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

✍ Scribed by Anil Kumar Sharma; Punit Kumar Mudgal; S. P. Bansal; K. S. Gupta

• Publisher: John Wiley and Sons
• Year: 2010
• Tongue: English
• Weight: 459 KB
• Volume: 42
• Category: Article
• ISSN: 0538-8066
• DOI: 10.1002/kin.20496

✦ Synopsis

Abstract

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 law

equation image

where 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 by

equation image

where K, 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~), Ni(NH~3~), and Ni(NH~3~), respectively. In unbuffered medium, the factor F reduces to

equation image

The 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