Physico-Chemical Studies on Vanadium(V), Niobium(V) and Tantalum(V) Complexes with Eriochrome cyanine-R and Methylene Blue

Abstract

The complexation of V(V), Nb(V) and Ta(V) with methylene blue (MB) and eriochrome cyanine-R(ECR) was investigated by conductometric titrations, conductometric measurements and spectrophotometric methods. The stoichiometry, formation constants and thermodynamic parameters (ΔGº , ΔH and ΔS) of the complexes were investigated. Effect of pH on the absorption spectra of complexes, effect of time and temperature on the stability of metal chelates were also studied. Vanadium, niobium and tantalum form complexes at pH 8.0 (λ ═ 660 nm), 3 (λ ═ 665) and 7(λ ═ 670 nm) with methylene blue and at pH 3.5 (λ ═ 540 nm), 4.53(λ ═ 520 nm) and 4.6(λ ═ 520 nm) with ECR for V, Nb and Ta-complexes, respectively. The complexes reach maximum development immediately and at 60º in the case of V-MB, immediately and at 50º in the case of Nb-MB, immediately and at 80º in the case of Ta-MB, after 30 min and at 30º in the case of Ta-ECR complexes.