potentiometric method notes

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CHAPTER-SIX
POTENTIOMETRY
Prepared by: MeLaKu M.(MSc)

2. Potentiometry
 Electro analytical techniques that measure or monitor electrode
potential utilize the galvanic cell concept.
 Examples include pH electrodes, ion-selective electrodes, and
potentiometric titrations.
 In these technique pair of electrodes are immersed, the potential
(voltage) of 1 of the electrodes is measured relative to the other, and
the conc of an analyte in the so/n into which the electrodes are dipped
is determined.
 One of the immersed electrodes is called the indicator electrode
and the other is called the reference electrode.
 Often, these two electrodes are housed together in one probe. Such
a probe is called a combination electrode.
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3. 6.1 Reference Electrodes
 The measurement of any voltage is a relative measurement and
requires an unchanging reference point.
 For voltage measurements in ordinary electronic circuitry, this
reference is usually ground.
 Ground is often a wire that is connected to the frame of the
electronic unit and also to the third prong in an electrical outlet,
which in turn is connected to a rod that is pushed into the earth,
hence the name ground.
 Thus an electronics technician measures voltages relative to
ground.
 In electro analytical chemistry, the unchanging reference is a half-
cell that, at a given temperature, has an unchanging potential.
 There are two designs for this half-cell that are popular the
saturated calomel electrode (SCE) and the Ag–AgCl electrode.
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4. I. The Saturated Calomel Reference Electrode
 The SCE is an example of a constant-potential electrode.
 It consists of two concentric glasses or tubes, each isolated from
the other except for a small opening for electrical contact .
 The outer tube has a porous fiber tip, which acts as the salt bridge
to the analyte solution and the other half-cell.
 A saturated solution of potassium chloride is in the outer tube.
 The saturation is evidenced by the fact that there is some
undissolved KCl present.
Fig. 6.1. A drawing of a commercial saturated calomel electrode. 4

5. Cont…
Within the inner tube is mercury metal and a paste-like material
known as calomel.
 Calomel is made by thoroughly mixing mercury metal (Hg) with
mercurous chloride (Hg2Cl2), a white solid.
 When in use, the ff half-cell rxn occurs:
The Nernst equation for this rxn is
Obviously the only variable on which the potential depends is [Cl–]
The saturated KCl present provides the [Cl–] for the rxn, and, since
it is a saturated so/n, [Cl–] is a constant at a given temperature
represented by the solubility of KCl at that temperature.
 If [Cl–] is constant, the potential of this half-cell, dependent only
on the [Cl–], is therefore also a constant.
As long as KCl is kept saturated and the temperature kept constant,
the SCE is useful as a reference against which all other potential
measurements can be made.
5

6. Cont…
Its standard reduction potential at 25oC is +0.2412 V.
 The SCE is dipped into the analyte solution along with the
indicator electrode.
 A voltmeter is then externally connected across the lead wires
leading to the two electrodes, and the potential of the indicator
electrode vs. that of the SCE is measured.
II. The Silver–Silver Chloride Electrode
The commercial Ag-AgCl electrode is similar to the SCE in that it
is enclosed in glass, has nearly the same size and shape, and has a
porous fiber tip for contact with the external solution.
 Internally, however, it is different.
 There is only one glass tube (unless it is a double-junction design)
and a solution saturated in AgCl and KCl is inside.
 A Ag wire coated at the end with a AgCl paste extends into this
so/n from the external lead. See Figure 6.2.
The half-rxn that occurs is
6

7. Cont…
 The Nernst equation for this is
 The standard reduction potential for this half-rxn is +0.22233 V.
 The potential is dependent only on the [Cl–], as was the potential of
the SCE, & once again [Cl–] is constant because the so/n is saturated.
Thus this electrode is also appropriate for use as a reference
electrode.
Fig. 6.2. A drawing of a commercial Ag–AgCl reference electrode. 7

8. 6.2. Indicator electrode
The other half is the indicator or working electrode, whose
response( potential) depends upon the analyte conc. develops.
There are a number of such indicator electrodes and analytical
experiments that are of importance.
Ecell=Eindicator-Ereference + Ejunction
It must be:
Two Broad Classes of Indicator Electrodes
Metal Electrodes:- Develop an electric potential in response to
a redox reaction at the metal surface, where the redox reaction takes
place at the electrode surface.
Ion-selective Electrodes:-Selectively bind one type of ion
to a membrane to generate an electric potential and where charge
exchange takes place at a specific surfaces and as a result a potential
is developed. 8
(a) give a rapid response and
(b) its response must be reproducible.

