In a two-electrode system, either a known current or potential is applied between the working and auxiliary electrodes and the other variable may be measured.
When a three-electrode cell is used to perform electroanalytical chemistry, the auxiliary electrode, along with the working electrode, provides a circuit over which current is either applied or measured.
Such isolation prevents any byproducts generated at the auxiliary electrode from contaminating the main test solution: for example, if a reduction is being performed at the working electrode in aqueous solution, oxygen may be evolved from the auxiliary electrode.
Such isolation is crucial during the bulk electrolysis of a species which exhibits reversible redox behavior.
Auxiliary electrodes are often fabricated from electrochemically active materials such as gold, platinum, or carbon, with a much higher surface area than the working electrode.