Cyanides of iron

Ferrocyanide of potassium contains cyanide of potassium, K C, N, and cyanide of iron, Fe C, N they are converted by the sulphuric acid into sulphates of iron and potassa, sulphate of ammonia, and carbonic oxide thus, in regard to the cyanide of potassium,... 

Cyanide of iron—Prussian blue—may be judged of by the following —... 

In most metallic solutions the two prussiates occasion precipitates of a characteristic color, which renders them of great importance in analytical chemistry, especially in qualitetlve analyses, In these reactions, the metal in solution takes the place of the potassium, and the resulting products are a soluble salt of potassa, and a double cyanide of iron and of the metal in solution, which, being usually Insoluble, is precipitated. 

The thoory of this manufacture is as follows i—The animal charcoal consists essentially of carbon, and contains, besides, a small quantity of nitrogen when it is heated per se it undergoes no alteration, but whon mixed with potassa and heated to redness, the affinity of the alkali for cyanogen determines the formation, of the latter by means of the nitrogen and a part of the carbon contaiued in the animal charcoal. When, in addition to potassa, iron enters into the mixture, there is formed cyanide of iron, which, having a groat affinity for the cyanide of potassium, combines with it to form a double cyanide, which is the prussiate of potassa,... 

Almost always, especially when the operation is arrested a little too soon, there ie not a sufficient quantity of cyanide of iron formed to saturate the cyanide of potassium, whioh is then present in exeess, and passes into the solution. To transform this excess of cyanide into ferrocyanide, before crystallizing the solution, a quantity of dissolved protosulphate of iron is poured in until a white or bluish and persistent prcoi-... 

Complex Cyanides of Iron. Cyanide ion added to a solution of ferrous or ferric ion forms precipitates, which dissolve in excess cyanide to produce the complexes. Yellow crystals of potassium ferrocyanide, K4Fe(CN)(./3H20, are made by heating organic material, such as dried blood, with iron filings and potassium carbonate. The mass produced by the heating is extracted with warm water, and the crystals are made by evaporation of the solution. Potassium ferricyanide, K3Fe(CN), is made as red crystals by oxidation of ferrocyanide. 

Hydrocyanic acid is most easily prepared from its potassium salt, K(CN), which is obtained principally by the decomposition of the complex double cyanides of iron as we shall soon consider. The acid is also obtained by the hydrolysis of certain glucosides, e.g., amygdalin, in bitter almonds. It is prepared synthetically by reactions to be discussed presently in connection with the constitution of it and its salts. It is a colorless liquid with a characteristic odor and burns with a violet flame. It boils at 26.1 and solidifies to crystals which melt at —14°. It is an extremely strong poison the best antidotes being chlorine and hydrogen dioxide. It is readily absorbed by metallic nickel which is thus used in gas masks for this purpose. It is stable in dry air but in presence of water is readily hydrolyzed yielding ammonia and formic acid as the chief products. 

Potassium cyanide is a white deliquescent solid readily soluble in water. It is easily decomposed by boiling the water solution and yields potassium formate and ammonia. Like hydrocyanic acid it is an extremely violent poison probably due to its hydrolysis into the free acid. It also yields hydrocyanic acid by the action of carbonic acid. It is prepared commercially by the decomposition of the double cyanide of iron and potassium, potassium ferro-cyanide, K4Fe(CN)e. In recent years it has been used extensively as a solvent for gold in the recovery of this metal from low-yielding ores. 

Aside from potassium cyanide in its use for the extraction of gold the most important cyanides commercially are the double cyanides of iron and potassium. 

In Experiment 112 it was shown that the double salt formed from ammonium sulphate and ferrous sulphate broke down in solution into its constituents the solution gave the test for a ferrous salt. In the experiment described below the complex cyanide of iron and potassium is studied. 

It is best obtained by heating in a retort 1 part of finely-powdered ferrocyanide of potassium (pmssiate of potash) along with 10 parts of oil of vitriol. The salt contains ( anide of potassium, K,C,N, and cyanide of iron, Fe,C,N. One or both of these salts acts on the sulphuric acid and water, yielding sulphate of potash (or of iron), sulphate of ammonia, and carbonic oxide, K, C,N- -2(HO, S0,)- -2H0=(K0,S0,)-l-(NH,0, SO,)-f-2CO. The gas may also be obteuned by heating a mixture of a formiate, MO,C,HO with an excess... 

Bersch (1901) describes Tessie du Motay s blue as prepared from 10 parts sodium tungstate, 8 parts tin crystals , 5 parts yellow prussiate and 1 part of iron(lll) chloride these are dissolved separately and mixed. The precipitate is washed and exposed to light in thin layers, with the blue colour developing over a period of a few days. According to the discoverer, the pigment consists of a compound of tungsten oxide with a double cyanide of iron and tin . 

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