The use of RRDE voltammetry to study acid-base reactions in unbuffered solutions

Abstract

Electrochemical mechanisms in unbuffered solutions are difficult to analyze because of the presence of second order reactions. If the protonation reaction is irreversible, then the second order reverse reaction is not kinetically relevant. For protonation on N- or O-atoms, the reaction is often reversible, and leads to an EC reversible mechanism. Rotating ring-disk electrode (RRDE) voltammetry is ideal to investigate this reaction because it is possible to focus on the limiting currents. The limiting currents are neither sensitive to the solution resistance, nor to electron transfer kinetics, but they are sensitive to reaction rates and equilibrium constants in solution. In this report, the theory for the EC reversible mechanism was developed for RRDE voltammetry. Two cases were examined. The first case (Case 1) is the situation where A is reduced to A−, and is then reversibly converted to a new product. An example of this is the reduction of A in a buffered media, where the reduction product is converted to its conjugate acid (HB + A− ⇆ B− + HA). In a buffered media, both the forward and reverse reactions are pseudo-first order. The second case (Case 2) is the situation where the forward reaction is generally (pseudo)first-order, and reverse reaction is second order. The theory was then applied to the reaction of Fe(OEP) (NO)- with 2,6-dichlorophenol (2,6-dcp) in THF in buffered and unbuffered media.