Analytical chemistry Electroanalytical analysis

2. VOLTAMETRY

3. • Voltammetry is a category of electroanalytical
methods used in analytical chemistry and
various industrial processes. In voltammetry,
information about an analyte is obtained by
measuring the current as the potential is
varied

4. Shape of Voltammograms
The shape of a voltammogram is determined by
several experimental factors, the most
important of which are how we measure the
current and whether convection is included as a
means of mass transport.

8. current
• When we oxidize an analyte at the working
electrode, the resulting electrons pass through
the potentiostat to the auxiliary electrode,
reducing the solvent or some other component
of the solution matrix. If we reduce the analyte at
the working electrode, the current flows from the
auxiliary electrode to the cathode. In either case,
the current from redox reactions at the working
electrode and the auxiliary electrodes is called
a faradaic current.

9. Influence of Mass Transport on the
Faradaic Current
• 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, migration, and
convection

10. • Diffusion occurs whenever the concentration
of an ion or molecule at the surface of the
electrode is different from that in bulk
solution.
• Convection occurs when we mechanically mix
the solution, carrying reactants toward the
electrode and removing products from the
electrode

11. • migration, which occurs when a charged
particle in solution is attracted to or repelled
from an electrode that carries a surface charge

13. charging current
• Due to the movement of ions and the
movement of electrons are indistinguishable,
the result is a small, short-lived nonfaradaic
current that we call the charging current

14. Residual Current
• Even in the absence of 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.

15. Quantitative and Qualitative Aspects
of Voltammetry
• Determining Concentration
• Determining the Standard-state Potential

16. Determining Concentration
• Let’s assume that the redox reaction at the
working electrode is
• O+ne−⇋R(11.37)(11.37)O+ne−⇋R
• where O is the analyte’s oxidized form and R is its
reduced form. Let’s also assume that only O is
present in bulk solution and that we are stirring
the solution. When we apply a potential causing
the reduction of O to R, the current depends on
the rate at which O diffuses through the fixed
diffusion layer shown in Figure 11.41. Using
equation 11.36, the current, i, is

17. • i=KO([O]bulk−[O]x=0)(11.38)(11.38)i=KO([O]bulk−[O]x=0)
• where KO is a constant equal to nFADO/δ. When we reach
the limiting current, il, the concentration of O at the
electrode surface is zero and equation 11.38 simplifies to
• il=KO[O]bulk(11.39)(11.39)il=KO[O]bulk
• Equation 11.39 shows us that the limiting current is a linear
function of the concentration of O in bulk solution. To
determine the value of KO we can use any of the
standardization methods covered in Chapter 5. Equations
similar to equation 11.39 can be developed for the
voltammograms shown in Figure 11.42b and Figure 11.42c.

19. Quantitative Applications

20. • Environmental Samples
• Voltammetry is one of several important
analytical techniques for the analysis of trace
metals in environmental samples, including
groundwater, lakes, rivers and streams, seawater,
rain, and snow. Detection limits at the parts-per-
billion level are routine for many trace metals
using differential pulse polarography, with anodic
stripping voltammetry providing parts-per-trillion
detection limits for some trace metal

21. • Clinical Samples
• Differential pulse polarography and stripping
voltammetry may be used to determine the
concentration of trace metals in a variety of
clinical samples, including blood, urine, and
tissue. The determination of lead in blood is of
considerable interest due to concerns about lead
poisoning. Because the concentration of lead in
blood is so small, anodic stripping voltammetry
frequently is the more appropriate technique

22. • Miscellaneous Samples
• In addition to environmental samples and clinical
samples, differential pulse polarography and stripping
voltammetry have been used for the analysis of trace
metals in other sample, including food, steels and
other alloys, gasoline, gunpowder residues, and
pharmaceuticals. Voltammetry is an important
technique for the quantitative analysis of organics,
particularly in the pharmaceutical industry where it is
used to determine the concentration of drugs and
vitamins in formulation

23. Scale of Operation
• Selectivity
• Selectivity in voltammetry is determined by the
difference between half-wave potentials or peak
potentials, with a minimum difference of ±0.2–0.3 V
for a linear potential scan and ±0.04–0.05 V for
differential pulse voltammetry. We often can improve
selectivity by adjusting solution conditions. The
addition of a complexing ligand, for example, can
substantially shift the potential where a species is
oxidized or reduced to a potential where it no longer
interferes with the determination of an analyte. Other
solution parameters, such as pH, also can be used to
improve selectivity.

24. • Time, Cost, and Equipment
• Commercial instrumentation for voltammetry
ranges from <$1000 for simple instruments, to
>$20,000 for a more sophisticated instrument. In
general, less expensive instrumentation is limited
to linear potential scans. More expensive
instruments provide for more complex potential-
excitation signals using potential pulses. Except
for stripping voltammetry, which needs a long
deposition time, voltammetric analyses are
relatively rapid.

