Question

Define the reference electrode.

Answer 1

Standard or normal hydrogen electrode is used as a reference electrode. It is used to calculate the absolute value of the electrode potential of a single electrode.

Complete answer:
The absolute value of the electrode potential of a single electrode called a single electrode potential cannot be determined because the oxidation half-reaction or reduction half-reaction cannot take place alone. It can only be measured by using some electrode as a reference electrode. The reference electrode used is the standard or normal hydrogen electrode (S.H.E or N.H.E). In the standard hydrogen gas electrode, hydrogen gas at 1 bar pressure is passed into 1 M HCl at 298 K in which a foil of platinum-coated with platinum black (finely divided platinum) remains immersed. It simply acts as an inert electrode through which the inflow or outflow of electrons takes place.

When in a cell, this electrode acts as the anode, i.e., oxidation takes place, the following reaction takes place:

${{H}_{2}}\to 2{{H}^{+}}+2{{e}^{-}}$

i.e., some hydrogen gas changes into ${{H}^{+}}$ ions which go into the solution.
When this electrode acts as cathode, i.e., reduction takes place, the following reaction occurs:

$2{{H}^{+}}+2{{e}^{-}}\to {{H}_{2}}$

i.e., some ${{H}^{+}}$ ions from the solution change into${{H}_{2}}$ gas. Thus, the electrode is reversible with respect to ions. This electrode is usually represented as:

$Pt,{{H}_{2}}(g)|{{H}^{+}}(Conc=c)$

The electrode potential of the standard hydrogen electrode is taken as 0.000 at 298 K.

The diagram of the standard hydrogen electrode is given below:

Note: The standard electrode potential of zinc was calculated with a standard electrode which was equal to 0.76 volts. The standard electrode potential of copper was calculated with a standard electrode which was equal to 0.34 volt. In common practice, all the electrode potentials are expressed as reduction potentials.