MEASUREMENT OF STANDARD POTENTIAL OF A METAL:
Standard potential of a metal, e.g Zn, is measured as follows:
i. One half cell is made up of hydrogen as reference while the other half cell is made up of the metal, i.e. Zn.
ii. Hydrogen electrode is placed in 1M H2S04 with the help of platinum plate and
Zn is placed in 1M ZnSO4.
iii. The two half cells are connected by a potentiometric bridge which is known as Salt Bridge. It is made of KCl jelly which is used to complete the circuit between half cells and to prevent the mixing of two solutions.
iv The electromotive force (e.m.f) of the cell is 0.76 volts. Since the hydrogen electrode has potential of 0.00 volts, hence 0.76 volts is the potential of Zn electrode. It is also observed that flow of electrons takes place from Zn to hydrogen electrode.
Eº reduction = Eº oxidation
Eº reduction: -0.76V
v. The whole cell can be symbolically represented as:
Zn1Zn2+ (1.0M) ——————–> H+ (1.0M) ; H2(g) (1atm) ; Pt
vi. In Zn half cell, oxidation is taking place;
Zn ———–> Zn2+ + 2 e- (oxidation)
In hydrogen halt cell, reduction is taking place:
2H+ + 2e- ——————->H2 (Reduction)
Therefore over all reaction can be represented as:
Zn(s) = 2H+(aq) ——————-> Zn+2 (aq) + H2 (g)