25. • Sensitivity
• In many voltammetric experiments, we can
improve the sensitivity by adjusting the
experimental conditions. For example, in
stripping voltammetry we can improve sensitivity
by increasing the deposition time, by increasing
the rate of the linear potential scan, or by using a
differential-pulse technique. One reason that
potential pulsetechniques are popular is that they
provide an improvement in current relative to a
linear potential scan.

26. • Accuracy
• The accuracy of a voltammetric analysis usually is limited by our
ability to correct for residual currents, particularly those due to
charging. For an analyte at the parts-per-million level, an accuracy
of ±1–3% is routine. Accuracy decreases when analyzing samples
with significantly smaller concentrations of analyte.
• Precision
• Precision is generally limited by the uncertainty in measuring the
limiting current or the peak current. Under most conditions, a
precision of ±1–3% is reasonable. One exception is the analysis of
ultratrace analytes in complex matrices by stripping voltammetry, in
which the precision may be as poor as ±25%.

27. Polarography

28. Polarography
• Polarography is a type of voltammetry where
the working electrode is a dropping mercury
electrode (DME) or a static mercury drop
electrode (SMDE), which are useful for their
wide cathodic ranges and renewable surfaces.
It was invented in 1922
by Czech chemist Jaroslav Heyrovský, for
which he won the Nobel prize in 1959.

29. operation
• polarography is a voltammetric measurement
whose response is determined by combined
diffusion/convection mass transport. The
simple principle of polarography is the study
of solutions or of electrode processes by
means of electrolysis with two electrodes,

31. 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.
Because we do not vary the potential,
amperometry does not result in a
voltammogram.

33. an amperometric biosensor responds
to glucose.

34. Types of voltammetry
 Linear sweep voltammetry
 Staircase voltammetry
 Cyclic voltammetry
 Chronoamperometry
 Differential pulse voltammetry
 Electrochemical stripping analysis

35. Linear sweep voltammetry
• Linear sweep voltammetry is a voltammetric
method where the current at a working electrode
is s measured while the potential between the
working electrode and refrence electrode is
swept linearly in time.
• Oxidation or reduction of species is registered as
a peak or trough in the current signal at the
potential at which the species begins to be
oxidized or reduced.

36. Method
• the experimental setup for linear sweep voltammetry
utilizes a potentiostat and a three-electrode setup to
deliver a potential to a solution and monitor its change
in current.
• The three-electrode setup consists of a working
electrode, an auxiliary electrode, and a reference
electrode. The potentiostat delivers the potentials
through the three-electrode setup. A potential, E, is
delivered through the working electrode. The slope of
the potential vs. time graph is called the scan rate and
can range from mV/s to 1,000,000 V/s

37. applications
•
In cases where the reaction is irreversible cyclic
voltammetry will not give any additional data that linear
sweep voltammetry would give us.
• In one example,[7] linear voltammetry was used to examine
direct methane production via a biocathode. Since the
production of methane from CO2 is an irreversible reaction,
cyclic voltammetry did not present any distinct advantage
over linear sweep voltammetry. This group found that the
biocathode produced higher current densities than a plain
carbon cathode and that methane can be produced from a
direct electric current without the need of hydrogen gas

38. Staircase voltammetry
• Staircase voltammetry is a derivative of linear
sweep voltammetry
• in staircase voltammetry the potential sweep
is a series of stair steps The current is
measured at the end of each potential change

39. Cyclic voltammetry
• Cyclic voltammetry (CV) is a type
of potentiodynamic electrochemical measuremen
t. In a cyclic voltammetry experiment, the
working electrode potential is ramped linearly
versus time. Unlike in linear sweep voltammetry,
after the set potential is reached in a CV
experiment, the working electrode's potential is
ramped in the opposite direction to return to the
initial potentia

40. • The current at the working electrode is
plotted versus the applied voltage (that is, the
working electrode's potential) to give the
cyclic voltammogram trace

42. •
Cyclic voltammetry (CV) has become an important and widely used
electroanalytical technique in many areas of chemistry.
• It is often used to study a variety of redox processes, to determine
the stability of reaction products, the presence of intermediates in
redox reactions,[11] electron transfer kinetics,[12] and the reversibility
of a reaction.[13] CV can also be used to determine the electron
stoichiometry of a system, the diffusion coefficient of an analyte,
and the formal reduction potential of an analyte, which can be used
as an identification tool. In addition, because concentration is
proportional to current in a reversible, Nernstian system, the
concentration of an unknown solution can be determined by
generating a calibration curve of current vs. concentration.[14]
• In cellular biology it is used to measure the concentrations in living
ones.[15] In organometallic chemistry, it is used to evaluate redox
mechanisms.[16]

43. Measuring antioxidant activity
• Cyclical voltammetry can be used to determine
the antioxidant capacity in food and even
skin.[17][18] Low molecular weight antioxidants,
molecules that prevent other molecules from
being oxidized by acting as reducing agents, are
important in living cells because they inhibit cell
damage or death caused by oxidation reactions
that produce radicals.[19] Examples of antioxidants
include flavonoids, whose antioxidant activity is
greatly increased with more hydroxyl groups

44. • Antioxidant capacity of chocolate and hops[edit]
• The phenolic antioxidants for cocoa powder, dark
chocolate, and milk chocolate can also be determined
via cyclic voltammetry. In order to achieve this, the
anodic peaks are calculated and analyzed with the
knowledge that the first and third anodic peaks can be
assigned to the first and second oxidation of
flavonoids, while the second anodic peak represents
phenolic acids.[22] Using the graph produced by cyclic
voltammetry, the total phenolic and flavonoid content
can be deduced in each of the three samples.

