Preparation of alloxan (mesoxalylurea; 2,4,5,6(1H,3H)-pyrimidinetetrone)
5 g of finely powdered alloxantin are added to a mixture of 3.5 ml of conc. nitric acid (d=1.4) and 7 ml of fuming nitric acid (d=1.5). The reaction mixture is kept for about 48 hours. Slight evolution of nitrous fumes occurs, and the alloxantin, which first remains at the bottom of the vessel, slowly changes into the more bulky crystals of alloxan, which gradually fill the liquid. The reaction is complete when a sample of the product dissolves readily in cold water. The crystalline mass is filtered, thoroughly washed in order to remove nitric acid and well dried in the air. Crude alloxan is purified by dissolving the product in the smallest quantity of hot water and allowing the solution to evaporate slowly in a desiccator over sulfuric acid. The crystals are liable to efflorescence in the air. Alloxan contains 4 molecules of water of crystallization.
A Class-Book of Organic Chemistry, by J. B. Cohen, 54, 1919