Kinetics and equilibria of 1 : 1 complex formation of hexa-aquatitanium(III) with malonic and methylmalonic acid

Abstract

Equilibria and kinetics of the 1 : 1 complex formation of Ti^III with malonic acid and methylmalonic acid have been investigated at 15 °C, ionic strength I= 0.5 mol dm^–3(LiCl). Spectrophotometric measurements yielded for the complex stability constant K_c=[TiL⁺]/[Ti³⁺][L^2–] values of 3.4 × 10⁶(malonic acid) and 4.0 × 10⁶ dm³ mol^–1(methylmalonic acid). The kinetics of the complex-formation process show simple behaviour within the acidity range [H⁺]= 0.05–0.30 mol dm^–3, the apparent second-order rate constant being of the form k_a=k[H⁺]. It is concluded that under these conditions only the reaction path Ti³⁺+ HL^– [graphic omitted] [TiL]⁺+ H⁺ contributes to the complex formation, with k₂= 4.2 × 10⁵(malonic acid) and 3.2 × 10⁵ dm³ mol^–1 s^–1(methylmalonic acid), respectively. Together with data obtained earlier, the results indicate an associativeinterchange mechanism for substitution at [Ti(H₂O)₆]³⁺.