Selenite reduction potential

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

The methodology most practiced is referred to here as codeposition, where a single solution contains precursors for all the elements being deposited and is reduced at a fixed potential or current density. The earliest report appears to be that by Gobrecht et al., which was published in 1963 [45]. Two anodes were used in the study, one of Se and one of Cd (or Ag), to form selenite and cadmium ions, respectively. CdSe was then formed by co-reduction of both species at the cathode. Reports of the formation of GaP in 1968 [46] and ZnSe in 1975 [47] via codeposition were subsequently published, and both involved molten salt electrolysis. 

In the aqueous phase, Umland and Wallmeier [80UML/WAL] studied the reduction of selenite at the mercury electrode in the presence of Zn by polarography, see Appendix A. The solubility product of ZnSe(s) was obtained from the position of the half-wave potential of the second reduction step, HgSe(s) + Zn "" + 2e" ZnSe(s) + Hg(l), to be log (ZnSe, s, 298.15 K) = -(23.212.0). Combined with CODATA for Zn and the selected value for Se l it corresponds to AfG° (ZnSe, s, 298.15 K) = - (151.0 11.8) kJ mol. The value is in disagreement with the value obtained by extrapolation of high temperature data. Since the validity of the polarographic method has not been documented, the value of AfG°(ZnSe, a, 298.15 K) obtained from the extrapolation of the high temperature data has been selected. 

In the aqueous phase, Umland and Wallmeier [80UML/WAL] studied the reduction of selenite at the mercury electrode in the presence of Cd by polarography, see Appendix A. The solubility product of CdSe(s) was obtained from the position of the half-wave potential of the second reduction step, HgSe(s) + Cd + 2e CdSe(s) +... 

Reaction 14 is particularly interesting because its end product is already present in the weak acid bleed. Removal of the selenite by this reaction would not cause the introduction of another potentially toxic by-product in the treated stream. The overall equation for the reduction of selenite to elemental selenium using sodium hydrosulfite is given by ... 

Selenites and selenates are quantitatively reduced to elementary selenium when their acid solutions are warmed with ferrous sulfate. In contrast, tellurite and tellurate solutions remain unaltered because the redox potential of iron is not sufhcient to accomplish the reaction Fe i + Teiv(vi) Fe -f Te°. However, the reducing power of Fe+ ions can be raised considerably if the Fe+ ions are removed as soon as they are produced. Phosphoric acid is excellent for this purpose it immediately converts the Fe+ ions into [Fe(P04)2] " ions. Fluorides act analogously through the formation of [FeF,]- ions. Consequently, when tellurites or tellurates are warmed with a ferrous sulfate-phosphoric acid mixture there is complete reduction to free tellurium. 

Only one study has been done specifically to examine the effect of a poised potential on the response of the bacterial biofilm. Finkelstein et al. (2006) set the potentials of anodes in sediment fuel cells at three potentials meant to mimic different reduction reaction potentials (vs. Ag/AgCl) at the cathode under acetate oxidizing conditions -0.058 V (arsenate, As0/7As03 ), 0.103 V (selenite, SeOs /Se ), and 0.618 V (O2/H2O). They found the microorganisms obtained most of the energy (95%) from oxidation of the acetate. When they switched each system to an open-circuit condition, the potentials dropped only 0.040-0.050 V in each case (i.e., they did not all drop to the same value). However, they allowed only 30 s for the system to obtain an OCV, and thus the final potential that each reactor would have reached given more time is not known. Still, this study does suggest that the mixed culture adapted in each case to the set potential—as the bacteria would have to do in order to use the electrode as a terminal electron acceptor. 

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