BASi PWR-3 Compliance Voltage

In voltammetric measurements, current is measured as a function of the applied potential (Eapp). A potentiostat tries to maintain Eapp between the working and reference electrodes and allows negligible current through the reference electrode. The auxiliary electrode acts as a source, or sink, of electrons to balance the redox reaction occurring at the working electrode. Therefore, the potential applied between the auxiliary and working electrodes (Eaux) must be sufficient both to drive the appropriate electron transfer reaction at the auxiliary electrode and to compensate for the potential drop due to the solution resistance between the auxiliary and reference electrodes. (This potential drop is given by Ohm's Law E = iRc, where i is the current generated by the electrochemical reaction, and Rc is the solution resistance between the auxiliary and reference electrodes.) The compliance voltage is the maximum potential that can be supplied between the working and auxiliary electrodes.

In a typical voltammetric experiment (e.g., cyclic voltammetry), µA currents are generated, and R_c is relatively small. Therefore, the potential required between the working and auxiliary electrodes rarely exceeds a few volts. However, in controlled potential electrolysis (bulk electrolysis) experiments, larger currents are generated (due to large surface area working electrodes), and R_c is increased by the presence of the frit that is needed to separate the auxiliary and working electrodes. In such experiments, a high compliance voltage may be required to maintain potential control (especially in non-aqueous electrolytes). For example, if 0.2 A is generated and the resistance through the frit is 100 ohms, then a compliance voltage of at least 20 V is required. If the compliance voltage of the potentiostat is not sufficient, then the actual potential applied between the working and reference electrodes is lower than the required value and the electrolysis time will increase.