Speciation of the Ternary Complexes of Vanadium(III)–Dipicolinic Acid with the Amino Acids Glycine, Proline, α-Alanine and β-Alanine Studied in 3.0 mol⋅dm−3 KCl at 25 °C

In this paper we present speciation results for the ternary vanadium(III)–dipicolinic acid (H2dipic) systems with the amino acids glycine (Hgly), proline (Hpro), α-alanine (Hα-ala), and β-alanine (Hβ-ala), obtained by means of electromotive forces measurements emf(H) using 3.0 mol⋅dm−3 KCl as the ionic medium and a temperature of 25 °C. The experimental data were analyzed by means of the computational least-squares program LETAGROP, taking into account hydrolysis of the vanadium(III) cation, the respective stability constants of the binary complexes, and the acid base reactions of the ligands, which were kept fixed during the analysis. In the vanadium(III)–dipicolinic acid–glycine system, formation of the ternary [V(Hdipic)(Hgly)]2+, [V(dipic)(Hgly)]+, [V(dipic)(gly)], [V(dipic)(gly)(OH)]− and [V(dipic)(gly)(OH)2]2− was observed; in the case of the vanadium(III)–dipicolinic acid–proline system the ternary complexes [V(Hdipic) (Hpro)]2+, [V(dipic)(Hpro)]+, [V(dipic)(pro)] and [V(dipic)(pro)(OH)]− were observed; in the vanadium(III)–picolinic acid–α-alanine were observed [V(Hdipic)(Hα-ala)]2+, [V(dipic) (Hα-ala)]+, [V(dipic)(αala)], [V(dipic)(α-ala)(OH)]− and [V(dipic)(α-ala)(OH)2]2−; and in the vanadium(III)–dipicolinic acid–β-ala system the complexes [V(dipic) (Hβ-ala)]+, [V(dipic)(β-ala)], [V(dipic)(β-ala)(OH)]− and [V(dipic)(β-ala)(OH)2]2− were observed. Their respective stability constants were determined, and we evaluated values of Δlog 10K″ in order to understand the relative stability of the ternary complexes compared to the corresponding binary ones. The species distribution diagrams are briefly discussed as a function of pH.