Licence: Public Domain Mark
Credit: Qualitative chemical analysis / by C. Remigius Fresenius. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![§ M. 13. Cyanide of Potassium, KC,N = KCy [KCN]. Preparation.—Commercial ferrocyanide of potassium (perfectly free from sulphate of potassa) is gently heated, with stirring, until the water of crystallization is completely expelled; it is then powdered, and 8 parts mixed with 3 parts of perfectly dry carbonate of potassa; the mixture is fused in a covered Hessian crucible or, better still, in a covered iron pot, until the mass is in a faint glow, appears clear, and a sample of it, taken out with a heated glass or iron rod, looks perfectly white, I'he crucible is then removed from the fire, tapped gently, and allowed to cool a little until the evolution of gas has ceased; the fused cyanide of potassium is poured into a heated tall, crucible-shaped vessel of clean iron or silver, or into a moderately hot Hessian crucible, with care, so as to prevent any of the minute particles of iron which have separated in the process of fusion and have subsided to the bottom of the crucible from running out with the cyanide. The product is allowed to cool slowly in a somewhat warm place. The cyanide of potassium prepared in this way is well adapted for analytical purposes, although it contains carbonate and cyanate of potassa ; when dissolved in water, the latter is converted into carbonate of ammonia and carbonate of potassa; KO,C,NO + 4H0 = KO,CO, + NHp,CO, [2KCNO + 4H2O = K,C03 + (N H,),C03]. The cyanide should be kept in the solid form in a well-stoppered bottle, and when required for use 1 part of it is dissolved in 4 parts of water, without application of heat. The mixture of the cyanides of potassium and sodium obtained by svibstituting 2 parts of dry carbonate of soda for the 3 parts of carbonate of potassa may also be employed. Tests.—Cyanide of potassium should be milk-white and quite free from particles of iron or charcoal. It should dissolve completely in water to a clear liquid. It must be free from silicic acid and sulphide of potassium; the precipitate which salts of lead produce in its solution should therefore be white, and the residue which its solution leaves on being supersaturated with hydrochloric acid and evaporated,* should dissolve completely in water to a clear solution. If fused with pure carbonate of soda in a gentle current of dry carbonic acid in a tube of glass free from arsenic, it should not give the slightest indication of an arsenical mirror (see § 132, 12). Uses.—Cyanide of potassium prepared in the mannefr described pro- duces in the solutions of most of the salts of metallic oxides precipitates of cyanides of the metals, or of oxides or carbonates. The precipitated cyanides are soluble in cyanide of potassium, and may therefore by further addition of the i-eagent be separated from the oxides or carbonates which are insoluble in cyanide of potassium. Some of the metallic cyanides redissolve invariably in the cyanide of potassium as double cyanides, even in presence of free hydrocyanic acid and on boiling; whilst others combine with cyanogen to form new radicles, which remain in solution in combination with the potassium. The most common compounds of the latter kind are cobalticyanide of potassium and ferro- and ferricyanide of potassium. These differ from * This snpersaturation with hydrochloric acid is attended with disengagement of hydrocyanic acid.](https://iiif.wellcomecollection.org/image/b21966953_0088.jp2/full/800%2C/0/default.jpg)
