Standard Hydrogen Electrode

For any half reaction, we can measure the potential (free energy) by comparing it to a standard reaction.  The reaction we choose is the following.

\[\rm{2H^+(aq) + 2e^- \rightarrow H_2(g)}\]

Where the concentration of the H⁺ is 1M and the pressure of the H₂ gas is 1 atm. Such an electrode is called a "standard hydrogen electrode" or SHE.

We can then make a cell with any other half reaction (under standard conditions where concentrations are 1M) and measure the potential (voltage).

For example here is a cell to compare the potential of the reaction of Ni to Ni²⁺ to the SHE.

We can then measure all such combinations and tabulate the data.  Note: for some reactions, we are measuring the oxidation and for others the reduction, but we will always tabulate the reduction potential for the half reaction.