IB Chemistry home > Syllabus 2016 > Redox processes > The standard hydrogen electrode

Syllabus ref: 19.1

An international standard is necessary to compare the electrode potentials of all of the possible redox half-equations. The standard hydrogen electrode is used as the reference against which all other electrode potentials can be measured.

The standard hydrogen electrode

The Standard Hydrogen Electrode (SHE)

This is the apparatus set-up that is used to compare the electrode potentials of metal and other electrode systems. It comprises hydrogen gas at atmospheric pressure brought into contact with a platinum black electrode in 1 molar (mol dm-3) acid solution. The hydrogen ions from the acid solution set up an equilibrium with the hydrogen gas. This set of conditions is known as standard conditions.

H2 2H+ + 2e

If the other half-cell connected to the standard hydrogen electrode releases electrons, it can be said to be relatively negative with respect to the standard hydrogen electrode (which is assigned a value of 0 volts)

For example, metals that are more reactive than hydrogen gas preferentially release electrons (forcing the SHE to accept the electrons) and have standard electrode potentials with negative values.