Kinetics of the simultaneous oxidation of nickel(II) and sulfur(IV) by oxygen in alkaline medium in Ni(II)–sulfur(IV)–O 2 system

In the Ni(II)–S(IV)–O 2 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 lawwhere k is the rate constant, K O is the equilibrium constant for the adsorption of O 2 on Ni(OH) 2 particle surface. In ammonia buffer, the factor F is defined bywhere K   SO   3, K OH , K 1 , K 2 , K 3 , and K 4 are the stability constants of NiSO 3 , NiOH + , Ni(NH 3 ) 2+ , Ni(NH 3 )   2 2+ , Ni(NH 3 )   3 2+ , and Ni(NH 3 )   4 2+ , respectively. In unbuffered medium, the factor F reduces toThe values of k and K sp 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 SO 3 2− and O 2 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