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 (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.