Speciation of the Vanadium (III) Complexes with 6-Methylpicolinic Acid,Salicylic Acid and Phthalic Acid

The complex species formed in aqueous solutions (25 °C, I=3.0 mol⋅dm⁻³ KCl ionic medium) between the V(III) cation and the ligands 6-methylpicolinic acid (MePic, HL), salicylic acid (H₂Sal, H₂L) and phthalic acid (H₂Phtha, H₂L) have been studied by potentiometric and spectrophotometric measurements. Application of the least-squares computer program LETAGROP to the experimental emf(H) data, taking into account the hydrolytic species and hydrolysis constants of V(III), indicates that under the employed experimental conditions the complexes VL]²⁺, V(OH)L]⁺, V(OH)₂L], V(OH)₃L]⁻, VL₂]⁺, VL₃] and V₂OL₄] form in the vanadium(III)–MePic system. Were observed the complexes VL]⁺, VL₂]⁻, V(OH)L₂]²⁻ and VL₃]³⁻ in the vanadium(III)–H₂Sal system, and the species VHL]²⁺, VL]⁺, V(OH)L], VHL₂], VL₂]⁻, V(OH)L₂]²⁻, V(OH)₂L₂]³⁻ and VL₃]³⁻ in the vanadium(III)–H₂Phtha system. The stability constants of these complexes were determined by potentiometric measurements, and spectrophotometric measurements were made in order to perform a qualitative characterization of the complexes formed in aqueous solution.