Potentiometry involves measuring electrode potentials using a reference electrode and indicator electrode. The reference electrode maintains a constant potential while the indicator electrode's potential varies with analyte concentration. Common reference electrodes include the saturated calomel electrode and silver-silver chloride electrode. Indicator electrodes include pH electrodes, ion-selective electrodes, and redox electrodes. Potentiometric measurements are used in clinical chemistry, environmental monitoring, titrations, and various industrial applications like food processing.

1. Potentiometry
Ayesha shafi

2. OBJECTIVES
• Introduction
• Principle
• Theory
• Types of the electrode
• Reference electrode

3. Introduction to potentiometry
Potentiometry is a branch of electrochemistry which deals with the
study and measurement of electrode potential.
Electrode potential is the measurement of tendency of an oxidation or
a reduction reaction to occur at an electrode.
Purpose:
 to measure the PH and potentiometric titration.
 Direct and selective measurement of analyte concentration
 Determination of equilibrium constant.
 Establishment of end point in various types of potentiometric
titration

4. Potentiometry
Principle: The principle involved in the Potentiometry is when the pair of
electrodes is placed in the sample solution it shows the potential difference by
the addition of the titrant or by the change in the concentration of the ions.
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5. Theory:
The main theory involved in the potentiometry is, when the known potential
electrode immersed in the sample solution then the potential is given by Nernst
equation:
E= E0 +(0.592/n) log c
Where E is the potential of the solution; E0 is the standard electrode potential; n
is the valency of the ions; c is the concentration of the sample solution; 0.592 is
the value obtained from the RT/F; where R is the gas constant, T is the
temperature in Kelvin, F is the faradays constant.
Electrodes: These are mainly used to measure the voltages. Mainly two
electrodes are used in the potentiometry .They are as follows:
•Reference electrode
•Indicator electrode
 Reference electrode: These are mainly used for the determination of the
analyte by maintaining the fixed potential.
Ex: Standard hydrogen electrode
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6. Continue…
• The reference electrode is the electrode which contains of its own potential value and it
is stable when dipped into sample solution. The salt bridge is used to prevent the
interference of the analyte solution with that of reference solution.
• Here analyte solution is the solution whose potential is to be measured.
• The indicator electrode is the electrode which responds to change in the potential of
analyte solution
•
• The electromotive force of the complete cell is given by the following equation:
• Ecell = Ereference + Eindicator + Ejunction
• where Ereference is the electromotive force of the reference electrode ,Eindicator is
electromotive force of indicator electrode, Ejunction is the electromotive force at the
junction of the liquid.

7. •The reference electrodes are classified into two main classes they are as follows:
Primary standard electrodes
ex: Standard hydrogen electrode
Secondary standard electrodes
ex: silver-silver chloride electrode
saturated calomel electrode
 Indicator electrode: It is used to measure the potential of the analyte solution
comparing with that of reference electrode . Its potential is directly proportional to ion
concentration.
Ex: Hydrogen electrode.
Glass electrode.
Antimony –antimony oxide electrode.
There are two classes of indicator electrodes:
Metal indicator electrodes
Ion-selective electrodes
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8. Properties of ideal reference electrode
• It should be accurately known and has the constant potential.
• It should be completely insensitive to the concentration of analyte
solution.
• It should maintained a constant potential irrespective of the current
that passes through it.
• It should be easy to assemble.
• It must be robust and unaffected by processes.

9. Saturated calomel electrode
• It contains of an inner jacket and outer sleeve
• Inner jacket has wire contact with Hg and plugged with a mixture of
calomel Hg2Cl2 & KCl.
• Outer sleeve – crystals of KCl & porous plug of asbestos.
• Space b/w inner jacket & outer sleeve is filled with either saturated
KCl or 1M KCl or 0.1M KCl.
• Merits of saturated calomel electrode
• Ease of construction
• Stability of potential

