Potentiometric titrations involve using a potentiometric indicator electrode to detect the analyte or titrant in a titration reaction. Polarography is a type of voltammetry where the working electrode is a dropping mercury electrode (DME). In polarography, a potential is applied to the DME causing current to flow from the reduction or oxidation of analyte ions. A polarogram plots the current versus the applied potential, providing qualitative and quantitative information about analytes present. Peak heights in polarograms can be used for quantitative calibration curves to determine analyte concentrations. Polarography is useful for determining both inorganic and organic compounds.

1. Potentiometric titrations
Titrations carried out using potentiometric indicators are
normally referred to as potentiometric titrations. This
form of titration may be applied across all of the types of
titration reactions, provided a suitable indicator
electrode is available that can detect either the analyte
or titrant. Figure bellow lists the measured species in
this form of titration and electrodes normally employed
to perform the measurement.
InstrumentalMethods of Analysis
1

2. InstrumentalMethods of Analysis
2

3. Electrochemical cell for a potentiometric measurement with an ISE
InstrumentalMethods of Analysis
3

4. Voltammetry and Polarography
Voltammetry
Voltammetry Comprises a group of
electroanalytical methods in which information's
about analysis is derived from current-voltage
curves, that is a plot of current as a function of
applied potential under conditions that encourage
polarization of indicator (working) electrode.
InstrumentalMethods of Analysis
4

5. InstrumentalMethods of Analysis
The term voltammetry is derived from the units
of electrical parameters measured volt-
ampere-metry
It is important to distinguish between
voltammetry (mm) and voltametry (m).
The term voltametry has been used to describe
the technique generally known as controlled
current potentiometric titration. 5

6. InstrumentalMethods of Analysis
The essential difference between voltammetric
and other potentiometric techniques such as
constant current coulometry is that in voltammetry
an electrode with a small surface area is used to
monitor the current produced by the species in
solution reacting at this electrode in response to
the potential applied to it.
6

7. Because the electrode used in voltammetry is
so small, minimal consumption of analyte takes
place (the amount of material reacting can be
ignored), this is in contrast to the case in
coulometry where large area electrode are used
so that all of a species in the cell may be
oxidized or reduced.
InstrumentalMethods of Analysis
7

8. InstrumentalMethods of Analysis
Voltammetry is widely used by inorganic, physical,
biological chemists for non-analytical purposes, including
fundamental studies of oxidation and reduction processes in
various media, adsorption process on surfaces, and electron
transfer mechanism at chemically modified electrode
surfaces.
At one time, voltammetry (particularly classical polarography)
was an important tool used by chemists for the determination
of inorganic ions and certain organic species in aqueous
solutions.
8

9. InstrumentalMethods of Analysis
Voltammetric measurements are
usually carried out in simple
electrochemical cells, similar to that
shown in Figures followed.
A common electrochemical cell consists
of a working electrode, a reference
electrode, and usually an auxiliary
(counter) electrode;
9

10. InstrumentalMethods of Analysis
1- The working electrode is an electron
conductor at which the reaction or transfer of
interest takes place. Working electrode is a
microelectrode whose potential is varied with
time
2- In all voltammetric cells, it is necessary to keep
one of the electrodes at a constant potential. This
electrode, designed to have a constant
(reversible) potential, is called a reference
electrode, (Ag/AgCl or calomel electrode).
10

11. InstrumentalMethods of Analysis
3- The auxiliary electrode (supporting
electrolyte) excess of nonreactive
electrolyte (inert metal) to conduct current.
It is one at which a counter reaction similar
to that takes place at the working electrode,
for the sake of balancing the total charge in
the system.
11

12. InstrumentalMethods of Analysis
In the presence of electroactive species
in the electrochemical working cell, the
applied potential will provoke a change
in the concentration of the monitored
electroactive species at the electrode
surface by electrochemically reducing
or oxidizing them.
12

13. InstrumentalMethods of Analysis
Changing the concentration of any
electroactive participant at the working
electrode surface will cause mass
transport toward the electrode, and
current (will flow through the
electrodes) that is directly proportional
to the analyte concentration.
13

14. This simple dependence between measured
current and analyte concentration makes
voltammetric techniques to be routinely
used for the quantitative determination of a
variety of inorganic and organic
compounds. Although there are many
voltammetric techniques they are all based
on the same electrochemical theory.
InstrumentalMethods of Analysis
14

