Current Oscillatory Phenomena Based on Redox Reactions at a Hanging Mercury Drop Electrode (HMDE) in Dimethyl Sulfoxide Md. Mominul Islam, Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ReceiVed: August 9, 2004 A systematic and comprehensive study on cyclic voltammetric anodic current oscillation (CVACO) at a hanging mercury drop electrode (HMDE) was carried out for the redox reactions of molecular oxygen (O 2 ), 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 (MV 2+ ), 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., BP 0 /BP •- ,O 2 0 /O 2 •- , AB 0 /AB •- , Ru(bpy) 3 2+ /Ru(bpy) 3 + , BTD 0 /BTD •- , NB 0 /NB •- , DNB 0 /DNB •- , DNB •- /DNB 2- , TMBQ 0 /TMBQ •- , MV 2+ /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 (g0.5 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. Introduction Recently, our group has reported cyclic voltammetric anodic current oscillation (CVACO) at a hanging mercury drop electrode (HMDE; Figure 1A) based on the redox reaction of the molecular oxygen (O 2 )/superoxide ion (O 2 •- ) couple in aprotic media. 1-4 The probable causes of this oscillation have been pointed out as follows: (i) the movement of a mercury electrode and its adjoining solution resulting from an inhomo- geneous polarization of the electrode (in this case the downward streaming) and (ii) the formation-destruction of a passive film of mercury compounds [e.g., Hg 2 (O 2 •- ) 2 ] on the electrode surface. However, the mechanism of this oscillation remains uncertain. Thus far, several research groups have also reported a similar CVACO at a HMDE based on the redox reactions of organic substances 5-8 and metal complex. 9 Santhanam and Bard 5 have suggested that the origin of CVACO observed during the reoxidation of the 9,10-diphenylanthracene anion radical is due to the downward streaming of the Hg surface and its adjoining solution, which leads to the concentration change of redox species in the vicinity of the electrode. According to the consideration of De Levie and Levich, 10,11 this streaming phenomenon is caused by the uneven current density (j) distribution, which arises from the shielding of an upper portion of the Hg drop by the tip of the capillary (Figure 1A) and leads to the uneven potential (E) distribution between the neck and the bottom portions of the Hg drop. Such uneven E distribution finally originates the uneven surface tension (γ) distribution (Figure 1B,C). 5,12 The streaming effects largely depend on whether the oxidation (or reduction) occurs at potentials more negative or positive than the potential of zero charge (PZC). For example, when the electrode reaction occurs at a potential more negative than the PZC, the downward and upward streaming can be expected for the oxidation and reduction processes, respectively (Figure 1C). On the other hand, they should become less, or should be completely absent around the PZC. Thus, in their paper, Santhanam and Bard 5 have also predicted that maximum and stirring behavior could be fre- quently observed in polarographic oxidations of hydrocarbon and other radicals occurring at potentials more negative than the PZC. In the present study, we have focused our attention on the current oscillation due to the streaming effect and investigated the oscillation at the HMDE on the basis of the redox reactions of various couples (Figure 2) having their formal potentials (E 0′ values) more negative or positive than the PZC (i.e., zone I or zone III of the ECC) or around the PZC (i.e., zone II) (Figure 1C). To the best of our knowledge, there has been no report of such a systematic and comprehensive study on the prediction of Santhanam and Bard 5 regarding the polarographic streaming, which induces the current oscillation. * Author to whom correspondence should be addressed (telephone +81- 45-9245404; fax +81-45-9245489; e-mail ohsaka@echem.titech.ac.jp). 19425 J. Phys. Chem. B 2004, 108, 19425-19431 10.1021/jp046421a CCC: $27.50 © 2004 American Chemical Society Published on Web 11/23/2004