Molar entropy of the selenium (VI)/(IV) couple obtained by cyclic voltammetry

An electrochemical investigation of selenium species, using cyclic voltammetry, has been carried out for the purpose of determining the molar entropy of the Se(VI)/(IV) couple. To obtain the ion interaction coefficient between HSeO4− and Na+, ϵT(HSeO4−, Na+), the following reaction involving HSeO4− as an oxidant and the uncharged species (H2SeO3) as the reductant was used: The Se(VI)/(IV) half-wave potentials were measured in acidic sodium nitrate solutions as a function of the molality of Na+ ranging from 0.500 to 2.00 mol kg−1. The temperatures for the measurement were 288, 298, 308 and 323 K. Specific ion interaction theory was used to calculate the HSeO4−/H2SeO3 standard redox potential, , and ϵT(HSeO4−, Na+) at each temperature. The following molar entropy was derived from the temperature dependence of : The value of ϵT(HSeO4−, Na+) at 298 K was determined to be 0.29 ± 0.03 kg mol−1. The following ∂ϵ/∂T was derived from the temperature dependence of ϵT(HSeO4−, Na+):