45. fast-scan cyclic voltammetry
• is cyclic voltammetry with a very high scan
rate (up to 1×106 V·s−1).[1] Application of high
scan rate allows rapid acquisition of
a voltammogram within several milliseconds
and ensures high temporal resolution of
this electroanalytical technique. An
acquisition rate of 10 Hz is routinely
employed.

46. Application
• Measurement of dopamine in vivo[edit]
• FSCV is used to monitor changes in concentration of
dopamine in mammalian brain in real time with
sensitivity down to 1 nM. Using an acquisition rate of
10 Hz is fast enough to sample dynamics of
neurotransmitter release and clearance.
Pharmacological action of dopaminergic drugs such
as D1 and D2 receptors agonists and antagonist (raclop
ride, haloperidol), dopamine transporter blockers
(cocaine, nomifensine, GBR ) could be evaluated with
FSCV. The fast acquisition rate also allows the study of
dopamine dynamics during behavior.

47. Measurement of other monoamine
neurotransmitters
• FSCV is used to study dynamics of exocytosis of
noradrenaline and adrenaline from chromaffin
cells; release of serotonin from mast cells; release
of 5-HT in brain slices; release of 5-HT in brain of
anesthetized rodents and fruit flies; release of
norepinephrine in brain of anesthetized and
freely moving rodents.

48. chronoamperometry
• chronoamperometry is an electrochemical
technique in which the potential of
the working electrode is stepped and the
resulting current from faradaic processes
occurring at the electrode (caused by the
potential step) is monitored as a function of
time.

50. Differential pulse voltammetry
differential pulse polarography, DPP is
a voltammetry method used to
make electrochemical measurements and a
derivative of linear sweep
voltammetry or staircase voltammetry, with a
series of regular voltage pulses superimposed
on the potential linear sweep or stairsteps.

52. • The current is measured immediately before
each potential change, and the current
difference is plotted as a function of potential.
By sampling the current just before the
potential is changed, the effect of the charging
current can be decreased.

53. Electrochemical cell
• The system of this measurement is usually the same as that
of standard voltammetry. The potential between
the working electrode and the reference electrode is
changed as a pulse from an initial potential to an interlevel
potential and remains at the interlevel potential for about 5
to 100 milliseconds; then it changes to the final potential,
which is different from the initial potential. The pulse is
repeated, changing the final potential, and a constant
difference is kept between the initial and the interlevel
potential. The value of the current between the working
electrode and auxiliary electrode before and after the pulse
are sampled and their differences are plotted versus
potential.

54. uses
• These measurements can be used to study
the redox properties of extremely small
amounts of chemicals because of the
following two features: 1) in these
measurements, the effect of the charging
current can be minimized, so high sensitivity is
achieved and 2) only faradaic current is
extracted, so electrode reactions can be
analyzed more precise

55. Electrochemical stripping analysis
• Electrochemical stripping analysis is a set
of analytical chemistry methods based
on voltammetry[ or potentiometry that are
used for quantitative determination of ions in
solution

56. •
Anodic stripping voltammetry is a voltammetric
method for quantitative determination of specific
ionic species. The analyte of interest
is electroplated on the working electrode during
a deposition step, and oxidized from the
electrode during the stripping step. The current is
measured during the stripping step. The oxidation
of species is registered as a peak in the current
signal at the potential at which the species begins
to be oxidized. The stripping step can be
either linear, staircase, squarewave, or pulse.

58. Cathodic stripping voltammetry
• Cathodic stripping voltammetry is a voltammetric
method for quantitative determination of specific ionic
species.[ It is similar to the trace analysis
method anodic stripping voltammetry, except that for
the plating step, the potential is held at an oxidizing
potential, and the oxidized species are stripped from
the electrode by sweeping the potential positively. This
technique is used for ionic species that
form insoluble salts and will deposit on or near
the anodic, working electrode during deposition. The
stripping step can be
either linear, staircase, squarewave, or pulse.

60. Adsorptive stripping voltammetry
Adsorptive stripping voltammetry is similar to
anodic and cathodic stripping voltammetry
except that the preconcentration step is not
controlled by electrolysis.[7] The
preconcentration step in adsorptive stripping
voltammetry is accomplished by adsorption on
the working electrode surface, or by reactions
with chemically modified electrodes.