It has been recently claimed [1] that the electroreduction of selenate on glassy carbon electrodes in neutral and acidic aqueous electrolytes can be accelerated by the presence of solution phase methyl viologen, MV2+(aq), at room temperature. This contribution will present evidence that the effects described in that report are due to the reduction of selenite, a common impurity found in commercial selenate salts. Also to be discussed are the results of our detailed studies involving the MV2+(aq)-mediated reduction of selenite, from which the onset for the overall process was found to occur at potentials far more positive than those associated with the reduction of MV2+(aq).
EXPERIMENTAL
Commercially available Na2SeO4 (Alfa Aesar, 99.9%) was purified by the method described by Gruebel et al. [2] which involved adsorption of SeO32- on high area TiO2 at low pH. A glassy carbon (GC) rotating disk electrode (RDE) was used as a working electrode, and a carbon rod and a Ag/AgCl as counter and reference electrodes, respectively. Cyclic voltammetric experiments were performed in aqueous 0.2 M sodium acetate buffer (pH 4.02), or 0.1 M sodium phosphate buffer (pH 7) in the presence of 1 mM MVCl2 (Sigma-Aldrich, 98%), 30 mM Na2SeO4, or 1 mM Na2SeO3 (ACROS organics, 44- 46% Se) or mixtures thereof prepared with ultrapure water (18.3 MΩ, Barnstead water purifier) using a bipotentiostat (Pine Instruments, RDE3).
RESULTS AND DISCUSSION
In marked contrast with the literature report [1], no changes were detected in the cyclic voltammogram of GC recorded at a scan rate of 100 mV/s under stagnant conditions after addition of 30 mM ultrapure Na2SeO4 into 0.2 M sodium acetate (pH 4.02) containing 1 mM MV2+ (see black and red lines, respectively in Fig. 1). However, a large increase in the negative current was observed after addition of 1 mM Na2SeO3. It may thus be concluded on this basis, that selenate does not undergo reduction on GC under the conditions of these experiments, and that the results reported earlier are consistent with the presence of selenite as an impurity in the commercial selenate. Interestingly, the onset for the reduction of SeO32- in the presence of MV2+, as found in dynamic polarization curves recorded at a scan rate of 10 mV/s with the GC RDE rotating at 900 rpm (blue line, Figure 2) was far more positive, ca. -0.2 V vs Ag/AgCl, than the onset of the reduction of either of SeO32- (black line, Fig. 2) or MV2+ (magenta line, Fig. 2) individually in the same buffered solution (ca. -0.55 V). One possible explanation for this phenomenon may be found in the formation of an adduct between the two reactive species for which the redox potential would be more positive than for either one of the constituents. However, no such species has been detected in our initial measurements involving solution phase NMR and UV-vis spectroscopies. Yet another plausible explanation involves the adsorption of either one of the species on GC which might lead to a change in its redox properties which might facilitate reduction of the other. These are currently being explored in our laboratory and will be reported in due course.
REFERENCES
1. Fumiya, Koshikumo., Wakana Murata., Akiyuki, Ooya; Shin-ichiro, Imabayashi. Electrochemistry. 81(2013), 350- 352.
2. Gruebel, K. A.; Davis, J. A.; Leckie, J. O. Environ. Sci. Tech. 29(1995), 586-594.
ACKNOWLEDGEMENTS
This work was supported by a Grant from NSF.