Kinetics and mechanism of the oxidation of titanium(III) by aqueous solutions of chlorine

Abstract

The kinetics of the non-complementary redox reaction 2Ti³⁺+ Cl₂+ 2H₂O → 2[TiO]²⁺+ 2Cl^–+ 4H⁺ have been studied in acid solution. Under the conditions 0·10 ⩽[H⁺] < 1·0M, 0·05 ⩽[Cl^–]⩽ 1·0M, and ionic strength of 1·0M, the reaction follows strictly second-order kinetics with an apparent second-order rate constant of 125·0 ± 8·0 l mol^–1 s^–1 at [H⁺]= 0·5, [Cl^–]= 0·5, I= 1·0M and 30 °C. The rate decreases as [H⁺] and [Cl^–] are independently increased, but addition of Ti^IV has no effect on the rate. The results are interpreted in terms of [Ti(OH)]²⁺+ Cl₂, [TiCl]²⁺+ HOCl, and Ti³⁺+ HOCl as the important activation paths. The rate constant for the Ti³⁺+ HOCl path is (1·02 ± 0·01)× 10⁴ l mol^–1 s^–1, with the activation parameters ΔH^‡= 16·0 ± 0·2 kcal mol^–1 and ΔS^‡= 12·58 ± 0·6 cal K^–1 mol^–1. For the [Ti(OH)]²⁺+ Cl₂ path, electron transfer via a series of one-electron changes is suggested. The role of prior hydrolysis in the kinetics of oxidation of aqua-ions to oxo-cations by acid solutions of halogens is discussed.