Hydrogen Electrode:
You know well that all the standard electrode potentials are quoted with reference to the standard Hydrogen Electrode, that is the most significant primary reference electrode and whose potential is equal to zero under fixed conditions namely hydrogen gas at 1 atm. unit and pressure activity of Hydrogen ions in the solution within contact along with the electrode. Upon linking the standard Hydrogen electrode with a metal electrode (a metal in contact with its ions in a solution of unit activity) through a bridge, the standard electrode potential of the metal may be obtained from the following.
The hydrogen ions of the solution and the gaseous hydrogen are kept in equilibrium and platinum (pt) electrode coated with a thin film of pt-black. The platinum electrode is cleaned with hot chromic acid and thoroughly washed with distilled water. Then it is electrolytically plated from a solution of chloroplatinic acid with pt foil as anode. It is important that a thin jet black deposit is formed and thick deposits are not satisfactory for use as hydrogen electrodes. Further platinizing, the electrode is to be freed from traces of chlorine. The mainly convenient source of hydrogen is the compressed gas available in the market in cylinders. The gas is purified through passing by all glass wash bottles holding 0.2 M KMnO4, alkaline Pyrogallol dilutes H2SO4 and distilled water before passing on to the electrode surface. Figure shows typical designs of commonly used hydrogen electrodes.
Figure: Common design of Hydrogen Electrodes