Известия Томского политехнического университета: Инжиниринг георесурсов (Dec 2021)
VOLTAMMETRIC DETERMINATION OF THIOSULFATE IONS AT A CARBON-CONTAINING COMPOSITE ELECTRODE MODIFIED WITH GOLD PARTICLES
Abstract
Relevance. Thiosulfates are widely used in industry (textile production, water treatment, metal extraction), agriculture (plant bloom control, soil fumigation, fertilizers), medicine (detoxification therapy), analytical chemistry. The areas of practical application of thiosulfates are determined by their high reducing and complexing activity, as well as by their oxidation products of practical significance (colloidal sulfur, polythionates). Despite the long experience of practical use of thiosulfates, the search for express, sensitive and accessible methods of their determination in technological media, environmental objects, and food is still ongoing. In this regard, the improvement of methods for the determination of thiosulfates is an urgent task. Purpose: to determine the conditions for obtaining an analytical signal during the electrooxidation of thiosulfate ions using a carbon-containing composite electrode modified with gold particles. Objects: sodium thiosulfate solutions. Methods: DC voltammetry with linear potential sweep, cyclic voltammetry, scanning electron microscopy, energy-dispersive X-ray spectroscopy, modeling of redox equilibria. Results. Electrooxidation of thiosulfate ions at a carbon-containing composite electrode modified with gold particles under conditions of direct current voltammetry with linear potential sweep (supporting electrolytes 0,05 M KNO3 or 0,05 M Na2SO4) proceeds at potentials of 0,2...0,6 and 0,8...1,3 V (vs. Ag/AgCl/KCl electrode) with anodic current maxima at Ep,1»0,3 and Ep,2»1,1 V. The value of the anode current of Ip,2 is an order of magnitude higher than the value of Ip,1, the concentration dependences of Ip,1 and Ip,2 are linear in the intervals c(S2O32–)=1×10–6...1×10–5 and 1×10–7...1×10–5 M, respectively. The interpretation of electrode processes was carried out using calculated diagrams of metastable ion-molecular species in the S–Au–H2O system, as well as literature data. It was shown that anodic current in the Ep,1 region is caused by a combination of electrode and chemical reactions involving intermediate products of thiosulfate oxidation (sulfite, polythionate ions, sulfur, Au complexes). The main process at Ep,2 is anodic oxidation of sulfur accompanied with electrocatalytic cycle involving Au (hydr)oxides. The use of the value of Ip,2 at Ep,2»1,.1 V to determine S2O32– ions of low concentrations was shown to be reasonable. The accuracy of the measurement results and the stability of the electrode function were evaluated during multiple registration of the analytical signal. The detection limit of S2O32– ions according to the proposed method is cmin=5×10–8 M (7,9 µg×l–1), the lower limit of the determined concentration is clim=1,2×10–7 M. The results obtained in the work can be used in the development (improvement) of methods for determining low concentrations of S2O32– ions in technological solutions, environmental objects, food.
Keywords
