Dissolution enthalpies of uric acid and uric acid dihydrate

The dissolution enthalpies of uric acid and its dihydrate in Tris–buffer solutions were measured calorimetrically for the first time. It was found that these quantities do not depend significantly on pH (7.2–8.9), temperature (t=25–37°C), ionic strength (I=0.010–0.045 mol dm−3) and uric acid 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), ΔrH=(56.3±0.4) kJ mol−1 H2U·2H2O(s)→H+(aq)+HU−(aq)+2H2O(aq), ΔrH=(64.5±0.2) kJ mol−1 are consistent with those obtained from the temperature dependence of the solubility equilibrium constants reported in our previous work.