ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 33, Number 1
JKCSEZ 33(1)
February 20, 1989 

Reduction and equilibrium of vanadium-DTPA (DTPA = diethylenetriaminepentaacetic acid, H5A) complexes at mercury electrodes are studied in 0.5M NaClO4 aqueous solution at 3.2 < pH < 10.5 and 25℃. At 3.2 < pH < 5.9, the reduction reaction is V·A2- + H+ + e- = V·HA2-, while at 5.9 < pH < 10.5 it is V·A2- + e- = V·A3-. The stability constants of V·HA2- and V·A3- are found to be 6.46 × 10⁹ and 3.09 × 10¹⁴, respectively. V(Ⅳ)-DTPA undergoes stepwise complexation as VO2+ + H2A3- = VO·HA2- + H+ and VO·HA2- = VO·A3- + H+, where acidity constant of VO·HA2- is pKa = 7.15. Stability constants of VO·HA2- and VO·A3- are found to be 1.41 × 10¹⁴ and 3.80 × 10¹⁷, respectively. It is detected that VO₂₊-DTPA is reduced irreversibly to V·A2- with the transfer coefficient of α = 0.43. At more cathodic overpotential, the reduction is stepwise as V(Ⅳ)→V(Ⅲ)→V(Ⅱ). The first one corresponds to VO·HA2- + e- → VO·HA3- at 3.2 < pH < 7.2 and VO·A3- + e- → VO·A4- at 7.2 < pH < 10.5. The second is identical to that of V(Ⅲ). Diffusion coefficients of VO·HA2- and VO·A3- are found to be (9.0±0.3) × 10^-6 cm2/s and (5.9±0.4) × 10^-6 cm2/sec, respectively.