Question

Describe the construction and working of the standard hydrogen electrode (S.H.E.). Give its advantages and disadvantages.

OR 

What is the standard hydrogen electrode 

OR 

Primary reference electrode? Write the construction and working of it.

Answer 1

A single electrode potential cannot be measured, but the cell potential can be measured experimentally. 

Hence, 

It is necessary to have a reference electrode. S.H.E. is a primary reference electrode.

(1) Construction :

(i) The standard hydrogen electrode (S.H.E.) consists of a glass tube at the end of which a piece of platinised platinum foil is attached as shown in Fig. 

Around this plate there is an outer jacket of glass which has a side inlet through which pure and dry hydrogen gas is bubbled at one atmosphere pressure. 

The inner tube is filled with a little mercury and a copper wire is dipped into it. 

This provides an electrical contact with the platinum foil. The outer jacket ends into a broad opening.

(ii) The whole assembly is kept immersed in a solution containing hydrogen ions (H⁺) of unit activity.

(iii) This electrode is arbitrarily assigned zero potential.

(iv) The platinised platinum foil is used to provide an electrical contact for the electrode. This permits rapid establishment of the equilibrium between the hydrogen gas adsorbed by the metal and the hydrogen ions in solution.

(2) Representation of S.H.E. :

H⁺ (1 M) | H₂ (g, 1 atm) | Pt

(3) Working :

Reduction : H⁺_(aq) + e^- ⇌ \(\frac{1}{2}\)H_2(g)  E⁰ = 0.00 V

H₂ gas in contact with H⁺_(aq) ions attains an equilibrium establishing a potential.

(4) Applications of SHE : 

A reversible galvanic cell with the experimental (indicator) electrode,

Zn²⁺ (1M) | Zn_(s) and SHE can be developed as follows :

Thus,

The potential can be directly obtained.

(5) Disadvantages (Drawbacks or Difficulties) :

• It is difficult to construct and handle SHE.
• Pure and dry H₂ gas cannot be obtained.
• Pressure of H₂ gas cannot be maintained exactly at 1 atmosphere.
• The active mass or concentration of H⁺ from HCl cannot be maintained exactly unity.

Answer 2

The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it.

H⁺(aq) + e^- → 1/2 H₂(g)

The concentration of both the reduced and oxidised forms of hydrogen is maintained at unity. This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the solution is one molar.

Above half-cell called standard hydrogen electrode (Fig.) represented by Pt(s)| H₂(g) |H⁺(aq), is assigned a zero potential at all temperatures corresponding to the reaction.