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

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 ==4.6, the rate increases with the pH due to the dissociation of H2NCSNH2 to the reactive H2NCSNH- ion. In dilute perchloric acid solutions the rate increases with increasing [H+] due to the participation of protonated NH2C+SHNH2 species. The seat of the reaction is thought to be the enolic S atom (not protonated S) rather than the protonated nitrogen of the NH2 group as in the oxidations of NH2OH, H2NNH2 and N3H where the rate is retarded by the increase in [H+]. The acid dissociation constant, Ka, of protonated NH2C+SHNH2 is estimated to be 0.048 mol dm-3 at 40 °C. The Marcus theory is used to estimate the self-exchange rate (k22) of the H2NCSNH2-(H2N)2CSSC(NH2)2 couple. The estimated k22 is 2.44 × 10^-10 dm^3 mol^-1 s^-1. The low value is attributed to the stable dimeric (H2N)2CSSC(NH2)2 species.