Purified Sodium Hydroxide: Caustic Soda Dissolved in Alcohol, Filtered and Dried in Silver

Commercial caustic soda carries alumina, carbonates, chlorides, phosphates, silicates and sulphates. For analysis, the British Pharmacopoeia of 1898 (quoted in the 1907 US Dispensatory) purified it with alcohol: sodium hydroxide dissolves in alcohol, most of those salts do not, so they are filtered off and the alcohol is evaporated in a silver dish. The result is free of phosphates and sulphates, nearly free of carbonate, but not quite free of alumina. The source gives no quantities, so none are given here. The picture: sodium hydroxide pellets (Walkerma, Wikimedia Commons, public domain).