Answer:
All standard reduction potentials are measured against that of the reduction of hydrogen ions to H2 gas.
Explanation:
The standard hydrogen electrode (SHE) serves as a sort of reference electrode. All standard electrode potentials of chemical systems are measured against the standard hydrogen electrode whose electrode potential has been arbitrarily set at 0.00 V.
The reduction potential of any other system is measured against the standard hydrogen electrode by connecting that half cell to the standard hydrogen electrode and observing the reading of the voltmeter. The standard hydrogen electrode consists of hydrogen ions of unit activity, hydrogen gas at 1 atm and the system is maintained at a temperature of 298 K.