Class 12- Electrochemistry / How electrode potential is measured by standard hydrogen electrode?

What is standard hydrogen electrode? How is it prepared? Explain with labelled diagram?

ANSWER

This consists of gas at 1 atmospheric pressure bubbling over a platinum electrode immersed in 1 M HCl at 25oC (298 K) as shown in figure. The platinum electrode is coated with platinum black to increase its surface. The hydrogen electrode thus constructed forms a half cell which on coupling with any other half cell begins to work on the principle of oxidation or reduction. Electrode depending upon the circumstances work both an anode or cathode.
Cell reaction of standard hydrogen electrode (SHE) when it acts as anode is:
H2(g)​→2H++2e−
It is as represented as
H2(g)​(1atm)Pt∣H3​O(aq)+​(1.0M)
When it acts as cathode, the cell reaction is
2H++2e−→H2(aq)​
and it is represented as
H3​O(aq)+​(1.0M)∣(1atm)Pt