Selenomethionine‐Catalyzed Nickel Ion Reduction at a Mercury Electrode: Applications in the Analysis of Nutritional Supplements

Selenomethionine (SeMet) is a catalyst for Ni²⁺ reduction at a mercury electrode in a borax buffer at pH around 9 and gives rise to a differential pulse voltammetric peak, A, at ?0.74 V vs. the Ag|AgCl, (3 M KCl) reference electrode. Peak current is directly proportional to SeMet concentration over the concentration range 0.4–10 μM. Alkali and alkali‐earth ions depress to some extent the sensitivity but the current‐concentration relationships remains linear even under these conditions. Differential pulse cathodic stripping voltammetry (DPCSV) in 0.01 M borax results in two partially overlapped peaks. The more negative (A, at about ?0.74 V) is similar to that recorded with no deposition and is due to the catalysis by nonadsorbed SeMet, whereas the more positive one (B, at about ?0.60 V) results from the catalysis by adsorbed SeMet. Only the DPCSV peak A appears if 0.1 M KNO₃ is also present along with 0.01 M borax. Stearic acid, which is present in nutritional supplement tablets, improves the separation of the DPCSV peaks. Consequently, the peak B recorded with 0.01 M borax buffer allows determining SeMet in nutritional supplement tablets by the standard addition method and enables discriminating between the organic and inorganic selenium forms.