Preparation of sodium monothioarsenate dodecahydrate
20 g of arsenic trioxide are dissolved in a solution of 24g of sodium hydroxide in 100ml of water. 6.5g of finely powdered roll sulfur are added and the mixture is boiled gently under reflux (or in an open vessel if water is added from time to time to replace that lost by evaporation) until virtually all the sulfur has dissolved (about 2.5 hours). The clear filtered solution is evaporated on the steam bath to crystallization and several batches of crystals are isolated by cooling and filtering without washing. The yield of product depends entirely on the extent of the evaporation and may be almost quantitative. The product is colorless, but turns pale yellow on standing. It melts at 46 °C and is somewhat efflorescent.
Inorganic laboratory preparations, by G. G. Schlessinger, 62, 1962