Potentiometry is the field of electro-analytical chemistry in which potential is measured without current flow. It is a method of analysis in which we determine the concentration of solute in solution and the potential difference between two electrodes.

1. PHARMACEUTICALANALYSIS
UNIT-5
TOPIC: POTENTIOMETRY
B.PHARM 1ST YEAR
Presented By:
MS. SHWETA SINGH
(Assistant Professor)
RBMI, Bareilly

2. CONTENT
Principle
Reference electrode
Standard Hydrogen electrode
Saturated calomel electrode
Silver-Silver Chloride electrode
Indicator electrode
Glass electrode

3. Definition
Potentiometry is the field of electro-analytical chemistry in
which potential is measured without current flow.
It is a method of analysis in which we determine the
concentration of solute in solution and the potential difference
between two electrodes.
Potentiometry is an instrument used to determine the
unknown concentration between the reference solution and
the sample.

4. 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 titrant.
 Metal atoms may dissolve in the solution as positive ions leaving
electrons on the electrode.
 Metal ions may take up electrons from the electrode and get deposited
as neutral atoms
 In this way, A POTENTIAL DIFFERENCE is set b/w electrode and
solution.

5. POTENTIOMETER
It is an instrument to determine the potential differences
between a reference electrode and an indicator electrode.
These two electrodes form an electrochemical cell that is
dipped in solution to be analyzed.
Measured potential can be used to determine the quantity of
analyte in terms of concentration.

6. REFERENCE ELECTRODE
 A reference electrode is a half-cell having a known potential.
 Has constant potential.
 Example: STANDARD HYDROGEN ELECTRODE (S.H.E), SILVER-SILVER CHLORIDE
ELECTRODE INDICATOR ELECTRODE
INDICATOR/TEST/SAMPLE ELECTRODE
 An indicator electrode is another half-cell with unknown potential.
 Immersed in analyte solution.
 Its potential is sensitive to the concentration of analyte.
 Potential is directly proportional to the ion concentration.
 EXAMPLE: METAL ELECTRODE, ION SELECTIVE ELECTRODE (I.S.E).

7.  The electromotive force of the complete cell is given by the
following equation:
Ecell = E(reference) + E(indicator) + E(salt bridge)
E(reference) = is the electromotive force of the reference electrode
E(indicator) = is the electromotive force of the indicator electrode,
E(salt bridge) = is the electromotive force at the salt bridge

8. Salt Bridge
 It is a laboratory device used to connect two half-cells of galvanic or voltaic
cells.
 Salt should be inert.
 It should not react with anode/cathode/solution.
 Salt should not be precipitate.
FUNCTION-
 To complete the circuit of the cell.
 To maintain electrical neutrality.
 Salt used: NaCl, KCl, KNO3, NH4OH, NH4Cl

10. Theory
 The main theory involved in potentiometry is, that when the known potential electrode is
immersed in the sample solution then the potential is given by the Nernst equation:
E= E0 + (2.303 RT) log10 Q
Where E is the electrode potential of the solution;
 E0 is the standard electrode potential;
 R= Gas constant (8.314 J/mol K)
 T= Temperature (Kelvin)
 n= No. of electrons transferred
 F = faradays constant (96500 coulombs)
 Q= Reaction quotient
nF

11. Electrodes
 These are mainly used to measure the voltages.
 Mainly two electrodes are used in the potentiometry. They are as follows:
•Reference/Standard electrode
•Indicator/test/sample electrode
Reference electrodes: These are mainly used for the determination of the analyte by
maintaining the fixed potential (0V).
Eg: • Standard hydrogen electrode
• Silver-silver chloride electrode
• Saturated calomel electrode
 The reference electrodes are classified into two main classes they are as follows:
1. Primary standard electrodes Eg: Standard hydrogen electrode (SHE)
2. Secondary standard electrodes Eg: Silver-silver chloride electrode and Saturated
calomel electrode (SCE).

12. Reference electrode
Ⅰ. Standard hydrogen electrode (SHE)
 Also known as Normal hydrogen electrode (NHE)
 It is a gas-ion electrode [H2(g)/H+
(aq)]
 It is a reference electrode set by IUPAC.
 Ideal characters of the Reference electrode:
1) Obeys the Nernst equation.
2) Exhibit constant potential (0V)
3) Exhibit little change with temperature.
 This electrode is conjugated with a sample/indicator electrode and measures the
concentration of analyte in the solution.

