Multicomponent diffusion with chemical reactions and its effect in voltammetry

Abstract

Situations are frequently encountered where a mixture of species, each of which is a participant in an electrode reaction, is to be investigated by voltammetry. In the simplest case, the species undergo no chemical reactions so they diffuse independently of one another. However, when reactions occur among the diffusing species, the fluxes of each can be affected. For example, an equilibrium mixture of two rapidly interconverting conformations of a molecule that react at an electrode can produce a diffusion current which differs from the case where interconversion is slow. Because the difference will be enhanced if the diffusion coefficient of one species is much different from that of the other, one can understand why such effects are usually undetectable for conformational changes or isomerizations but can be important for metal ions and their complexes with bulky ligands [I], host-guest interactions with large hosts [2,3], intercalation of probe ions by DNA [4] and inclusion of electroactive solutes in micelles (5-131. In these examples, the reaction is often conducted under pseudo-first-order conditions and corresponds to the conversion of one species to another, e.g., complexed metal ion to free metal ion. Another type of chemical reaction among diffusing species is the exchange reaction (A + B + B + A) in which there is no net change in the concentration of either species due to the reaction. Nevertheless, conditions exist where such reactions can affect the apparent diffusion rate of the species. Electron exchange reactions have played a prominent role in the interpretation of transport in polymer films. The theory has been reviewed recently and refined by Buck [14,15]. Exchange reactions will not be considered in this paper whose purpose instead is to analyze the effect of interconversion reactions among diffusing species in voltammetry and to define those conditions where the effects of such reactions will be important. The conclusions will be discussed and illustrated through reference to published results.