Why is the standard reduction potential of hydrogen ions set to zero volts?

The standard reduction potential of hydrogen ions is set to zero volts by convention to serve as a universal reference point for measuring the relative electrode potentials of other chemical species.123

Because it is impossible to measure the absolute potential of a single half-cell in isolation, scientists established the hydrogen electrode as the standard basis for the hydrogen scale. By arbitrarily assigning this electrode a value of 0.00 volts, all other half-reactions can be compared against it.123

This system allows chemists to determine the relative tendency of different substances to gain or lose electrons. Under these standard conditions—where hydrogen ions are at a concentration of one mole per liter and hydrogen gas is at a pressure of one atmosphere at 25°C—the potential of any other electrode reflects its ability to be reduced or oxidized relative to the hydrogen standard. Consequently, species with positive reduction potentials are more easily reduced than hydrogen ions, while those with negative potentials are more difficult to reduce.1324