Exemplifying performance of kinetics-sensitive double-step voltcoulometry:redox reactions of protons in unsupported acids

An alternative electroanalytical technique to the widely used differential pulse voltammetry (DPV) is presented in connection with monitoring proton r edox reactions in strong acids (HCl, H2SO4, HNO3) and in L-ascorbic acid (A A) in the absence of any supporting electrolyte. Contrary to the DPV method , the current flowing through the working electrode in response to a double -step change of the applied potential is first integrated and subsequently processed by a three-channel correlator. Expressing the faradaic transient charge as Q(t) proportional to t(beta), the ratio R-beta = [Q(t(1)) - 2Q(5t (1)) + Q(9t(1))/(I(lim)t(1)), where t(1) is the delay of the first sampling event with respect to the trailing edge of the potential double step and I ,,, is the limiting current of the corresponding steady-state voltammetric wave, it is calculated and then compared to the values found experimentally . The sensitivity to the kinetics represented as dR(beta)/d beta has an opt imum around beta = 0.5, a value consistent with the Cottrell equation. The experimental data point to a crucial role of CO2 (H2CO3) dissolved in the a cid solution, envisaged as the reversed sign of the measured charge. After deaerating the solution by argon the sign became positive, nevertheless the experimental R-beta values were systematically higher than the predicted o nes. The reaction of the protons of AA at negative potentials seems to be o f the EC type when proceeding from negative to more positive potentials. Mo reover, there is a dominant voltammetric wave of AA at positive potentials coming from an irreversible reaction. accompanied by a relatively weak peak of the correlated charge. (C) 2001 Elsevier Science B.V. All rights reserv ed.