In a 500-ml. three-necked flask, provided with a stirrer, are placed 15 g. (0.09 mole) of finely powdered uric acid, 30 g. (25.2 ml.) of concentrated hydrochloric acid, and 40 ml. of water. The mixture is warmed to 30°, the stirrer is started, and 4 g. (0.014 mole) of finely powdered potassium chlorate is added in small portions during a period of not less than 45 minutes (Note 1). Most of the uric acid dissolves; any undissolved material is removed by filtration through a fritted-glass filter. The clear filtrate is diluted with 30 ml. of water, and a rapid stream of hydrogen sulfide is led into it until it is saturated (about 10–15 minutes). Sulfur and alloxantin separate, and the mixture is cooled for 2–3 hours in an ice bath until the separation is complete.

The solid is collected on a Büchner funnel and washed with three 30-ml. portions of cold water. The alloxantin is dissolved by boiling the wet solid for 15 minutes with 250 ml. of water, and the hot solution is filtered to remove the sulfur (Note 2). Alloxantin dihydrate crystallizes from the filtrate in glistening plates which should be pressed as dry as possible on a Büchner filter, washed with about 30 ml. of ether, and dried in a vacuum desiccator (Note 3). The yield is 8–10 g. (55–69%) (Note 4). The product melts with decomposition at 234–238° (Note 5), and is pure enough for most purposes.