The standard hydrogen electrode (SHE) is a primary reference electrode used to determine the electrode potential of a cell (Standard Hydrogen Electrode......; Reference Electrodes........). In SHE, 1M HCl is taken, and hydrogen gas is taken at 1atm pressure and temperature is maintained at 25℃. In these conditions its standard reduction potential and standard oxidation potential is always zero. This is the reason it can be used as a reference electrode (Standard Hydrogen Electrode - Construction, Representation, Uses and ....).
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Source:
(1) Standard Hydrogen Electrode - Construction, Representation, Uses and .... https://www.vedantu.com/chemistry/standard-hydrogen-electrode.