Mechanistic studies of oscillatory copper(II) catalyzed oxidation reactions of sulfur compounds

Trace amounts of copper ion catalyst induce exotic phenomena in the oxidation of several inorganic sulfur compounds by peroxides in aqueous solution. Simple and complex oscillations and several kinds of bistability are observed when the copper(II)-catalyzed oxidation of S(2)O(3)(2-) by either H(2)O(2) or S(2)O(8)(2-) is carried out in a CSTR and when SCN(-) ions are oxidized with H(2)O(2) in the presence of copper ions under either batch or flow conditions. For the S(2)O(3)(2-)-H(2)O(2)-Cu(II) reaction, a four-step model is proposed, in which formation of the intermediate HOS(2)O(3)(-) and attack on that species by S(2)O(3)(2-) and H(2)O(2) play key roles. When this core of reactions is extended with additional steps, computer simulations yield good agreement between the experimentally observed and calculated pH oscillations, bistability and batch behavior. In the oscillatory S(2)O(3)(2-)-S(2)O(8)(2-)-Cu(II) flow reaction, Cu(I), Cu(II) and Cu(III) species as well as SO(4)(.-) and S(28)O(3)(.-), are postulated to participate in a free radical mechanism, which successfully simulates the oscillations. To model the experimentally observed oscillations and bistability in the H(2)O(2)-SCN(-)-Cu(II) system, we have proposed a complex mechanism involving 30 reactions and 26 independent species. (C) 1999 Elsevier Science Ltd. All rights reserved.