Precipitation diagrams and solubility of uric acid dihydrate
1987; Elsevier BV; Volume: 83; Issue: 4 Linguagem: Inglês
10.1016/0022-0248(87)90253-3
ISSN1873-5002
AutoresVesna Babić-Ivančić, H. Füredi‐Milhofer, W. E. Brown, Thomas Grégory,
Tópico(s)Freezing and Crystallization Processes
ResumoThe solubility of uric acid dihydrate (UA·2H2O) and the precipitation of UA·2H2O and anhydrous uric acid (UA) from solutions containing sodium hydroxide and hydrochloric acid have been investigated. For the solubility studies, crystals of pure UA·2H2O were prepared and equilibrated with water and with solutions of HCl or NaOH for 60 min or 20 h, respectively. The equilibrium pH (pH = 2–6.25) and uric acid concentration were determined. For the precipitation experiments, commercial UA was dissolved in NaOH in a 1:1.1 molar ratio and UA·2H2O and/or UA were precipitated with hydrochloric acid. The precipitates and/or supernatants were examined 24 h after sample preparation. The results are represented in the form of tables, precipitation diagrams and “chemical potential” diagrams. Solubility measurements with 60 min equilibration times yielded the solubility products of UA·2H2O, Ksp(298 K) = (0.926 ± 0.025) × 10-9 mol2 dm-6 and Ksp(310 K) = (2.25 ± 0.05) × 10-9 mol2 dm-6 and the first dissociation constants of uric acid, K1(298 K) = (2.45 ± 0.07) × 10-6 mol dm-3 and K1(310 K) = (3.63 ± 0.08) × 10-6 mol dm-3. Precipitation diagrams show that under the given experimental conditions, at 298 K, UA·2H2O is stable for 24 h while at 310 K this was true only for precipitates formed from solutions of high supersaturations. At lower supersaturations, mixtures of UA·2H2O and UA formed. Consequently, while the Ksp value determined from precipitation data obtained at 298 K (Ksp = 1.04 × 10-9 mol2 dm-6) was consistent with the respective solubility product, the 310 K precipitation boundary yielded an ion activity product, AP, the value of which fulfills the conditions Ksp(UA) < AP < Ksp (UA·2H2O). Similar ion activity products were obtained from solubility measurements in pure water at 20 h equilibration time.
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