15. InstrumentalMethods of Analysis
15

16. InstrumentalMethods of Analysis
Three-electrode setup: (1) working electrode (2) counter electrode; (3) reference electrode
16

17. In voltammetric techniques the potential
applied at the reference electrode increased
gradually by a mean of external battery until it
reaches the decomposition of active ion
(analyte), then an induced current formed as a
result of oxidation-reduction of the analyte ions.
A plot of the current against the applied
potential gives an S shape curve which called
voltammogram.
InstrumentalMethods of Analysis
17

18. Voltammogram
From the voltammogram; informations of
qualitative and quantitative data can be obtained
for the oxidized or reduced analyte.
At the beginning, there is small current which is
called residual current ir which is presents even in
absence of active ions; residual current is due to
the presence of interferences.
InstrumentalMethods of Analysis
18

19. A small increase in diffusion current by increasing
the applied potential, when the applied potential
reaches the decomposition potential of the analyte,
a sudden increase in the current is observed then
the current will remain constant and independable
on the potential and the current is called limiting
current il because it is limited by the average
movement of ions through the medium to the
electrode surface.
InstrumentalMethods of Analysis
19

20. InstrumentalMethods of Analysis
Have wave potential is determined at half value of
diffusion current (E1/2) is the potential at the point of
inflection of current-voltage curve, one half the
distance between the residual current and the final
limiting current plateau).
Half wave potential is a characteristic and constant for
ion under investigation (qualitative information), while
the amount of diffusion current is proportional to
concentration of analyte (quantitative information).
20

21. InstrumentalMethods of Analysis
Voltammogram
21

22. In the case of presence of several
reducible ions in the same solution, it is
possible to know about the nature and
approximate quantities of various ions
by single run.
The method can be also used for
determination of a number of organic
compounds e.g. aldehydes, ketones,
and nitroso compounds.
InstrumentalMethods of Analysis
22

23. Current in Voltammetry
•When an analyte is oxidized at the working electrode, a
current passes electrons through the external electric circuitry
to the auxiliary electrode.
•This current flows from the auxiliary to the working
electrode, where reduction of the solvent or other
components of the solution matrix occurs .
•The current resulting from redox reactions at the working
and auxiliary electrodes is called a faradaic current.
InstrumentalMethods of Analysis
23

24. Sign Conventions
A current due to the analyte's reduction is called a cathodic
current and, by convention, is considered positive. Anodic
currents are due to oxidation reactions and carry a negative value.
Oxidation-Reduction processes in electrochemical reactions
There are three types of these processes:
•Mass transfer
•Charge transfer.
•Transfer of oxidized and reduced substances to the bulk solution
InstrumentalMethods of Analysis
24

25. Voltammetric techniques
A number of voltammetric experiments are routinely used in quantitative and
qualitative analysis. Some of these methods; are DC polarography, AC
polarography, pulse polarography, cyclic polarography, stripping
voltammetry…….
Cyclic voltammetry
Anodic stripping voltammetry
Linear sweep voltammetry
Cathodic stripping voltammetry
Rotated electrode voltammetry
Normal pulse voltammetry
Differential pulse voltammetry
Adsorptive stripping voltammetry
Alternating current voltammetry
Chronoamperometry
Polarography
InstrumentalMethods of Analysis
25

26. •Cathodic stripping voltammetry - A quantitative, analytical
method for trace analysis of anions. A positive potential is
applied, oxidizing the mercury electrode and forming insoluble
precipitates of the anions. A negative potential then reduces
(strips) the deposited film into solution.
•Adsorptive stripping voltammetry - A quantitative, analytical
method for trace analysis. The analyte is deposited simply by
adsorption on the electrode surface (i.e., no electrolysis), then
electrolyzed to give the analytical signal. Chemically modified
electrodes are often used.
•Alternating current voltammetry
InstrumentalMethods of Analysis
26

27. •Polarography - a subclass of voltammetry where the working
electrode is a dropping mercury electrode (DME), useful for its wide
cathodic range and renewable surface.
•Rotated electrode voltammetry - A hydrodynamic technique in which
the working electrode, usually a rotating disk electrode (RDE)
or rotating ring-disk electrode (RRDE), is rotated at a very high rate.
This technique is useful for studying the kinetics and electrochemical
reaction mechanism for a half reaction.
•Normal pulse voltammetry
•Differential pulse voltammetry
•Chronoamperometry
InstrumentalMethods of Analysis
27

