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

Abstract

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 = (k⁰ + 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 = k⁰[H₂O₂] with a value k⁰ = (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.