13. Construction:
 It consists of a platinum electrode is covered in finely powdered platinum black (platinized platinum
electrode).
 Hydrogen gas is passed through the tube at 1atm pressure.
 A platinum foil is attached at the end of the wire.
 The electrode is immersed in 1M HCl ion solution at 25°C.
Working:
 The electrode potential of SHE is ZERO at all temperatures.
 Half cell reaction:
½ H2(g) H+ + e-
 It can act as an anode half-cell and a cathode.
 Its standard reduction potential (SRP) EO H+/H2= 0, as well as standard oxidation potential (SOP)
EO H2/H+=0.
Diagram of Standard Hydrogen
Electrode (SHE)

14. Ⅱ. SATURATED CALOMEL ELECTRODE (SCE)
Construction:
 Also known as Mercury-Mercurous electrode.
 It consist of an outer tube containing saturated solution of KCl.
 At the bottom of the tube pure mercury (Hg) is placed, which is covered with a slurry or paste of Hg-
Hg2Cl2 which is known as Calomel.
 The remaining portion of the cell is filled with a solution of normal saturated KCl (1M).
Working:
 This electrode is used as a reference electrode.
 SCE is used as both cathode and anode.
Hg2Cl2 Hg2
+ + 2Cl-
 At Cathode: Reduction (GOE): Hg2
2+ + 2e- 2Hg
 At Anode: Oxidation (LOE): 2Hg Hg2
2+ + 2e-
 Electron (e-) exchange between ionic Hg and molecular Hg takes place.
Concentration of
KCl
Potential
0.1 M KCl 0.337V
1M 0.280V
Saturated KCl 0.242V

15. Saturated Calomel Electrode

16. Ⅲ. Silver, Silver Chloride (Ag, AgCl) Electrode
 The silver/silver chloride reference electrode is composed of a silver wire, sometimes coated with a
layer of solid silver chloride, immersed in a solution that is saturated with potassium chloride and silver
chloride.
Construction:
• Ag, AgCl electrode consists of Silver wire coated with silver chloride (AgCl) and is fully placed in a
3M KCl solution saturated with AgCl.
Working:
• This electrode is used within the range of -10 to 110 C.
• The half-reaction will be at the Ag-AgCl electrode.
• Reduction (GOE): Ag+ + e- Ag(s)
• Oxidation (LOE): Ag (s) Ag + + e-

17. Glass tube
Silver- Silver Chloride (Ag, AgCl)
Electrode

18. Difference between Indicator and Reference electrode

19. Indicator Electrode
 Definition: An electrode that is useful for measuring the potential or pH of a solution is
known as an indicator electrode.
 It is connected to the reference electrode and the potential of indicator electrodes is
measured compared to the potential of the reference electrode.
There are two types of Indicator electrodes:
1. Metal indicator electrode
 First-order electrode
 Second order electrode
2. Ion-selective electrode
 Glass electrode

20. 1. Ion-Selective Electrodes (ISE)
 It is also known as Membrane electrodes.
 Ion-selective electrodes are not based on redox processes but on selective binding of one type of ion to a
membrane, which generates a potential.
 The combination glass pH electrode contains a complete galvanic cell with two electrodes in a single device.
 Glass membrane is a most common example of ISE.
 Glass membrane electrode
 It is a type of ion-selective electrode made of a doped glass membrane (Doping is done to increase the electrical
conductivity (and thus resistance is decrease) that is sensitive to a specific ion. The most common application of
ion-selective glass electrodes is for the measurement of pH of any solution.
Construction:
• It consists of a hard glass tube, one internal electrode (usually Ag wire coated with AgCl), and a reference
electrode (usually calomel electrode).
• A sensing part of the electrode which is a bulb filled with 0.1 M solution of HCl, where transfer of ion occurs
through thin and semipermeable membrane.
• An Ag wire is placed between both the electrodes.
• 0.1M AgCl solution as internal solution and reference internal solution usually 0.1 M KCl (calomel electrode).

