Ammonium sulfate forms many double salts (ammonium metal sulfates) when its solution is mixed with equimolar solutions of metal sulfates and the solution is slowly evaporated. With trivalent metal ions, alums such as ferric ammonium sulfate are formed. Double metal sulfates include ammonium cobaltous sulfate, ferrous diammonium sulfate, ammonium nickel sulfate which are known as Tutton's salts and ammonium ceric sulfate. Anhydrous double sulfates of ammonium also occur in the Langbeinites family. The ammonia produced has a pungent smell and is toxic.