Describe the construction and working of standard hydrogen electrode. 

Explanation

i) A rectangular platinum plate coated with platinum black is welded to a platinum wire and the wire is fused in to a glass tube with of help of mercury.
ii) This tube issealed in to a jacket having a side tube at the top. It acts as the hydrogen inlet.
iii) Two  holes are provided at the bottom of the outer jacket, which act as the hydrogen outlet
iv) The outer jacket is immersed in a glass vessel containing 1M HCI.

Working: Hydrogen gas is passed from the top inlet. The pressure inside the jacket increases. Due to this, acid level is pushed down and the platinum plate gets exposed to hydrogen gas. Pt absorbs hydrogen. As a result, pressure falls and acid level increases back. Now, the absorbed film of hydrogen comes in contact with hydrogen ions of the solution and equilibrium is established and potential is developed. The potential developed is taken as zero.
`H_(2)hArr 2H^(+)+2e^(-)`
The half cell reaction is `H^(+)(aq)+e^(-)rarr(1)/(2)H_(2)(g)`