Construction, Working and Limitations of Hydrogen Electrode

chemistry-assignment:(Q.2)

This is a gas electrode. It consists of a thin rectangular platinum foil which is coated with fresh platinum black to increase the adsorption capacity of the metal. This is welded and the end of it is fused into the inner tube through the base. In the inner tube, little mercury is taken and a Cu wire is introduced to make the external electrical contact. The inner tube is enclosed in an outer jacket having an inlet tube for sending in H₂ gas and has a perforated wider base for the escape of excess of H₂. This unit is dipped in HCl taken in a beaker such that the metal foil remains in the solution.

Working

When pure and dry H₂ gas is passed through the inlet tube, a part of the gas gets adsorbed and the excess bubbles out through the perforations. Between the H₂ gas absorbed on the surface of the metal and H⁺ of the solution, an equilibrium is established and an electrical double layer of opposite charges is formed. The potential developed is called H₂ electrode potential.

In the above system, when the H₂ gas at a pressure of 1atm is bubbled through 1M HCl, the electrode (constructed) or formed is called STANDARD HYDROGEN ELECTRODE (SHE) or Normal H₂ electrode (NHE). This is represented as Pt, H₂ / H⁺(760 mm of Hg) (IM)

The standard H₂ electrode potential is defined as the potential that is developed between the H₂ gas adsorbed on the pt metal and H⁺ of the solution when the H₂ gas at a pressure of 760 mm of Hg is in equilibrium with H⁺ of unit concentration

The magnitude of SHE potential is considered to be zero. It is used-

1) For the determination of electrode potential of metal electrode system.

2) For the determination of pH of the solution.

Limitations

1) It is rather difficult to regulate the pressure of the H₂ gas to be at exactly 1atm throughout the experiment.

2) 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.

3) 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.

4) If the solution contains any oxidizing agent, the H₂ electrode cannot be used.

Determination of Single Electrode Potential

chemistry-assignment:(Q.1)

It is not possible to measure directly the potential developed at interface of the metal gas and its ions in solution using voltmeter. Hence the electrode whose single electrode potential has to be determined is coupled with another electrode of known potential and the emf of the cell is determined by the potentiometeric method.

Normally SHE is used for coupling and in the electrochemical cell constructed, it acts either as a positive or negative electrode depending on the nature of the other electrode.

When SHE acts as positive electrode, then,

E.m.f = E^+ve - E^-ve

This means when the electrode potential is negative with respect to SHE, oxidation takes place at that electrode.

When SHE acts as negative electrode, then,

Emf = E^+ve - E^-ve

This means when the electrode potential is positive reduction takes place at the metal electrode.

Example:

electrode a cell made of a Zn electrode immersed in 1M ZnSO₄ is coupled with a S.H.E. The E.M.F. of the above cell is 0.76V. The direction of flow indicates that oxidation takes place so a standard reduction potential of -0.76V is given to it.