Unified Electroanalytical Chemistry: Application of the Concept of Electrochemical Equilibrium

Abstract

Electroanalytical chemistry, encompassing both static and dynamic techniques, can be unified and best explained by invoking the concept of electrochemical equilibrium. Parallel to chemical equilibrium that occurs when -TΔStot = ΔG = 0, electrochemical equilibrium is established when -TΔStot = {ΔG + ΔUel} = 0; that is, when the tendency to pump electrons from the electrical subsystem is exactly counterbalanced by the tendency to pump electrons from the chemical subsystem. Under this condition, the applied voltage (Vel) equals the chemical cell potential (electromotive force, E), the electrical current (I ) is essentially zero, and static techniques such as potentiometry are practiced. A full description of electrochemical equilibrium is supported by novel diagrams and a classroom demonstration. Dynamic techniques such as voltammetry require electrochemical nonequilibrium, where {ΔG + ΔUel} is not equal to 0, Vel is not equal to E, and I is not equal to 0. Depending on the values of these quantities, the chemical reaction may proceed as written or in the reverse direction. Systems in which electrolysis causes chemical reactions to occur in reverse are interpreted in a way different from that in analytical textbooks: namely, by assigning z, the moles of reaction variable, a negative value. The total entropy or {ΔG + ΔUel} is a continuum upon which dynamic (nonzero values) and static (at the zero point) electrochemical techniques can be placed and compared.