Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown of purine nucleotides, and it is a normal component of urine. High blood concentrations of uric acid can lead to gout and are associated with other medical conditions, including diabetes and the formation of ammonium acid urate kidney stones. Chemistry Uric acid was first isolated from kidney stones in 1776 by Swedish chemist Carl Wilhelm Scheele. In 1882, the Ukrainian chemist Ivan Horbaczewski first synthesized uric acid by melting urea with glycine. Uric acid displays lactam–lactim tautomerism (also often described as keto–enol tautomerism). Although the lactim form is expected to possess some degree of aromaticity, uric acid crystallizes in the lactam form, with computational chemistry also indicating that tautomer to be the most stable. Uric acid is a diprotic acid with pKa1 = 5.4 and pKa2 = 10.3, thus at physiological pH, it predominately exists as the monoionic urate ion. {| cellpadding="5" border="0" style="text-align:center; float: center; background: transparent; border: 1px solid darkgray;" |+ Tautomers of uric acid and urate |- | || || || pKa1 || |- | Lactam form || || Lactim form || || Urate ion |} Water solubility In general, the water solubility of uric acid and its alkali metal and alkaline earth salts is rather low. All these salts exhibit greater solubility in hot water than cold, allowing for easy recrystallization. This low solubility is significant for the etiology of gout. The solubility of the acid and its salts in ethanol is very low or negligible. In ethanol/water mixtures, the solubilities are somewhere between the end values for pure ethanol and pure water. {| class="wikitable sortable" |+ Solubility of urate salts (grams of water per gram of compound) ! data-sort-type="text" | Compound ! data-sort-type="number" | Cold water ! data-sort-type="number" | Boiling water |- | Uric acid | align=right | 15,000 | align=right | 2,000 |- | Ammonium hydrogen urate | align=center | — | align=right | 1,600 |- | Lithium hydrogen urate | align=right | 370 | align=right | 39 |- | Sodium hydrogen urate | align=right | 1,175 | align=right | 124 |- | Potassium hydrogen urate | align=right | 790 | align=right | 75 |- | Magnesium dihydrogen diurate | align=right | 3,750