Aluminum ammonium alum. Chemical symbol: NH4Al (S04)2 • 12H20. It is obtained by crystallization of concentrated solutions |ammonium sulfate with aluminum sulfate (crystalline ammonium alum). After the water has evaporated from the crystalline alum (at a temperature of 100-120 ° C) anhydrous alum is obtained. Aluminum-ammonium alum has properties and applications very similar to those of aluminum-potassium alum..