The cell measurements away from equilibrium

75 The cell measurements away from equilibrium 7.5 The cell measurements away from equilibrium 

In electrochemical research, auxiliary electrodes are frequently made of platinum foil or gauze, sometimes placed in a compartment separated from the rest of the solution by a porous frit, so as to avoid contamination arising from the reaction occurring at the auxiliary electrode. This is an oxidation if reduction is occurring at the working electrode and vice versa, but the identity of the auxiliary electrode reaction is not important in this type of experiment. 

Exceptions to the three-electrode system in the laboratory are now few but important. A pool of mercury at the bottom of a cell in contact with an electrolyte solution containing chloride or bromide can serve as auxiliary and reference electrodes simultaneously owing to the very large surface area exposed. Another exception is experiments with microelectrodes the current that passes is so small that the effect on the reference/auxiliary electrode potential is negligible. 

Quasi-reference electrodes can be employed in situations where the high reproducibility of potential is not necessary, such as in many voltammetric analysis experiments. Mercury pools (referred to above) or silver wires in aqueous halide media are examples. Platinum wires can also be used. The advantage of wires, apart from their small size, is in reducing the uncompensated resistance in resistive media, relative to conventional reference electrodes. 

Normally electrode reactions take place in solutions, or sometimes in molten salts (e.g. aluminium extraction). In order to minimize the phenomenon of migration of the electroactive ions caused by the electric field (Chapter 2) and to confine the interfacial potential difference to the distance of closest approach of solvated ions to the electrode (Chapter 3), the addition of a solution containing a high concentration of inert electrolyte, called supporting electrolyte, is necessary. This has a concentration at least 100 times that of the electroactive species and is the principal source of electrically conducting ionic species. The concentration of supporting electrolyte varies normally between 0.01m and 1.0 m, the concentration of electroactive species being 5 mM or less. The 

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