Dissolution enthalpies of uric acid and uric acid dihydrate

The dissolution enthalpies of uric acid and its dihydrate in Tris-buffer so lutions were measured calorimetrically for the first time. It was found tha t these quantities do not depend significantly on pH (7.2-8.9), temperature (t=25-37 degrees C), ionic strength (I=0.010-0.045 mol dm(-3)) and uric ac id concentration (c=3.4-8.5 mmol dm(-3)). After subtracting the protonation enthalpy of Tris, the present results H2U(s) --> H+(aq) + HU-(aq), Delta(r)H = (56.3 +/- 0.4) kJ mol(-1) H2U . 2H(2)O(s) --> H+(aq) + HU-(aq) + 2H(2)O(aq), Delta(r)H = (64.5 +/- 0. 2) kJ mol(-1) are consistent with those obtained from the temperature dependence of the s olubility equilibrium constants reported in our previous work. (C) 2000 Els evier Science B.V. All rights reserved.