Kinetics and mechanism of oxidations by peroxodiphosphate ions. Part 7. Role of trace metal ions, and catalysis by ethylenediaminetetra-acetatocopper(II) in the oxidation of hexacyanoferrate(II) in acid perchlorate media

Abstract

Oxidation of [Fe(CN)]₆^4– by peroxodiphosphate [equation (i)] is influenced by trace iron(II) and copper(II) ions 2[Fe(CN)₆]^4–+ H₂P₂O₈^2–→ 2[Fe(CN)₆]^3–+ 2HPO₄^2–(i) present in the distilled water and the reagents used. A detailed study has shown that Fe²⁺(aq), Fe³⁺(aq), and [Cu(edta)]^2– catalyse the reaction, [Fe(edta)]^2– and [Fe(edta)]^–(edta = ethylenediaminetetra-acetate) are not reactive, and Cu²⁺(aq) inhibits the reaction. The increase in hexacyanoferrate(III) concentration has been followed colorimetrically and rate of reaction in the presence of edta and added Cu^II is given by (ii), where [Fe^II], d[Fe(CN)₆^3–]//_dt=k₁′[Cu(edta)^2–][Fe^II][peroxodiphosphate]//_{k2′[Fe^III]+k3′[peroxodiphosphate]}(ii)[Fe^III], and [peroxodiphosphate] are the total concentrations of the iron(II), iron(III), and peroxodiphosphate species respectively. The hydrogen-ion dependent values of k₁′/k₂′ and k₃′/k′₂ are 5.9 ± 1.1 and (1.09 ± 0.24)× 10^–1 respectively at 35 °C and [H⁺]= 0.05 mol dm^–3. The reactive species and the mechanism are as in equations (iii) and (iv). [Cu(edta)]^2–+[HFe(CN)₆]^3– [graphic omitted] [Cu(edta)]^3–+[HFe(CN)₆]^2–(iii), [Cu(edta)]^3–+ H₄P₂O₈ [graphic omitted] [Cu(edta)]^2–+ products (iv)