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)-O-2 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)(2)(2+), and Cu(NH3)(3)(2+) appear to be equally reactive and Cu(NH3)(4)(2+) appears to be unreactive. The kinetics obey the rate law: k(obs) = [alpha(1){beta(1)[NH3] + beta(2)[NH3](2) + beta(3)[NH3](3)} + lambda(1) {beta(1)[NH3] + beta(2)[NH3](2) + beta(3)[NH3](3)} [O-2]] / {1 + K-SO3 [SO32-] + K-OH[OH-] + beta(1)[NH3] + beta(2)[NH3](2) + beta(3)[NH3](3) + beta(4)[NH3](4)} where alpha(1) and gamma(1) are the rate constants for O-2-dependent and O-2-independent pathways, respectively, for Cu(NH3)(2+), Cu(NH3)(2)(2+), and Cu(N H-3)(3)(2+) complexes, which appear to be equally reactive. The values of alpha(1) and gamma(1) were found to be (1.32 +/- 0.21) x 10(6) L-2 mol(-2) s(-1) (1.74 +/- 0.40) x 10(9) L-3 mol(-3) s(-1) respectively at 30 degrees 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. (C) 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 379-392, 2011