The pK(a) for the equilibrium Cr(III)(H2O)3(OH)3(OH)3 reversible Cr(III)(H2O)2(OH)4- + H+ was determined to be 12.8 at 25-degrees-C. The dimerization of the two monomeric forms was studied in alkaline solutions using the stopped-flow method: k2[Cr(III)(H2O)3(OH)3 + Cr(III)(H2O)3(OH)3] = (2.5 +/- 0.3) x 10(4) dm3 mol-1 s-1; k3[Cr(III)(H2O)3(OH)3 + Cr(III)(H2O)2(OH)4-] = (3.8 +/- 0.5) x 10(3) dm3 mol-1 s-1 and k4[Cr(III)(H2O)2(OH)4- + Cr(III)(H2O)2(OH)4-] = (1.2 +/- 0.2) x 10(2) dm3 mol-1 s-1. The absorption spectra of the monomeric and dimeric species are also reported. The oxidation of chromium monomer Cr(III) and dimer (Cr)2III,III by hydroxyl radicals was studied by the pre-mix pulse radiolysis technique. The kinetic parameters were established for: (a) Cr(IV) monomer formation; (b) formation of (Cr)2III,IV dimer; (c) dissociation of (Cr)2III,IV dimer and (d) formation of polymers e.g. [Cr(VI)-(O-Cr(III))n]. Furthermore, a second-order reaction between two Cr(IV) monomers to yield a species which may be either a (Cr)2IV,IV or a (Cr)2III,V mixed-valence dimer was observed. The corresponding spectra in both the UV and visible range were determined.
|Journal||Radiation Physics and Chemistry|
|Publication status||Published - 1995|