Alkaline Selenide, Polyselenide Electrolytes: Concentrations, Absorption-Spectra and Formal Potentials

Abstract

Spectrophotometric, titrimetric, electron spin resonance and ⁷⁷Se nuclear magnetic resonance studies made on the alkaline selenide, selenium electrolyte showed that soluble selenium species existed in four oxidation states, none of the species was a radical, and the mean state of oxidation of the Se-saturated solution was equivalent to Se_3.8². It was concluded that the species were Se_n^2- with n = 1, 2, 3 and 4, or their protonated forms.

On the basis of the Nernst equation, we analysed independently (A) the titration E_redox against the mean state of oxidation of the solution and (B) the absorption spectra of solutions with different mean states of oxidation. Thus formal redox potentials E′ were determined as E′₁₂ = -0.935, E′₁₃ = -0.931 and E′₁₄ = -0.914 V against s.c.e. for the couples Se^2-, Se₂^2-; Se^2-, Se₃^2-; and Se^2-, Se₄^2- respectively in 1 mol dm^-3 KOH supporting electrolyte at 304 K. It followed that E′₂₃ = -0.919 for Se₃^2-,Se₃^2- and E′₃₄ = -0.778 for Se₃^2-,Se₄^2-. The concentration of each species at each redox potential and the absorption spectrum of each distinct oxidation state were found. Excellent fits and agreement between the two analyses were observed.

Ultraviolet absorption spectra showed that the reduced ion was approximately 50% protonated in 1 mol dm^-3 KOH supporting electrolyte.