9. Metal Electrodes
a- First-order electrodes for cations:
e.g. in determination of Ag+ a rode or wire of silver metal is the
indicator electrode, it is potential is:
It is used for determination of Ag+ with Cl-, Br- and CN-. Copper,
lead, cadmium, and mercury
b) Second order electrodes for anions
A metal electrode is also indirectly responsive to anions that form
slightly soluble precipitates or stable complexes with its cation.
The electrode reaction is AgCl + e = Ag+ + Cl-, and the electrode
potential is given by:
E25 = EoAg/AgCl - 0.059 log [Cl-]

10. c) Inert electrodes for redox reaction:
Electrodes formed from platinum or gold inert and the
potential it developed depends upon the potential of
oxidation-reduction systems of the solution in which
it is immersed for example the potential of a platinum
electrode in a solution containing Ce(III) and
Ce(IV)ions is given by

11. 3. indicator electrodes for neutralization reaction
Glass Membrane
Electrode

12. Glass Membrane Electrode
E = K + 0.059 (pH1 - pH2)
K= constant known by the asymmetry potential.
PH1 = pH of the internal solution 1.
PH2 = pH of the external solution 2.
The final equation is:
E = K + 0.059 pH

13. Glass Membrane Electrode
• Advantages of glass electrode:
It can be used in presence of oxidizing, reducing, complexing
• Disadvantage:
1. Delicate, it can’t be used in presence of dehydrating agent e.g. conc.
H2SO4, ethyl alcohol….
2. Interference from Na+ occurs above pH 12 i.e. Na+ exchange
together with H+ above pH 12 and higher results are obtained.
3. It takes certain time to come to equilibrium due to resistance of glass
to electricity.

14. 2. pH Electrodes
 pH meters use electrochemical reactions. Curiously, the measurement is
based on the potential of a half-cell, the pH electrode.
 The pH electrode consists of a closed-end glass tube that has a very thin
fragile glass membrane at the tip.
 Inside the tube is a saturated so/n of AgCl that has a particular pH.
 It is typically a 1 M solution of HCl.
• Ion selective probes: respond to the presence of
a specific ion. pH probes are sensitive to H3O+.
• Specific reactions:
Hg2Cl2(s) + 2e- 2Hg(l) + 2Cl-(aq) E°1/2 = 0.27 V
Hg2Cl2(s) + H2(g) 2Hg(l) + 2H+(aq) + 2Cl-(aq)
H2(g) 2H+(aq) + 2e- E°1/2 = 0.0 V

15. Measurement of pH (cont.)
Hg2Cl2(s) + H2(g) 2Hg(l) + 2H+(aq) + 2Cl-(aq)
• What if we let [H+] vary?

Q  H
 
2
Cl
 
2
Ecell = E°cell - (0.0591/2)log(Q)
Ecell = E°cell - (0.0591/2)(2log[H+] + 2log[Cl-])
Ecell = E°cell - (0.0591)(log[H+] + log[Cl-])
saturate
constant

16. Measurement of pH (cont.)
Ecell = E°cell - (0.0591)log[H+] + constant
• Ecell is directly proportional to log [H+]
electrode

17. Cont…
A Ag wire coated with AgCl is dipped into this so/n to just inside
the thin membrane.
While this is almost the same design as that of the Ag–AgCl
reference electrode, the presence of HCl and the fact that the tip is
fragile glass and does not have a porous fiber plug point out the
difference. See Figure 6.3.
Fig. 6.3. A drawing of a pH electrode.
The purpose of the Ag–AgCl combination is to prevent the
potential that develops from changing due to possible changes in the
interior of the electrode.
The potential that develops is a membrane potential. 17

18. Cont…
o Since the glass membrane at the tip is thin, a potential develops due
to the fact that the chemical composition inside is different from the
chemical composition outside.
o Specifically, it is the difference in the conc of the H-ions on
opposite sides of the membrane that causes the potential (the
membrane potential) to develop.
o There is no half-cell rxn involved.
The Nernst equation is
or, since the internal [H+] is a constant, it can be combined with Eo,
which is also a constant, giving a modified Eo, E*, and eliminating
[H+](internal): E = E* - 0.0592 log [H+](external)
In addition, we can recognize that pH = –log [H+] and substitute this
into the above equation: E = E* + 0.0592 pH
The beauty of this electrode is that the measured potential (measured
against a reference electrode) is thus directly proportional to the pH
of the so/n into which it is dipped.
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19. Cont…
 A specially designed voltmeter, called a pH meter, is used.
 A pH meter displays the pH directly, rather than the value of E.
 The pH meter is standardized (calibrated) with the use of buffer
so/ns. Usually, 2 buffer solutions are used for maximum accuracy.
 The pH values for these so/ns should bracket the pH value
expected for the sample.
 For example, if the pH of a sample to be measured is expected to
be 9.0, buffers of pH = 7.0 and pH = 10.0 should be used.
 Buffers with pH values of 4.0, 7.0, and 10.0 are available
commercially specifically for pH meter standardization.
 Alternatively, of course, homemade buffer solutions may be used.
 In either case, when the pH electrode & RE are immersed in the
buffer so/n being measured and the electrode leads are connected to
the pH meter, the meter reading is electronically adjusted (refer to
manufacturer’s literature for specifics) to read the pH of this so/n. 19

