Kinetics and equilibria of the interaction of titanium(III) with oxalic acid

Abstract

Rate and equilibrium data have been determined for the complex-formation reactions of Ti^III with oxalate in aqueous solution. In order to suppress the hydrolysis of Ti^III, fairly high concentrations of hydrogen ion have been used. At 10.0 °C and ionic strength I= 1.0 mol dm^–3(NaCl) the following stability constants have been obtained by spectrophotometry: Ti³⁺+[C₂O₄]^2–⇌[Ti(C₂O₄)]⁺, K₁= 2.8 × 10⁶ dm³ mol^–1; [Ti(C₂O₄)]⁺+[C₂O₄]^2–⇌[Ti(C₂O₄)₂]^–, K₂= 1.2 × 10⁶ dm³ mol^–1. The kinetics of the complex-formation reactions have been studied by the temperature-jump relaxation technique. At [H⁺]= 0.2–0.8 mol dm^–3 complex formation occurs predominantly via reaction with the monoprotonated ligand. For the first step, Ti³⁺+[HC₂O₄]^– [graphic omitted] [Ti(C₂O₄)]⁺+ H⁺, a rate constant of k_1b= 3.9 × 10⁵ dm³ mol^–1 s^–1 is evaluated and for [Ti(C₂O₄)]⁺+[HC₂O₄]^– [graphic omitted] [Ti(C₂O₄)₂]^–+ H⁺, k_2b= 8.0 × 10⁶ dm³ mol^–1 s^–1(10.0 °C, I= 1.0 mol dm^–3). The kinetic data available indicate that substitution at the [Ti(OH₂)₆]³⁺ ion proceeds by an associative mechanism.