The Solubility of Allopurinol in Aqueous Solutions of Polyvinylpyrrolidone

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Abstract
Saturation solubilities of allopurinol have been determined in aqueous solutions at pH 1.0 containing different molecular weight fractions of polyvinylpyrrolidone (P.V.P.). The P.V.P. was characterized using light scattering techniques. Over the temperature range 15-40° the solubilities increased linearly with concentration of P.V.P. For equal concentrations of different P.V.P. species the lower the molecular weight the greater is the increase in solubility of allopurinol. Solubility increases were never more than a twofold increase for any polymer species at any temperature. Binding ratios indicated a low affinity of allopurinol to all the P.V.P. species with the greatest affinity being for the lower molecular weight fraction. The solubility data has been used to calculate enthalpy, free energy and entropy values for allopurinol/P.V.P. systems. Changes in these thermodynamic parameters did not suggest complex formation between allopurinol and P.V.P.