All electrochemical cells require at least two electrodes, since the potential of a given electrode can only be measured relative to another electrode, the potential of which must be constant (a reference electrode). In potentiometric measurements (such as measurement of pH), there is no current through the cell, and these two electrodes are sufficient (it should be noted that many pH and ion-selective electrodes used in potentiometric measurements are combination electrodes – both electrodes are contained within the same body). However, in a cyclic voltammetry experiment, an external potential is applied to the cell, and the current response is measured. Precise control of the external applied potential is required, but this is not possible with a two electrode system, due to the potential drop across the cell due to the solution resistance (potential drop (E) = current (i) x solution resistance (R)) and polarization of the counter electrode that is required to complete the current measuring circuit. Better potential control is achieved using a potentiostat and a three electrode system, in which the potential of one electrode (the working electrode) is controlled relative to the reference electrode, and the current passes between the working electrode and the third electrode (the auxiliary electrode).
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