Mechanism of complex formation: a kinetic study of the gallium(III)–salicylate ion system

Abstract

The kinetics of the reaction between gallium(III) and salicylic acid have been investigated by the temperature-jump technique at 25 °C. The measurements have been made in acid solution (pH 1.3–3) in order to avoid precipitation and to minimise polymerisation. The results are interpreted according to a reaction scheme consisting of five parallel paths : the species Ga³⁺, [Ga(OH)]²⁺, and [Ga(OH)₂]⁺ react with the salicylate ion, and [Ga(OH)]²⁺ and [Ga(OH)₂]⁺ react with salicylic acid, all giving rise to a 1 : 1 complex [Ga(OC₆H₄CO₂)]⁺. The path involving [Ga(OH)₂]⁺ and salicylate ion proceeds through an intermediate [Ga(OH)(OC₆H₄CO₂)]. Among the reaction paths proposed, two pairs exhibit proton ambiguity; therefore, only upper limits far their rate constants can be estimated. The rate constants for the hydrolysed species are considerably larger than those for Ga³⁺. The equilibrium constant for the complex formation has been determined, both spectrophotometrically and kinetically, in the range pH 1.8–3.2.