice and standard addition of 0,
200, 400, and 600 μdm3 of 0.2% ascorbic acid.
5.Dilute to mark with distilled water.
6.Record polarograms under the same conditions as in the calibration step.
7.Draw the standard additions plot and determine the concentration of ascorbic acid. Report the
concentration of ascorbic acid (Vitamin C) in the original sample (juice) in mol/l and also ppm.

28. Amperometry
❑ In Amperometry, we apply a constant potential to the working electrode and measure current as a function of
time.
❑ In Amperometric titration, the analyte is dissolved in an appropriate volume of suitable solvent.
❑ Then the electrical connections are completed (as we discussed in polarography)
❑ The electro reactive material (sample) is diffused into the electrode surface from the bulk of the solution (as we
discussed in polarography)
❑ As the diffusion process is governed by the concentration gradient, the diffusion current is proportional to the
concentration of the electro reactive material present in the solution.

29. where (il)avg is the average diffusion current
n is the number of electrons in the redox reaction,
D is the analyte’s diffusion coefficient,
m is the flow rate of the Hg,
t is the drop’s lifetime and
Kavg is constant.
So, from Ilkovic equation--
Amperometry
❑ Now, if some electro-reactive materials are removed by the interaction with the titrant, the diffusion current
will be gradually decreased.
❑ The observed diffusion current at a suitable applied voltage is measured as a function of the volume of titrant
added.

30. End point determination--
Amperometry
❑ The observed diffusion current is plotted against
volume of the titrant. The intersection of two lines
indicates the end point of the titration.
❑ Electroactive material is being removed from the
solution by precipitation or complex formation
with an inactive titrant at that potential applied.
❑ e.g. Pb+2 vs Oxalate or Zn2+ vs EDTA.
Titration Curves in Amperometry:
A) analyte is reducible but titrant and product not: When
solution containing Pb+2 ion is titrated against SO4-2 ion. A
precipitate of PbSO4 is formed. The titration can be
performed at fixed potential -0.8 Volt v/s saturated calomel
electrode. As titration is proceeds concentration of Pb+2 ion
decreases and diffusion current also decreases till it becomes
minimum at equivalence point. The diffusion current remains
constant beyond end point. The values of diffusion current is
plotted against the volume of titrant added. The resulting
titration curves is straight line levelling off at end point . The
intersection of two extra plotted portions of the curves gives
the end point.

31. Amperometry
❑ Substance to be determined does not give diffusion
current but the titrant (reagent) gives current at the
applied potential. (e.g: SO4
2− ions titrated with Pb2+
ions).
B) Titrant is reducible but analyte and product not:
When solution containing Mg+2 ion is titrated against
with the reducible species such as 8- hydroxy quinoline
because Mg+2 ion does not undergoes reduction.
Beyond the end point the 8- hydroxyl quinoline
undergoes reduction. As its concentration increases
diffusion current also increases.
As long as the substance is available to the titrant, that
titrant reacts with substance and current remains
constant. After the substance is completely removed by
reagent the excess reagent gives its diffusion current in
proportional to the excess volume added. So, the
reverse L shaped curve obtained.

32. Amperometry
Both substance to be determined and the titrating reagent give diffusion currents at the potential selected (eg.
Pb2+ titrated with Cr2O7
-2 at -0.08 V; SCE)
C) Analyte and titrant both are reducible but product not:
When solution containing Pb+2 ion is titrated against K2Cr2O7 .
The titration is performed at potential of -0.8 Volt v/s SCE .
Diffusion current is decreases due to removal of Pb+2 ion as
lead dichromate. The current is minimum at the end point. On
further addition of the titrant the current once again increases.
V shaped curve is obtained.

33. ❑ Wider range of applications than polarography because even electro-inactive substances can be
determined using electro- active titrant .
• Determination of water using KF reagent (bioampereometry)
• Quantification of ions or mixture of ions.
• Amperometric titration are quantitative in nature they are used to determine the end point of such
titration.
• Can be used to measure dissolved O2 in blood
Application of amperometry

34. Advantages of Amperometry
1. The apparatus is very simple, no indicator required and characteristics of electrodes are less important
3. A number of amperometric titrations can be carried out at higher dilutions (ca 10−4 M) at which many
visual or potentiometric titrations no longer yield accurate results.
4. ‘Foreign’ salts may frequently be present without interference and are, indeed, usually added as the
supporting electrolyte in order to eliminate the migration current which is not possible in case of
conductometric titration.
5. The results of the titration are independent of the characteristics of the capillary.
6. The temperature need not be known provided it is kept constant during the titration.
2. The titration can usually be carried out rapidly, since the end point is found graphically; a few current
measurements at constant applied voltage before and after the end point are sufficient.

35. 7. Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual
indicator methods are unsatisfactory.
For example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed
(acid-base titration). This is because the readings near the equivalence point have no special significance in
amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which
points the solubility or hydrolysis is suppressed by the Mass Action effect; the point of intersection of these lines
gives the equivalence point.
Advantages of Amperometry
8. Although a polarograph is convenient as a means of applying the voltage to the cell, its use is not essential in
amperometric titrations. The constant applied voltage may be obtained with a simple potentiometric device.