Saturation molalities and standard molar enthalpies of solution of cytidine(cr), hypoxanthine(cr), thymidine(cr), thymine(cr), uridine(cr), and xanthine(cr) in H 2O(l)

Abstract

Saturation molalities m(sat) in H 2O(l) have been measured for the substances cytidine(cr), hypoxanthine(cr), thymidine(cr), thymine(cr), uridine(cr), and xanthine(cr) by using h.p.l.c. The states of hydration were established by performing Karl-Fischer analyses on samples of these substances, which had been allowed to equilibrate with their respective aqueous saturated solutions for several days at T≈298 K and then dried with air at T≈296 K for ≈24 h. The crystalline forms of the substances were identified by comparison of the results of X-ray diffraction measurements with results from the literature. Also, molar enthalpies of solution Δ sol H m(cal) for these substances were measured by using an isoperibol solution calorimeter. A self-association (stacking) model was used to estimate values of the activity coefficients γ and relative apparent molar enthalpies L φ for these substances. These γ and L φ values were used to adjust the measured values of m(sat) and Δ sol H m(cal) to the standard state and thus obtain values of the standard molar Gibbs free energy Δ sol G ∘ m and enthalpy changes Δ sol H m ∘ for the dissolution reactions of these substances. The values of the p Ks and of the standard molar enthalpies of the ionization reactions were also used to account for speciation of the substances in the calculations of Δ sol G m ∘ and Δ sol H m ∘. Values of standard molar enthalpies of formation Δ f H m ∘, standard molar Gibbs free energies of formation Δ f G m ∘, and standard partial molar entropies S 2,m ∘ for the aqueous species of hypoxanthine and xanthine were calculated. A detailed summary and comparison of thermodynamic results from the literature for these substances is presented.