20. Cont…
• The electrodes can then be immersed into the solution being tested
and the pH directly determined.
2.The Combination pH Electrode
• In order to use the pH electrode described above, two half-cells
(probes) are needed the pH electrode itself and a reference electrode,
either the SCE or the silver–silver chloride electrode and two
connections are made to the pH meter.
• An alternative is combination pH electrode.
• This electrode incorporates both the reference probe and pH probe
into a single probe and is usually made of epoxy plastic.
• It is by far the most popular electrode today for measuring pH.
• The reference portion is a silver–silver chloride reference.
• A drawing of the combination pH electrode is given in Figure 6.4.
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21. Cont…
Fig. 6.4. A drawing of a commercial
Combination pH Electrode.
o The pH electrode is found in the center of the probe as shown.
o It is identical to the pH electrode described above a Ag wire coated
with AgCl immersed in a so/n saturated with AgCl and having a [H+]
of 1.0 M.
o This solution is in contact with a thin glass membrane at the tip.
o The RE is in an outer tube concentric with the inner pH electrode.
o It has a Ag wire coated with AgCl in contact with a so/n saturated
with silver chloride and potassium chloride. 21

22. Cont…
 A porous fiber strand serves as the salt bridge to the outer tube
with the solution tested.
 The drawing in Figure 6.4 shows this strand on the side of the
outer tube, but some designs have it on the bottom of the probe next
to the glass membrane.
 Notice the epoxy plastic sheath surrounding the thin glass
membranes.
The external end of the porous fiber strand must be in full contact
with the solution being tested when the measurement is made.
The connection of this electrode to the pH meter is either the bnc
type or another type, depending on the pH meter used.
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23. 3. Ion-Selective Electrodes
The internal solution must contain a constant concentration of the
analyte ion, as with the pH electrode.
 Today we utilize electrodes with: 1) glass membranes of varying
compositions, 2) crystalline membranes, 3) liquid membranes, and 4)
gas-permeable membranes.
 In each case, the interior of the electrode has a silver–silver
chloride wire immersed in a solution of the analyte ion.
 Examples of electrodes that utilize a glass membrane are those for
lithium ions, sodium ions, potassium ions, and silver ions.
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24. Cont…
 Varying percentages of Al2O3 and B2O3, along with oxides of the
metal analyte, are often found in the membrane, as well as other
metal oxides.
 The selectivity and sensitivity of these electrodes vary.
 With crystalline membranes, the membrane material is most often
an insoluble ionic crystal cut to a round, flat shape and having a
thickness of 1 or 2 mm and a diameter of about 10 mm.
 This flat disk is mounted into the end of a Teflon or polyvinyl
chloride (PVC) tube.
 The most important of the electrodes with crystalline membranes
is the fluoride electrode.
 The membrane material for this electrode is lanthanum fluoride.
 The fluoride electrode is capable of accurately sensing fluoride ion
concentrations over a broad range and to levels as low as 10–6 M.
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25. Cont…
Other electrodes that utilize a crystalline membrane but with less
impressive success records are chloride, bromide, iodide, cyanide,
and sulfide electrodes.
 The main difficulty with these is problems with interferences.
Fig. 6.5. A drawing of an ion-selective electrode with a liquid
membrane.
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26. Cont…
As the gases diffuse in, the pH of the so/n constituting the thin film
changes, and thus the response of the pH electrode changes
proportionally to the amount of gas diffusing in.
Calibration of ion-selective electrodes for use in quantitative
analysis is usually done by preparing a series of standards as in most
other instrumental analysis methods, since the measured potential is
proportional to the logarithm of the concentration.
The relationship is
in which z is the signed charge on the ion.
 The analyst can measure the potential of the electrode immersed in
each of the standards and the sample (vs. the SCE or silver–silver
chloride reference), plot E vs. log [ion], and find the unknown
concentration from the linear regression procedures.
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27. 2. Potentiometric titration
It is possible to monitor the
course of a titration using
potentiometric measurements.
 The pH electrode, for example,
is appropriate for monitoring an
acid–base titrn and determining an
end point in lieu of an indicator.
The procedure has been called a
potentiometric titration and the
experimental setup is shown in
Figure 6.5.
The end point occurs when the
measured pH undergoes a sharp
change when all the acid or base in
the titration vessel is reacted

28. It is used for all types of volumetric analysis: acid base,
precipitimetry, complexometry and redox
It is used when it is not easy or impossible to detect the
end point by ordinary visual methods i.e:
1. For highly coloured or turbid solutions.
2. For very dilute solutions 10-3, 10-6 M.
3. When there is no available indicator
2. Potentiometric titration contd…

29. Potentiometric titration contd…
• Fig. 6.5 A setup for a potentiometric titration.
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31. Application of potentiometric titration in
a) Neutralization reactions: glass / calomel
electrode for determination of pH
b) Precipitation reactions: Membrane electrodes for the
determination of the halogens using silver nitrate reagent
c) Complex formation titration: metal and membrane
electrodes for determination of many cations (mixture of Bi3+,
Cd2+ and Ca2+ using EDTA)
d) Redox titration: platinum electrode For example
for reaction of Fe3+/ Fe2+ with Ce4+/Ce3+

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