POTENTIOMETRY: NERNST EQUATION & TYPES OF ELECTRODES and MCQ for GPAT, Pharmacist, Drug Inspector

Cell is a device which convert chemical energy into electrical energy. cell is consists of two half cell.

ELECTRODE POTENTIAL:– Difference between the potential of two electrodes.

POTENTIOMETRY consists of measuring potential or emf of solution using set of indicator & reference electrode.

potential of solution depends on nature, concentration , temperature of ions of drug substance.

POTENTIAL AND CONCENTRATION – THE NERNST EQUATION:-

The potential of a metal electrode at 25°c immersed into a solution of its ions is given by

E=Eº+0.0591/n log[mⁿ⁺]

where Eº = Standard potential of the metal

n = valency of ions

c= concentration of ions

TERMINOLOGY :-

1. SALT BRIDGE :- A connection between two solution that allows the movement of current in the form of ionic charge.
2. Anode:-The electrode where oxidation occurs.
3. Cathode :- The electrode where reduction occurs.
4. Shorthand Notation :- A Vertical slash (/) indicates a phase boundray where a potential develops, and a comma(,)separates species in the same phase, or two phases where no potential developes. The double vertical slash (//) indicates the salt bridge the constant of which are normally not indicated.
5. Liquid junction potential :- A potential that develops at the interface between two ionic solutions that differ in composition , because of a difference in the mobility of the ions (Elj).

TYPES OF ELECTRODES :-

A. Based on the components :-

1. metallic electrode :- Pt, Ag electrode
2. metal ion:- Cu/CuSO4
3. Amalgam electrode : mix of Hg with any other metal eg. Na/Hg
4. Gas electrode :- one components of electrode is gaseous components eg. H-electrodes
5. micro-electrode:- Very small size electrode Eg dropping mercury electrode
6. ion sensitive electrode

B. Based on working principle :-

1. Working / Indicator / polarisable electrode
2. Reference / Unpolarisable electrode

REFERENCE ELECTRODE :-

1. HYDROGEN ELECTRODE :- 

Hydrogen electrode is the primary standard against which the potentials of other electrodes are measured. it can be used as indicator as well as reference electrode. It consists of a platinum foil coated with platinum black and has wire contacts through mercury. This assembly is enclosed in a glass covering through which hydrogen is passed at 1 atmospheric pressure continuously and dipped into the solution of standard acid or unknown solution. At all time the electrodes surface and the solutions should be saturated with the gas.

the potential of this electrode is given as folloes:

E=Eº +0.0592 LOG[H‍⁺]

= Eº-0.0592

= -0.0592

MERITS :-

1. It is primary reference standard against which potential of other electrode are measured.
2. Exhibit no salt error& used over entire pH range.
3. Used as reference electrode when dipped with std. acid solution & as indicator electrode when dipped into a sample solution.

DEMERITS :-

1. Factor like purity of H2 , pressure of H2 affect potential of electrode.
2. Affected by presence of oxidising and reducing substance.
3. Affected due to poisoning of platinum surface.
4. Can not be used in solution containing metal ions that are below H2 in potentials series

E=-0.0591 pH

MCQ:-

1.which technique is used to measure emf ?

a. acid base titration

b. mass spectroscopy

c. TGC

d.potentiometry

2. which part of the cell is responsible to between two solution that allows the movement of current in the form of ionic charge ?

a. electode

b.platinum wire

c.salt bridge

d.none of the above

3.which device is converted physical or chemical energy to electrical energy ?

a.cell

b.galvonometer

c.ECG

d.none of the above

4.In hydrogen electrode which material are using coating of platinum foil ?

a. pb

b.copper

c.platinum black

d.iron

5. which is dis advantage of hydrogen electrodes?

a.It is primary reference standard against which potential of other electrode are measured.

b.Exhibit no salt error& used over entire pH range.

c.Used as reference electrode when dipped with std. acid solution & as indicator electrode when dipped into a sample solution.

d.Factor like purity of H2 , pressure of H2 affect potential of electrode.

ANSWER KEY 

1.d

2.c

3.a

4.c

5.d

REFERENCE :-

TEXT BOOK OF PHARMACEUTICAL ANALYSIS OF THIRD EDITION OF DR. S. RAVI SANKAR (PAGE NO. 9.3-9.17)