The auxiliary electrode, often also called the counter electrode, is an

electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials d ...

used in a three electrode electrochemical cell for voltammetric analysis or other reactions in which an electric

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

is expected to flow. The auxiliary electrode is distinct from the

reference electrode A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each ...

, which establishes the electrical potential against which other potentials may be measured, and the

working electrode The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode sys ...

, at which the cell reaction takes place. 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. The auxiliary electrode functions as a cathode whenever the working electrode is operating as an anode and vice versa. The auxiliary electrode often has a surface area much larger than that of the working electrode to ensure that the half-reaction occurring at the auxiliary electrode can occur fast enough so as not to limit the process at the working electrode. 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. Here, the potential of the auxiliary electrode is usually not measured and is adjusted so as to balance the reaction occurring at the working electrode. This configuration allows the potential of the working electrode to be measured against a known reference electrode without compromising the stability of that reference electrode by passing current over it. The auxiliary electrode may be isolated from the working electrode using a glass frit. 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 inert materials such as gold, platinum, or carbon.

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