11. Application of saturated calomel electrode
• The SCE is used in pH measurement, cyclic
voltammetry and general aqueous
electrochemistry.
• This electrode and the silver/silver chloride
reference electrode work in the same way. In both
electrodes, the activity of the metal ion is fixed by
the solubility of the metal salt.
• The calomel electrode contains mercury, which
poses much greater health hazards than the silver
metal used in the Ag/AgCl electrode

12. Silver-silver chloride electrode
• The silver/silver chloride reference electrode is a widely used
reference electrode because it is simple, inexpensive, very stable
and non-toxic.
• it is mainly used with saturated potassium chloride (KCl)
electrolyte, but can be used with lower concentrations such as 1
M KCl and even directly in seawater.
• Silver wire coated electrolytically with silver chloride and dipped
into KCl
• It has the advantage that it is easy to use.
• Demerit is that it is difficult to prepare.

13. Standard hydrogen electrode
• Is an important electrode as the potential of other electrode is measured
with respect to it.
• Platinum coil coated with platinum black
to provide the large surface area for
adsorption of hydrogen gas.
• Platinum takes no part in electrochemical
reaction.
The electrolyte solution is HCL of unit activity
(1.18M HCL at 25◦c) kept saturated with
hydrogen gas.

14. Application and Limitation
• 1) For the determination of electrode potential of metal electrode system.
• 2) For the determination of pH of the solution.
• Limitation:
• It is rather difficult to regulate the pressure of the H2 gas to be at exactly
1atm throughout the experiment.
• Excess of H2 bubbling out carries little HCl with it and hence the H+
concentration decreases. In such a system, it is difficult to maintain the
concentration of HCl at 1M.
• 4) Platinum foil gets easily poisoned by the impurities present in the gas
and HCl. In fact, the attainment of equilibrium is ensured by trial and error.

15. APPLICATIONS OR IMPORTANCE OF REFERENCE
ELECTRODES
•PH sensors
•Glucose sensors
•Gas sensor
• Field effect transistor

16. INDICATOR ELECTRODE
• An indictor Electrode is the electrode system having a potential that
varies with the variation in the concentration of analyte.
Characteristic of the ideal electrode:
• An ideal electrode responds rapidly.
• It can bring changes in the concentration of the analyte group/ ion.

17. INDICATOR ELECTRODE
• The potential of this electrode is proportional to the concentration of
analyte.
• Two classes of indicator electrodes are used in potentiometry:
• Metallic electrodes
• Electrodes of the first kind
• Electrode of the second kind
• Redox electrode
• Membrane electrodes (ion-selective electrodes)
• Glass pH electrode

18. Metallic electrodes
Electrodes of the first kind
• A metal in contact with a solution containing its cation.
• The most common ones:
• Silver electrode (dipping in a solution of AgNO3)
Ag+ + e ↔ Ag
• Copper electrode
Cu+2 + 2e ↔ Cu
• Zn electrode
Zn+2 + 2e ↔ Zn

19. Electrode of the second kind
• A metal wire that coated with one of its salts precipitate.
• Respond to changes in ion activity through formation of complex.
• A common example is silver electrode and AgCl as its salt precipitate.
• This kind of electrode can be used to measure the activity of chloride
ion in a solution.
Electrode of the third kind
A metal can respond to the concentration of another metal ion in which
it is in contact.
These electrodes are rarely used.
Metallic electrodes

20. Redox electrode
• An inert metal is in contact with a solution containing the
soluble oxidized and reduced forms of the redox half-
reaction.
• The inert metal usually is platinum (Pt).
• The potential of such an inert electrode is determined by
the ratio of the reduced and oxidized species in the half-
reaction.
• A very important example of this type is the hydrogen
electrode.

21. Ion selective electrode
An ion-selective electrode (ISE), also known as a specific ion
electrode (SIE), is a transducer (or sensor) that converts the
activity of a specific ion dissolved in a solution into an
electrical potential, which can be measured by a voltmeter or
pH meter.
indicator electrode based on determination of cations or
anions by the selective absorption of these ions to a
membrane surface.

