An Experimental Evaluation of Cyclic Voltammetry of Multicharged Species at Macrodisk Electrodes in the Absence of Added Supporting Electrolyte

Abstract

The reversible reduction of [S2Mo18O62]4- to [S2Mo18O62]5- and [S2Mo18O62]6- at a glassy carbon macrodisk electrode has been studied by cyclic voltammetry in acetonitrile as a function of the concentration of [(C6H13)4N]4[S2Mo18O62] in the absence and presence of [(C6H13)4N]ClO4 as the added supporting electrolyte. Consideration is given to the influence of scan rate, reference-working electrode distance, [(C6H13)4N]4[S2Mo18O62], and electrolyte concentrations. Experimental data confirm theoretical predictions that cyclic voltammetry at a macrodisk electrode is a viable technique for studies of multiply charged electroactive species without added electrolyte, provided the influence of enhanced complexities associated with effects of increased solution resistance, the mass transport contribution from migration, and convection arising from enhanced density gradients are considered. Enhanced density gradients present in the absence of added supporting electrolyte give rise to a more marked dependence of voltammograms on the angle of the electrode and hence lead to significant distortion of wave shapes at low scan rates. The summation of all these obstacles implies that quantitative evaluation of cyclic voltammograms of multiply charged species requires significantly greater care in the absence than in the presence of added supporting electrolyte.

Theory of linear sweep voltammetry with diffuse charge: Unsupported electrolytes, thin films, and leaky membranes

“…Since the 1980s, various experiments have focused on the role of supporting electrolyte [2]. For example, Bond and co-workers performed linear sweep [55] and cyclic [56,57] voltammetry using the ferrocene oxidation reaction on a microelectrode while varying supporting electrolyte concentrations. One of the primary theoretical concerns at that time was modeling the extra Ohmic drop in the solution from the electromigration effects which entered into the physics due to low supporting electrolyte concentration.…”

Linear Sweep and Cyclic Voltammetry

“…2.5.5.6). In these cases, the transport of charge in the electrolyte is realized by small amounts of impurities [48], by ions of the substrate material itself [52], or those generated in the electrode reaction [39]. Thus, migration has to be considered as an additional mode of transport, in particular for multiply charged species [52].…”

Cyclic Voltammetric Studies on [SMo12O40]2- and [SMo12O40]3- at Macrodisk Electrodes in Acetonitrile With and Without Added Supporting Electrolyte

“…A comparison of the experimental voltammogram given in Figure 5a and a simulated voltammogram, which accounts for diffusion, migration and IR u drop effects, is shown in Figure 6. As has been detected for the reduction of [S 2 Mo 18 O 62 ] 4À in the absence of ASE [8], the slight difference in simulated and experimental voltammograms is likely due to density gradients generated by the electrolytic production of a new product at the working electrode surface, an effect for which the simulation does not account. These density gradients result in natural convection, which enhances mass transfer of the electroactive analyte from bulk solution to the electrode surface.…”

“…The auxiliary electrode was a 1 mm diameter platinum wire. The quasi-reference electrode used in studies without ASE consisted of a hookshaped platinum wire (0.5 mm diameter) arranged so that the tip was 2 AE 0.5 mm from the working electrode surface [8]. This avoided leakage of adventitious electrolyte.…”

“…The predominant problems with voltammetric studies without added supporting electrolyte (ASE) are related to difficulties encountered with enhanced IR u (ohmic) drop and migration current. However, recent work in our laboratory has defined experimental conditions under which well-defined cyclic and rotated disk voltammograms may be obtained for charged species in the absence of ASE at macrodisk electrodes [8,9] …”