The oxidation of chromium(III) by hydroxyl radical in alkaline solution. A stopped-flow and pre-mix pulse radiolysis study

Abstract

The p K a for the equilibrium Cr III(H 2O) 3(OH) 3⇌Cr III(H 2O) 2(OH) − 4 + H + was determined to be 12.8 at 25°C. The dimerization of the two monomeric forms was studied in alkaline solutions using the stopped-flow method: k 2[Cr III(H 2O) 3(OH) 3 + Cr III(H 2O) 3(OH) 3] = (2.5 ± 0.3) × 10 4 dm 3 mol −1 s −1; k 3[Cr III(H 2O) 3(OH) 3 + Cr III(H 2O) 2(OH) − 4] = (3.8 ± 0.5) × 10 3 dm 3 mol −1 s −1 and k 4[Cr III(H 2O) 2(OH) − 4 + Cr III(H 2O) 2(OH) − 4] = (1.2 ± 0.2) × 10 2 dm 3 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) III,III 2 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) III,IV 2 dimer; (c) dissociation of (Cr) III,IV 2 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) IV,IV 2 or a (Cr) III,V 2 mixed-valence dimer was observed. The corresponding spectra in both the UV and visible range were determined.