21.  Working of the glass membrane potential:
1. For the electrode to become operative, it must be soaked in water.
2. During this process, the outer surface of the membrane is coated with a hydrogel which has Na+.
3. The metal cation (Na+) in hydrogel diffuses out of the glass and into the solution, while H+ from the solution can
diffuse into the hydrogel.
4. H+ does not cross through the glass membrane of the pH electrode it is the Na+ that crosses and leads to a change in
energy.
5. When an ion diffuses from a region of activity to another region of activity there is an energy change and this is
what a pH meter measures.
6. It is the hydrogel that makes the pH electrode an ion-selective electrode.
7. If external conc. of H+ is high and internal H+ conc is low than, pH<7.
8. If external conc. of H+ is low and internal H+ conc is high than, pH>7.
9. If external conc. of H+ is equal to internal H+ conc than, pH=7.
 PROPERTIES:
1. Potential not affected by the presence of oxidizing or reducing agents.
2. Operates over a wide pH range.
3. It responds fast and functions well in the physiological system.
4. Very selective.
5. Long lifespan.

22. Analyte

23. Metal electrodes
 Metal electrodes develop an electric potential in response to a redox reaction
at the metal surface.
 Platinum and gold electrodes are very common because they are not very
reactive.
 Their purpose is to transfer electrons to and from the solution.
 Such electrodes work best when:
 They have a very large surface area.
 They are very clean (clean the electrodes with hot 8.0 M HNO3 and rinse with
distilled water)

24. METAL INDICATOR ELECTRODE
 Metal electrode develops electric potential as a result of redox reaction at its surface.
1. First-order electrode: A first-order electrode involves the metal involved the metal in
contact with its ions, such as Ag, silver nitrate (Ag+) or Zn, Zinc chloride (Zn²+), and Copper rod in a
copper sulfate solution.
Ag+ + e- Ag(s) Eº = 0.799
Construction:
 Electrode is made up of the sample element.
 Electrode is dipped in a solution containing the sample in ionic form.
 KCl-filled tube acts as a salt bridge.

25.  These are a kind with two interfaces.
 Such as a metal rod coated with a thin layer of its sparingly soluble salt.
 The reaction involved on its surface.
 This category will include silver coated with a thin deposit of silver chloride or mercury with
mercurous chloride.
 Electrode of this type can be used for direct determination of the activity of metal ions or the
anion used in coating.
 E.g.- Chloride ion in Silver chloride.
AgCl(s) + e- Ag (s) + Cl- Eº = 0.222V
2. Second-order electrode

26. Limitation
 Metallic indicator electrodes are not very selective and respond not only to their own cations
but also to other more easily reduced cations.
 Many metal electrodes can be used only in neutral or basic solutions because they dissolve
in the presence of acids.

27. METHODS FOR DETERMINING THE ENDPOINT BY
POTENTIOMETRIC TITRATIONS
 It’s a volumetric method in which potential between two electrodes
(reference & indicator) is measured as a function of added reagent volume.
 There are various methods to locate the endpoint or equivalence point. It is
generally found graphically (curve).
The methods for determining the endpoint by potentiometric titration are as
follows:
 Normal titration curve
 First derivative curve
 Second derivative curve

28. 1. Normal titration curve
 Values of EMF or pH are recorded after the addition of each volume of titrant.
 The graph is plotted between the volume of titrant added v/s values of pH or EMF.
 At the endpoint there is a sharp increase in potential or there is a maximum slope of the curve.
 Generally S-shaped curve is obtained.
 Eg: Weak acid vs strong base (CH3COOH Vs NaOH)
Normal titration curve of potentiometric
titration

29. 2. First derivative curve
 It indicates the change in pH per ml of titrant addition.
 Here change in pH for the volume of titrant added is plotted as ordinate (Y-axis) vs the
volume of titrant (v) added as abscissa (X-axis)
 From the shape of the graph, it becomes clear that the maximum change in pH occurs at
the equivalence point
 The end point can be readily recorded by drawing perpendicular from the peak of the graph
on the volume axis (X-axis).
First derivative titration curve of
potentiometric titration

30. 3. Second derivative curve
 This method uses the second derivative square of the difference in potential or pH of the
volume added vs the volume of titrant added is plotted on the graph
 The endpoint is shown as a zero point where the slope or curve of ∆pH2/∆2V is in the
middle of the curve.
Second derivative titration curve of
potentiometric titration

31. Advantages of endpoint detection by potentiometry over
normal visual indicator method
 pH meter is versatile as compared to an indicator method because
each indicator has a certain pH range. So same indicator cannot be
used for all types of acid-base titrations but the same pH meter can
be used for all types of acid-base titration.
 pH meter does not require any indicator.