Equilibria and mechanism of the interaction of InIII with the indicator ferron

Abstract

The interaction between In^III and the indicator ferron (H₂L) has been investigated at 25 °C, ionic strength of 0.2 mol dm^–3, and [H⁺]= 0.008–0.2 mol dm^–3. Two complexes were observed, namely, [InL]⁺ with K₁= 237 ± 7 and [In(HL)]²⁺ with K₁′= 688 ± 65 dm³ mol^–1. The formation of the complexes proceeds via three parallel paths differing in their [H⁺] dependences. One of these can be uniquely assigned to the reaction between In³⁺ and H₂L with a forward rate constant of (1.1 ± 0.1)× 10³ dm³ mol^–1 s^–1; each of the other two is in fact a proton-ambiguous pair, one involving In³⁺ and HL^–, and/or [In(OH)]²⁺ and H₂L, and the other (which makes a smaller contribution) involving [In(OH)]²⁺ and HL^–, and/or [In(OH)₂]⁺ and H₂L. For these pairs, only upper limits for the rate constants can be given. Nevertheless, it can be concluded unequivocally that the hydrolysed species react faster than In³⁺. The results are compared with previous data involving In^III and are discussed in terms of the dissociative Eigen mechanism vs. an associative mechanism.