Speciation of the Ternary Complexes of Vanadium(III)-Dipicolinic Acid and the Amino Acids Cysteine, Histidine, Aspartic and Glutamic Acids in 3.0 mol.dm-3 KCl at 25 °C

In this work we present results for the speciation of the ternary complexes formed in the aqueous vanadium(III)-dipicolinic acid and the amino acids cysteine (H(2)cys), histidine (Hhis), aspartic acid (H(2)asp) and glutamic acid (H(2)glu) systems (25 degrees C; 3.0 mol.dm(-3) KCl as ionic medium), determined by means of potentiometric measurements. The potentiometric data were analyzed with the least-squares program LETAGROP, taking into account the hydrolysis of vanadium(III), the acid-base reactions of the ligands, and the binary complexes formed. Under the experimental conditions (vanadium(III) concentration = 2-3 mmol.dm(-3) and vanadium(III):dipicolinic acid:amino acid molar ratio 1:1:1, 1:1:2 and 1:2:1), the following species [V(dipic)(H(2)asp)](+), [V(dipic)(Hasp)], [V(dipic)(asp)](-), [V(dipic)(asp)(OH)](2-), and [V(dipic)(asp)(OH)(2)](3-) were found in the vanadium(III)-dipicolinic acid-aspartic acid system. In the vanadium(III)-dipicolinic acid-glutamic acid system [V(Hdipic)(H(2)glu)](2+), [V(dipic)(H(2)glu)](+), [V(dipic)(Hglu)], [V(dipic)(Hglu)(OH)](-), and [V(dipic)(Hglu)(OH)(2)](2-) were observed. In the vanadium (III)-dipicolinic acid-cysteine system the complexes [V(dipic)(H(2)cys)](+), [V(dipic)(Hcys)], [V(dipic)(cys)](-), and [V(dipic)(cys)(OH)](2-) were present. And finally, in the vanadium(III)-dipicolinic acid-histidine system the complexes [V(Hdipic)(Hhis)](2+), [V(dipic) (Hhis)](+), [V(dipic)(his)], [V(dipic)(his)(OH)](-), and [V(dipic)(his)(OH)(2)](2-) were observed. The stability constants of these complexes were determined. The species distribution diagrams as a function of pH are briefly discussed.