Preparation of potassium octacyanomolybdate(IV) dihydrate
Eighty-three grams of potassium molybdate are dissolved in 100ml of water; or 50g of pure molybdenum (VI) oxide are treated with a solution of 40g of potassium hydroxide in 100ml of water. Two hundred and fifty milliliters of concentrated hydrochloric acid are now added slowly from a dropping funnel with constant stirring and the entire mixture is heated on the steam bath while a solution of 150g of potassium thiocyanate in 150ml water is slowly stirred into the hot liquid. The resulting blood-red solution is now heated on the steam bath in the hood for two hours with frequent stirring. The liquid is filtered by suction while hot to remove a black by-product and cooled in an ice bath while 70ml of pyridine are added slowly (with good stirring) from a separatory funnel. A red oily layer precipitates which soon becomes very viscous in the ice bath. The supernatant liquid, which is still pale red in color, is decanted and the oil is rinsed twice with two 50ml portions of ice-cold water. A solution of 200g of potassium cyanide in 300ml of water is added to the oil obtained in the preceding step. As poisonous cyanotic gases are evolved, this must be done in a well-ventilated hood. The oil dissolves rapidly to give first a green and then a yellow-brown solution which is heated on a steam bath for one-half an hour with occasional stirring. It is then suction-filtered (hood) to remove a black material. The filtrate is evaporated on the water bath to one-half its original volume, cooled in ice, and filtered. The filtrate may be reserved for further treatment by purification; otherwise it is discarded. The yellowish-black crystalline solid obtained in the preceding step is dissolved in the minimum amount of boil ing water needed; a black impurity remains behind. Five grams of decolorizing charcoal are stirred in for about five minutes and the hot material is suction-filtered and cooled in ice. About two volumes of ethanol are added and crystalliza tion is completed by allowing the mixture to stand in ice for about thirty minutes. The golden-yellow crystals are filtered off, washed with two 50ml portions of alcohol and a similar quantity of ether, and dried in air. Yield about 80g.
Inorganic laboratory preparations, by G. G. Schlessinger, 91, 1962