Solubility equilibria in the uric acid-ammonium urate-water system

Solubility constants reported in the literature for ammonium hydrogenurate (NH4HU), an occasional constituent of urinary calculi in humans, show a large scatter, probably due to the tendency of hydrogenurates to form supersaturated solutions that persist over long periods of time. In this study, the solubility of NH4HU(s) was measured as a function of p[H] = - lg{[H+]/mol dm(-3)} (4.5-8) and temperature (25 and 37 A degrees C) at constant ionic strength, I (c) = 0.300 mol dm(-3) NH4Cl, which is similar to the ionic strength of urine. Highly reproducible values for the solubility product (K (s0)) of NH4HU(s) were obtained. The first dissociation constant (K (1)) of uric acid determined in this work agrees well with values obtained in our previous studies of uric acid solubilities in electrolyte solutions relevant to urolithiasis. This study confirms our earlier findings that in the ionic strength range of urine, the solubilities of uric acid and hydrogen urates as a function of p[H] can be described consistently by unique values of their solubility constants (K (s), K (s0)) and the first dissociation constant of uric acid (K (1)). In addition, no appreciable complexation between uric acid or hydrogenurates and other urine constituents has been found.