Current oscillatory phenomena based on redox reactions at a hanging mercury drop electrode (HMDE) in dimethyl sulfoxide

A systematic and comprehensive study on cyclic voltammetric anodic current oscillation (CVACO) at a hanging mercury drop electrode (HMDE) was carded out for the redox reactions of molecular oxygen (02), nitrobenzene (NB), 1,4-dinitrobenzene (DNB), benzoquinone (BQ), 2,3,5,6-tetramethylbenzoquinone (TMBQ), benzophenone (BP), azobenzene (AB), 2,1,3-benzothiadiazole (BTD), 7,7,8,8-tetracyanoquinodimethane (TCNQ), methyl viologen dichloride (MV2+), and tris(2,2'-bipyridine)ruthenium(II) dichloride [Ru(bpY)(3)(2+)] in dimethyl sulfoxide (DMSO) solutions containing 0.1 M tetraethylammonium perchlorate (TEAP). From the electrocapillary curve (ECC) obtained using a dropping mercury electrode as well as the capacitance versus potential curves measured using electrochemical impedance technique, the value of the potential of zero charge (PZC) was estimated to be -0.27 V versus Ag\AgCl\NaCl (sat.) in a DMSO solution containing 0.1 M TEAP. CVACO was found to occur only for the redox couples (i.e., BP0/BP.-, O-2(0)/O2(.-), AB(0)/AB(.-), Ru(bPY)(3)(2+)/Ru(bpY)(3)(+), BTD0/BTD.-, NB0/NB.-, DNB0/DNB.-, DNB.-/DNB2-, TMBQ(0)/TMBQ(.-), MV2+/MV.+, and BQ(.-)/BQ(2-)) having the formal potentials (E-0' values) more negative than the PZC. CVACO was largely dependent on the concentrations of redox species and TEAP; for example, in the case of BTD the intensity of CVACO increased with increasing concentration, and CVACO ceased at high concentrations of TEAP ( greater than or equal to0.5 M). Furthermore, CVACO was not observed for the BQ(0)/BQ(.-) redox couple having E-0'(= -0.31 V) near the PZC, and a pronounced cathodic maximum was observed for the TCNQ(.-)/TCNQ(2-) redox couple with E-0' (= -0.16 V) more positive than the PZC. These observations and the factors governing the CVACO are discussed on the basis of the theory presented for the polarographic maxima of the first kind. The observed CVACO and the cathodic maximum obtained for the TCNQ(.-)/TCNQ(2-) redox couple could be explained in terms of the so-called streaming effect.