Preparation of sodium tetrathiophosphate octahydrate
80g of sodium sulfide 9-hydrate are melted in a suitable evaporating dish over a small flame. Under constant stirring, 8g of powdered phosphorus pentasulfide are sifted into the molten salt under the hood. After 10-20 minutes, during which the dark grey-brown fusion is continuously stirred and gently heated, the phosphorus pentasulfide has dissolved. 80 ml of water at 70-80°C are added and the hot solution is rapidly filtered by suction through a steam-jacketed Buchner funnel. The filtrate is allowed to stand in a stoppered flask cooled in ice for 30-45 minutes and the crude crystalline product is filtered off. In order to obtain pure sodium tetrathiophosphate the solid is rapidly dissolved below 10°C in five times its weight of 2% aqueous sodium sulfide containing 1ml of 2N sodium hydroxide. The crystals are washed with 100ml of ice-cold 1:1 alcohol- water, followed by 95% alcohol and by acetone; the product is briefly air-dried. An equal volume of ice-cold 95% alcohol is added during the cooling. After standing in ice until precipitation is complete (about 10 minutes), the salt is immediately suction-filtered and washed, first with cold 50% alcohol, then with 95% alcohol, followed by ether. The colorless crystals are dried briefly in air and preserved in a tightly stoppered bottle to prevent hydrolysis due to atmospheric moisture. Yield 50%.
Inorganic laboratory preparations, by G. G. Schlessinger, 69-70, 1962