Significance of Controlled Potential Electrolysis

In many descriptions of electrochemical preparations of organic substances, only the overall current and voltage applied across the cell have been specified. It must be emphasized that this information is generally inadequate for a proper electrochemical specification of the experimental conditions and a characterization of the reaction mechanism. Under constant current conditions, as consumption of the reactant occurs, the potential normally becomes increased (greater polarization) until eventually some new electrode process becomes predominant (see Section 5.1). This may either be decomposition of the solvent or supporting electrolyte or, in some cases, a further reaction with the substrate involved in the electroorganic preparation. In the latter case, it is clear that the preparation will yield more than one principal product. A classical case, first investigated by Haber, is the electroreduction of nitrobenzene referred to above and also the Kolbe reaction.  

It is evident that controlled potential electrolysis is, in principle, the preferred technique. In large-scale preparations, potentiostatic electrolysis with wholly electronic equipment may be economically unfeasible, but electromechanical systems may be constructed more easily. 

From the electrode-kinetic point of view (Section 4), operation of electrochemical reactions at constant potential enables the rates to be treated exactly like rates of other heterogeneous processes with, e.g., corresponding evaluation of reaction orders leading to mechanism elucidation (see Section 6.3). This approach can be applied at various selected, controlled potentials. 

For preparative purposes it is usually not necessary to control the potential more closely than 20-100 mV, depending on the type of reaction and the possibility of parallel reaction pathways (Section 5.1). 

Various potentiostats are available commercially, the best known being constructed by the following companies  

---