Kinetics of the simultaneous oxidation of nickel(II) and sulfur(IV) by oxygen in alkaline medium in Ni(II)–sulfur(IV)–O2 system

In the Ni(II)–S(IV)–O2 system in the region of pH > 8.4, both Ni(II) and S(IV) are simultaneously autoxidized, and when sulfur is consumed fully NiOOH precipitates. At pH > 8.4, ethanol has no effect on the rate, whereas ammonia strongly inhibits the reaction when pH > 7.0. The kinetics of the reaction, in both the presence and the absence of ethanol, is defined by the rate law where k is the rate constant, KO is the equilibrium constant for the adsorption of O2 on Ni(OH)2 particle surface. In ammonia buffer, the factor F is defined by where K, KOH, K1, K2, K3, and K4 are the stability constants of NiSO3, NiOH+, Ni(NH3)2+, Ni(NH3), Ni(NH3), and Ni(NH3), respectively. In unbuffered medium, the factor F reduces to The values of k and Ksp were found to be (1.3 ± 0.08) × 10−1 s−1 and (4.2 ± 3.5) × 10−16, respectively, at 30°C. A nonradical mechanism that assumes the adsorption of both SO32− and O2 on the Ni(OH)2 particle surface has been proposed. At pH ≤ 8.2, Ni(II) displays no catalytic activity for sulfur(IV)-autoxidation and it is also not oxidized to NiOOH. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 464–478, 2010