Exercise 58. Sodium Nitroprusside

Put 20 g. of potassium ferrocyanide in a 250-cc. Erlenmeyer flask, adding 30 cc. of water. Place the flask on the water bath and heat till the ferrocyanide dissolves. Add 35 cc. of nitric acid (sp. gr., 1.24) and continue heating until a test portion shows no blue color but a dark green precipitate when treated with ferrous sulfate. Let stand for a day or two and filter off the tarry impurities. Now neutralize with sodium carbonate, taking care that no excess is added. Warm and, when cool, add from one to two times the volume of alcohol (ethyl). Set the flask in a dry place for a day or two, after which the impurities (chiefly crystals of potassium nitrate) may be filtered off, and the nitro-prusside crystallized out. Sometimes two or three treatments with alcohol are necessary. In order to expedite the operation of evaporating down to crystallization, the solution may be evaporated by heating in a suction flask under reduced pressure. Quick crystallization increases the yield. The yield should be above 75 per cent. 

NaAg3(S203)2, formed in developing photographic-plates. 

K4Fe(C204)2, used as a reducing agent in photography. 

K3Co(N02)6, the precipitate obtained with DeKoenig s solution. 

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