Describe the construction and working of Normal Hydrogen Electrode.

Explanation

The absolute value of a single electrode potential cannot be determined experimentally because neither the oxidation nor the reduction can occur by itself. Therefore, we can find only the relative electrode potentials. For this purpose, hydrogen electrode has been selected as the reference electrode and is given arbitrary value of zero in its standard state i.e., `2H^(+)(C=1" mol "l^(-1))+2e^(-)toH_(2)(g)(1atm):E^(@)=0.00V`

The standard hydrogen electrode consists of platinum wire sealed in a glass tube and has a platinum foil attached to it. The foil is coated with finely divided platinum and acts as platinum electrode. It is dipped into an acid solution containing `H^(+)` ions in 1 M concentration (1 M HCI). Pure hydrogen gas at 1 atmospheric pressure is constantly bubbled into the solution at constant temperature of 298 K. The surface of the foil acts as a site for the reaction.
The following reactions occur in this half cell depending upon whether it acts as an anode or as a cathode.
If S.H.E. acts as anode
`H_(2)(g)to2H^(+)+2e^(-)`
If S.H.E. acts as cathode
`2H^(+)+e^(-)toH_(2)(g)`
The electrode potential of an electrode can be measured by connecting this half cell with standard hydrogen electrode, whose potential is taken as zero.