28. InstrumentalMethods of Analysis
Polarography
Polarography is a special type of voltammetry in which the
working electrode is "Dropping Mercury electrode (DME). Figure
shows a typical dropping mercury electrode (DME). The cell of
polarography is made up of three electrodes system immersed
in a solution containing the analyte
•First electrode is dropping mercury electrode, DME, is the
working electrode whose potential is varied linearly with time.
•The second one is the reference electrode.
•The third electrode is nonreactive electrolyte called a
supporting electrolyte.
28

29. InstrumentalMethods of Analysis
29

30. InstrumentalMethods of Analysis
Drops of mercury fall from the orifice of this capillary at a constant
rate (5-30 drops/min).
Each drop is the electrode while attached to the capillary end,
where oxidation and reduction occur in its surface allowing current
to pass.
Thus polarography is used for the determination of electro-active
substances that either accepts or loss electrons.
The drop is hanged by the power of surface tension; the
accumulation of Hg increasing the size of Hg drops will lead to
gravity overcoming the surface tension resulting in the fall of the
drop. This usually takes 3-6 sec.
30

31. InstrumentalMethods of Analysis
Since, in the case of the DME, the electrode surface is being
constantly renewed in a cyclic fashion, the current increases
from a small value as the drop beings to form to a maximum
value as the drop falls. A suitable recorder is used to
measure the current; the characteristic saw-toothed record
is obtained. The limiting current (il) is the sum of residual
and diffusion currents. The residual current (ir) is subtracted
from the limiting current to give the wave height [diffusion
current (id)].
31

32. InstrumentalMethods of Analysis
Ilkovic equation: the linear relationship between the diffusion current and the
concentration C of electro-active species is shown by the Ilkovic equation:
id = 706 n D1/2 m2/3 t1/6 C
where id is the maximum current in microamperes,
n is the number of electrons current required per mole of electro-active
substance,
D is the diffusion coefficient in square cm per second,
m is the rate of mercury flow from the capillary in mg/sec,
t is the drop time between two successive drops in sec,
C is the concentration in millimoles per liters.
To obtain an expression for the average current rather than maximum current,
the constant in the forgoing equation becomes 607 rather than 706. That is
id =607 n D1/2 m2/3 t1/6 C
32

33. InstrumentalMethods of Analysis
the product m2/3 t1/6 in Ilkovic equation, called the
capillary constant, describes the influence of dropping
electrode characteristics upon the diffusion current;
both m and t readily evaluated experimentally, n , D,
and t are constants during the analysis, so diffusion
current is proportional to concentration;
id = KC,
K represents 607, n, D1/2, m2/3 and t1/6.
33

34. Polarogram
A polarogram is a plot of current as a function of
potential applied to the polarographic cell, the
microelectrode (DME) is connected to the negative
terminal of the power supply, convection the applied
potential is given a negative sign, also the current is
designated (labeled) as positive when the flow of
electrons from power supply to microelectrode, that is
the electrode that behave as cathode.
InstrumentalMethods of Analysis
34

35. InstrumentalMethods of Analysis
35

36. InstrumentalMethods of Analysis
Polarogram
36

37. Removal of dissolved oxygen
In as much as oxygen is reduced at the DME in two steps, first to
hydrogen peroxide and then to water, it interfere where polarograms
are to be made at potentials more negative than about 0 volts versus
SCE, and must be removed. This may be removed. This may be
accomplished by bubbling oxygen-free nitrogen through the solution for
10 to 15 minutes immediately before recording the wave. It is necessary
that the solution be quiet and vibration-free during the time is recorded
to ensure that the current is diffusion current. Therefore the nitrogen
aeration should be stopped and the gas to flow over the surface of the
solution before a polarogram is recorded.
InstrumentalMethods of Analysis
37

38. InstrumentalMethods of Analysis
Applications of Polarography
Generally quantitative applications are based upon calibration
curves in which peak heights are plotted as a function of
analyte concentration.
1- Inorganic applications, the polarographic method is widely
applicable to the analysis of inorganic substances. Most metallic
cations, for example, are reduced at the dropping mercury
electrode. Even the alkali and alkaline earth metals are
reducible at DME. The polarographic method is also applicable
to the analysis of such inorganic anions as bromate, iodate,
dichromate, vanadate, selenite and nitrite.
38

39. 2- Organic polarographic analysis.
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.
InstrumentalMethods of Analysis
39

40. InstrumentalMethods of Analysis
40