Comparative Study of Thermodynamic Parameters of Vanadium (IV) and (V) Acetohydroxamate Complexes

Acetohydroxamic acid, CH_3CONHOH, forms highly stable complexes with vanadium (V) and vanadium (IV) in 1:1, 1:2 and 1:3 mole ratios. The stability of these complexes can be determined in terms of thermodynamic parameters; \Delta G,\Delta H and \Delta S. The preliminary data, obtained through pH titration at various temperatures, was processed and analyzed by the computer program BEST for the refinement of graphically calculated log \beta values. Graphs of \ln\beta versus 1/T, gave a straight line, with a slope -\Delta H/R and intercept \Delta S/R. Enthalpy and free energy changes for V(V) complexes were found in the order of ML>ML_2>ML_3 with a negative sign. Whereas entropy change was found to be in the same order but positive, for vanadium (IV) acetohydroxamic acid complexes, the order of \Delta G, \Delta H and \Delta S was ML>ML_3>ML_2. The \Delta S is most positive for a 1:1 complex, while \Delta G and \Delta H are more negative for the same.