Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

Saturation molalities m(sat) in H2O(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 DsolHm(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 c and relative apparent molar enthalpies L/ for these substances. These c and L/ values were used to adjust the measured values of m(sat) and DsolHm(cal) to the standard state and thus obtain values of the standard molar Gibbs free energy DsolGm and enthalpy changes DsolHm for the dissolution reactions of these substances. The values of the pKs and of the standard molar enthalpies of the ionization reactions were also used to account for speciation of the substances in the calculations of DsolGm and DsolHm . Values of standard molar enthalpies of formation DfHm , standard molar Gibbs free energies of formation DfGm, and standard partial molar entropies S2;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.