Strength of a reductant (reducing agent) or oxididant (oxidising agent) can be found out by measuring the relative electrode potential when it’s connected in a solution using a cell. Element under consideration can be made electrode of a standard cell whose electrode potential is already known. For example Fe³⁺/Fe is the element we want to test with Standard Hydrogen electrode (SHE). The half-cell reaction for Fe and H are given below.

When any element needs to be evaluated it is placed as an electrode with SHE. The amount of emf it generates in the cell can be considered as the potential of the element.

The tendency of Fe³⁺ ion to undergo reduction is more as compared to hydrogen. Hence the above assumed configuration of Fe being anode can be reversed and hence strength Fe as a reductant can be established. Hence the strength of an oxidant can be determined.