The oxidation of thiosulfates with copper sulfate. Application to an industrial fixing bath

This paper presents the transformation of thiosulfate using Cu(II) salts, such as copper sulfate, at pH between 4 and 5. The nature and kinetics of this process were determined. In the experimental conditions employed, the reaction between thiosulfates and Cu(II) ions produces a precipitate of CuS and the remaining sulfur is oxidized to sulfate, according to the following stoichiometry: 1 mol thiosulfate reacts with 1 mol Cu2+ and 3 mol H2O, generating 1 mol copper(II), 1 mol sulfate and 2 mol H3O+. In the kinetic study, the apparent reaction order was ≈ 0 with respect to H3O+ concentration, in the interval 1.0 · 10− 4–1.0 · 10− 5M H3O+; of order 0.4 with respect to Cu2+ in the interval 0.21–0.85 g L− 1 Cu2+; and of order 0 with respect to S2O32− in the interval 0.88–2 g L−1 S2O32−. The apparent activation energy was 98 kJ mol− 1 in the interval 15–40 °C. On the basis of this behavior an empirical mathematical model was established, that fits well with the experimental results. The thiosulfate transformation process using copper(II) sulfate was applied to an industrial fixing bath that proceeded from the photographic industry; after this, the resulting effluent contained less than 10 mg L− 1 of thiosulfates.