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

摘要

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 Cu~(2+)and 3 mol H_2O, generating 1 mol copper(II), 1 mol sulfate and 2 mol H_3O~+. In the kinetic study, the apparent reaction order was approx = 0 with respect to H_3O~+ concentration, in the interval 1.0 centre dot 10~(-4)-1.0 centre dot 10~(-5)M H_3O~+; of order 0.4 with respect to Cu~(2+) in the interval 0.21-0.85 g L~(-1) Cu~(2+); and of order 0 with respect to S_2O_3~(2-) in the interval 0.88-2 g L~(-1) S_2O_3~(2-). The apparent activation energy was 98 kJ mol~(-1) in the interval 15-40 deg 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.