Three conditions influence the potential for uric acid stone formation : the quantitative excretion of uric acid , the volume of urine as it affects the urinary concentration of uric acid, and the urinary pH which seems to be the most important pathogenic factor . Uric acid is a weak acid with a pKa of 5.35 at 37° C . At that pH , half of the uric acid is present as the urate salt and half as free uric acid . Because sodium urate is approximately 20 times more soluble than the free acid, the relative proportion present as free uric acid strongly determinates the risk of stone formation . Low urinary pH increases concentrations of sparingly soluble undissociated uric acid, which leads to direct precipitation of uric acid .