Abstract
The kinetics of vanadyl complex formation with vanillomandelic, mandelic, and thiolactic acids was studied at 25 °C and ionic strength 0.5 M (KN03) by temperature jump. The complex stability constant for thiolactic acid with vanadyl was determined by utilizing the Bjerrum titration technique. Due to the unavailability of acid-dissociation constants for the -hydroxyl groups, the complex stability constants for vanillomandelic and mandelic acids with vanadyl could not be directly determined by the Bjerrum method. For all three ligand systems an iterative procedure was carried out in which apparent forward and reverse rate constants for the complexation reaction were used to calculate the complex stability constant. The stability constants, K, = [V0L][H+]/[V02+][HL"], obtained were (4.45 ± 0.37) X 10"1
(1.79 ± 0.06) X 10~2 for the V02+-vanillomandelic acid, V02+-mandelic acid, and V02+-thiolactic acid complexes, respectively. Vanadyl complexation with all three ligands occurred with the loss of two protons, indicating bidentate chelation. Inclusion of the VOOH+ species in the overall reaction scheme led to rate constants consistent with a dissociative (Eigen) mechanism. For the vanillomandelic, mandelic, and thiolactic acid systems (ligand = HU), the V02+ complex formation rate constants obtained were (1.13 ± 0.07) X 103, (1.09 ± 0.06) X 103, and (6.76 ± 0.19) X 102 M"1 s"1, respectively; and those determined for VOOH+ complex formation were (4.17 ± 0.20) X 104, (5.86 ± 0.25) x 104, and (7.93 ± 0.14) x 104 M"1 s"1, respectively. Reanalysis of the relaxation data of earlier vanadyl studies reporting high V02+ complexation rate constants resulted in lower values.