Uric acid has 4 ionizable hydrogen ions (positions 1, 3, 7, and 9). Only the hydrogen ion on position 9 (
pKa = 5.8) is ionizable at physiologic pH. Sodium urate is 15 times more soluble than uric acid. When pH equals pKa (5.8), uric acid and sodium urate are present in equal quantities. As pH increases, the ratio of sodium urate to uric acid increases. At a pH of 6.8, 10 times more sodium urate is present than uric acid; whereas, at a pH of 7.