A mixture of 26.5 g of pure anhydrous sodium carbonate and 35 g of anhydrous urea is prepared in a mortar. The mixture is then heated in a tall porcelain crucible with a burner. The mixture should be tamped down from time to time. The mixture melts partially and a large volume of ammonia escapes. It then re-solidifies and finally melts to a clear liquid. The flame is lowered and 0.5 g of urea is stirred in quickly. When the evolution of gas stops, the molten fusion is poured into a large dry mortar and ground while hot during the process of solidification. The crude product can be purified with a hot ethanol treatment.
(NH2)2CO = HOCN + NH3
2 HOCN + Na2CO3 = 2 NaOCN + H2O + CO2