Solid-State Dehydration of Uric Acid Dihydrate

Abstract

Uric acid dihydrate (UAD) is a crystalline constituent present in a significant fraction of human renal precipitates. Using a combination of techniques including powder X-ray diffraction, hot-stage light microscopy, differential scanning calorimetry, and thermogravimetric analysis, the mechanism and kinetics of its irreversible dehydration to polycrystalline anhydrous uric acid (UA) is analyzed as a function of intrinsic sample parameters (e.g., crystal size, shape, structure) and environmental conditions (e.g., temperature and humidity). The highly anisotropic dehydration of UAD to UA is rationalized on the basis of its crystal structure and morphology, and appears to involve no other crystalline intermediate phases. Mechanistic models derived from both isothermal and nonisothermal kinetic data support a one-dimensional-phase boundary mechanism. Increasing relative humidity conditions were found to decrease the dehydration kinetics up to a point, after which a dissolution−recrystallization mechanism is enabled, and the conversion of UAD to UA is significantly accelerated.