The kinetics of the oxidation of thiourea by 12-tungsto-cobaltate(iii) ion: evidence for anionic, neutral and protonated thiourea species in acetic acid-acetate buffer and perchloric acid solution

摘要

The kinetics of the redox reaction between 12-tungstocobaltate(iii) ion, [Co(iii)W]~(5-), and thiourea is studied in acetic acid-acetate buffered solutions (3.4 ≤ pH ≤ 5.6), and dilute perchloric acid solution. The reaction in buffered solution is first order both in [Co(iii)W]~(5-) and low [H_2NCSNH_2]. A simultaneous first- and second order dependence is observed at high [H_2NCSNH_2]. The rate is independent of pH ≤ 4.6 due to the participation of H_2NCSNH_2. In solutions of pH ≥ 4.6, the rate increases with the pH due to the dissociation of H_2NCSNH_2 to the reactive H_2NCSNH~- ion. In dilute perchloric acid solutions the rate increases with increasing [H~+] due to the participation of protonated NH_2C~+SHNH_2 species. The seat of the reaction is thought to be the enolic S atom (not protonated S) rather than the protonated nitrogen of the NH_2 group as in the oxidations of NH_2OH, H_2NNH_2 and N_3H where the rate is retarded by the increase in [H~+]. The acid dissociation constant, K_a, of protonated NH_2C~+SHNH_2 is estimated to be 0.048 mol dm~(-3) at 40 ℃. The Marcus theory is used to estimate the self-exchange rate (k_(22)) of the H_2NCSNH_2-(H_2N)_2CSS(NH_2)_2 couple. The estimated k_(22) is 2.44 * 10~(-10) dm~3 mol~(-1) s~(-1). The low value is attributed to the stable dimeric (H_2N)_2CSSC(NH_2)_2 species.