DETERMINATION OF POLYSULFIDE IONS IN ALKALINE MEDIUM UNDER CONDITIONS OF CATHODIC STRIPPING VOLTAMMETRY WITH MERCURY-FILM ELECTRODE

Relevance. Polysulfides of alkaline, alkaline earth metals and ammonium are components of technological solutions in large-capacity industries (pulp and paper industry; blackening and bluing of metals; production of polysulfide elastomers), they are used to obtain new functional materials (sealants) and play a decisive role in functioning of chemical current sources of a new generation. In this regard, the study of the electrode processes involving polysulfides is relevant. The aim of the research is to establish conditions for determining polysulfide ions in alkaline solutions using cathodic stripping voltammetry with a mercury-film electrode. Objects: sodium polysulfides (Na2S2, Na2S3) solutions, sodium hydroxide solutions. Methods: direct and alternating current cathodic stripping voltammetry, cyclic voltammetry, accumulative electrolysis. Results. The authors have proposed empirical equations to calculate the Gibbs energy of formation of S-n(2-)-ions in aqueous solutions and their standard potentials depending on the degree of polysulfidity in the range n=1.8. The equilibrium activities of ion-molecular forms in the Hg-S-H2O system at different pH and potentials were calculated. The authors established the optimal conditions for determining polysulfide ions S-2(2-) and S-3(2-) in alkaline medium (0,1 M NaOH) at a mercury-film electrode using the direct and alternating current cathodic stripping voltammetry: preelectrolysis potential -0,5 V (vs. Ag/AgCl); preelectrolysis duration 1...2 min; subsequent cathodic polarization to E-end=-1,0 V; preliminary deaeration of solutions with nitrogen. Under these conditions, the concentration dependence of the maximum cathodic current I-cat at E-max=-0,8...-0,9 V is linear in the range of polysulfide concentrations 1.10(-7)...1. 10(-3) M. The method can be used to determine the total content of polysulfides in solutions and to calculate the average degree of polysulfidity for Sn2--ions.