Preparation of potassium hexathiocyanatochromate(III)
Twelve grams of potassium thiocyanate are heated very gently in an evaporating dish until the salt just begins to melt, around 170°C. Gentle heating is continued while 10g of finely powdered potassium chrome alum is sifted onto the fused salt in about 1-gram portions from a spatula. The mixture is stirred well after each addition and the evolution of steam is allowed to subside before more alum is added. The reddish-violet reaction mixture, which is almost solid towards the end of the fusion, is finally allowed to cool in a desiccator to avoid deliquescence. The cold melt is quickly scraped into 75ml of 95% ethanol contained in a mortar, and thoroughly triturated with the liquid. The residue of potassium sulfate is removed by suction-filtration and washed with 10ml of alcohol. The combined filtrate and washing are evaporated on the steam bath almost to dryness and the semi-solid residue is dried in vacuo. The crude material is recrystallized by dissolving it in the minimum of absolute methanol (filtering if necessary), and allowing the solution to evaporate spontaneously at atmospheric pressure over concentrated sulfuric acid. The purple-red product (4-hydrate) is stored in dark-colored, tightly sealed vessels because it is very deliquescent and photo-sensitive, especially when hydrated. Yield = 8g (96% based on the alum)
Twenty grams of chromium nitrate 9-hydrate, or 0.05 moles of the hydrated sulfate or chloride, are fused with 30g of potassium thiocyanate as in the procedure described above, and the melt al lowed to cool in a desiccator and then triturated with 25ml portions of ethyl acetate until no more material dissolves. The violet liquid is filtered; a small green residue remains behind together with the potassium nitrate, sulfate, or chloride. The ethyl acetate extract is evaporated to dryness on the steam bath and the violet residue is further dried in vacuo. The anhydrous salt is violet. Yield (based on the chromium salt) 22g.
Inorganic laboratory preparations, by G. G. Schlessinger, 89-91, 1962