Kinetics of copper(II)-catalyzed oxidation of S(IV) by atmospheric oxygen in ammonia buffered solutions

To understand the chemistry of Cu(II)–NH3–S(IV)–O2 system, the kinetics of the oxidation of sulfur(IV) catalyzed by amminecopper(II) complexes has been studied in the ammonia-buffered solutions. Sulfur(IV) is oxidized to sulfate. The complexes, Cu(NH3)2+, Cu(NH3)22+, and Cu(NH3)2+3 appear to be equally reactive and Cu(NH3)42+ appears to be unreactive. The kinetics obey the rate law: where α1 and γ1 are the rate constants for O2-dependent and O2-independent pathways, respectively, for Cu(NH3)2+, Cu(NH3)2+2, and Cu(N H3)32+ complexes, which appear to be equally reactive. The values of α1 and γ1 were found to be (1.32 ± 0.21) × 106 L2 mol−2 s−1 (1.74 ± 0.40) × 109 L3 mol−3 s−1respectively at 30°C. The reaction rate is not influenced by the presence of ethanol—a free radical scavenger, so a nonradical mechanism has been proposed. The results of this study are useful in understanding the atmospheric chemistry of aqueous phase autoxidation of dissolved sulfur dioxide in copper(II)–ammonia–sulfur(IV)–oxygen system at high pH. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 379–392, 2011