Question

Explain normal or standard hydrogen electrodes.

Answer 1

Hint: Normal or standard hydrogen electrode (SHE) is a primary standard reference electrode. The SHE is the universal reference for reporting values of relative half-cell potentials. It is a type of gas electrode and was widely used as the determination of pH values.

Complete solution:
Reference electrode: The electrode which acts as both anode and cathode is known as reference electrode.
Hydrogen electrode is also known as primary standard hydrogen electrode (SHE) which is at STP conditions.
Construction and working of standard hydrogen electrode:

1) It consists of a small platinum strip coated with platinum black as to absorb hydrogen gas.
2) A platinum wire is welded to the platinum strip and sealed in a glass tube to make contact with the outer circuit through mercury.
3) The platinum strip and glass tube which has an inlet for hydrogen gas at the top and a number of holes at the base for the escape of excess of hydrogen gas at 1atm pressure.
4) The temperature of the cell is maintained at 298K.
5) The Pt strip is placed in an acid solution which has H+ ion concentration with 1M.
6) The hydrogen electrode can act as a cathode or anode with respect to other electrodes.
7) Based on international agreement the standard hydrogen electrode is arbitrarily assigned a potential exactly +0.00V or -0.00V.
8) Thus hydrogen electrode obtained forms one of two half cells of a voltaic cell.
9) When this half cell is connected with any half cell, a voltaic cell is constructed.
10) By measuring the potentials of various electrodes versus standard hydrogen electrodes (SHE), a series of standard electrode potentials has been established.
11) The standard hydrogen electrode is also known as Normal hydrogen electrode.
12) This is a type of primary reference electrode.

Note: However, the hydrogen electrode method is not appropriate for daily use because of the effort and expenses involved, with the inconvenience of handling hydrogen gas and great influence of highly oxidizing or reducing substances in the test solution.