22. TYPES OF ION SELECTIVE ELECTRODE
•Glass Membrane Electrode
•Solid State Electrode
•Liquid Membrane Electrode
•Gas Sensing Electrode

23. GLASS MEMBRANE ELECTRODE
• Responsive to univalent cations ( H+ , Na+)
• Glass electrodes available for Na+, K+, Li+, Ag+(cations only) by
varying glass composition
• The selectivity for this cation by varying the composition of a thin
ion sensitive glass membrane.
• Glass membrane manufactured from SiO2 with negatively charged
oxygen atom.
• Inside the glass bulb, a dilute HCl solution and silver wire coated
with a layer of silver chloride.
• The electrode is immersed in the solution and pH is measured
• Example: pH electrode

24. Continue….
Glass pH electrode
Advantages over other electrodes for pH measurements:
• Its potential is essentially not affected by the presence of oxidizing or
reducing agents.
• It operates over a wide pH range.
• It responds fast and functions well in physiological systems.
• Selective for monovalent cations only because polyvalent ions can not
penetrate the surface of membrane.

25. pH electrode
 Selective for the
detection of hydrogen
ions.
 The measuring or
indicator electrode has a
“glass membrane”
 pH is then determined
from potential between
the pH electrode and a
standard reference
electrode.

26. SOLID STATE ELECTRODE
• Solid state electrode are selective primarily to anions.
• It may be a
- homogenous membrane electrode
- heterogeneous membrane electrode.
• Homogenous membrane electrode: ion-selective electrodes in
which the membrane is a crystalline material (AgI/Ag2S).

27. • Heterogeneous membrane electrode: ion-selective electrodes
prepared of an active substance, or mixture of active
substances (silicone rubber or PVC)
• Example: Fluoride ion selective electrode.

28. LIQUID MEMBRANE ELECTRODE
• Liquid membrane is a type of ISE based on water- immiscible
liquid substances produced in a polymeric membrane used for
direct potentiometric measurement.
• Used for direct measurement of several polyvalent cations (Ca
ion) as well as a certain anions.
• Inner compartment of electrode contains reference electrode
& aqueous reference solution.
• Outer compartment – organic liquid ion exchanger

29. •The polymeric membrane made of PVC to
separate the test solution from its inner
compartment which contains standard solution of
the target ion.
•The filling solution contains a chloride salt for
establishing the potential of the internal Ag/AgCl
wire electrode.

30. GAS SENSING ELECTRODE
• Available for the measurement of ammonia, carbon dioxide
and nitrogen oxide.
• This type of electrode consist of permeable membrane and an
internal buffer solution.
• The pH of the buffer changes as the gas react with it.
• The change is detected by a combination pH sensor.
• This type of electrode does not require an external reference
electrode.

31.  Measurement of PCO2 in
routine blood gases
 A modified pH electrode
with a CO2 permeable
membrane covering the
glass membrane surface
 A bicarbonate buffer
separates the
membranes
 Change in pH is
proportional to the
concentration of
dissolved CO2 in the
blood
pco2 electrode

32. Application of Potentiometric Measurement
• Clinical Chemistry
• Ion-selective electrodes are important sensors for
clinical samples because of their selectivity for
analytes.
• The most common analytes are electrolytes, such as
Na+, K+, Ca2+,H+, and Cl-, and dissolved gases such as
CO2.
• Environmental Chemistry
• For the analysis of of CN-, F-, NH3, and NO3
- in water
and wastewater.

33. • Potentiometric Titrations
• pH electrode used to monitor the change in pH
during the titration.
• For determining the equivalence point of an
acid–base titration.
• Possible for acid–base, redox, and precipitation
titrations, as well as for titrations in aqueous
and non-aqueous solvents.
• Agriculture
• NO3, NH4, Cl, K, Ca, I, CN in soils, plant
material, fertilizers.
• Detergent Manufacture
• Ca, Ba, F for studying effects on water quality

34. • Food Processing
• NO3, NO2 in meat preservatives
• Salt content of meat, fish, dairy products, fruit juices, brewing
solutions.
• F in drinking water and other drinks.
• Ca in dairy products and beer.
• K in fruit juices and wine making.
• Corrosive effect of NO3 in canned foods.