Mechanism of tetrachloroplatinate(II) oxidation by hydrogen peroxide in hydrochloric acid solution

Oxidation of tetrachloroplatinate(II) by hydrogen peroxide in hydrochloric acid was studied by UV-Vis spectrophotometry. Oxidation takes place via two parallel reactions with hypochlorous acid and hydrogen peroxide, respectively, according to the overall rate law d[Pt(IV)]/dt = (k(0) + k(H2O2)[Pt(II)])[H2O2]. For oxidation of [PtCl4](2-) at relatively low concentrations, [PtCl4](2-) ≪ 0.5 mM, hypochlorous acid formation is fast relative to the oxidation of [PtCl4](2-) by hydrogen peroxide, as a result of the rate determining reaction H2O2 + H(+) + Cl(-) → HOCl + H2O, resulting in a rate law d[Pt(IV)]/dt = k(0)[H2O2] with a value k(0) = (8 ± 2) × 10(-7) s(-1) at 35 °C. For concentrations of [PtCl4](2-) > 0.5 mM, oxidation by hydrogen peroxide becomes dominant, resulting in the pseudo-first order rate law d[Pt(IV)]/dt = k(H2O2)[Pt(II)][H2O2] with the value k(H2O2) = (1.5 ± 0.1) × 10(-2) M(-1) s(-1) at 35 °C. The final oxidation product is a mixture of [PtCl5(H2O)](-) and [PtCl6](2-), with [PtCl6](2-) formed as a result of [PtCl4](2-) assisted chloride anation reactions.