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.
89/526 page 65
![•'§§ 55, 50.] tlie double cyanides particularly in that dilute acids fail to precipitate the metallic cyanides which they contain. Cyanide of potassium may iiccorchno-ly be used to separate the metals which form compound- radicles,1from others the cyanides of which are precipitated on adding an acid to their solution in cyanide of potassium. In the course of analysis, this reagent is of great importance, as it serves to separate cobalt from nickel; also copper from cadmium, the sulphide of the former being soluble in it, whilst that of the latter is insoluble. § 55. 14. Ferrocyanide of Potassium, 2E:,C3N3Fe = 2K,Cfy [K,Fe (CN),]. Crystallized, 2K,Cfy + 3aq [K,Cfy,3KO]. Preparation.—Commercial ferrocyanide of potassium is sufliciently pure. 1 part of the salt is dissolved in 12 parts of water for use. Uses.—^With most metals, ferrocyanogen forms compounds insoluble in water, which frequently exhibit highly characteristic colours. These ferrocyanides are formed when ferrocyanide of potassium is brought into contact with soluble salts of metallic oxides, with chlorides, &c., the potassium changing places with the metals. Ferrocyanide of copper and ferrocyanide of iron exhibit the most characteristic colours of all: ferrocyanide of potassium serves therefore particularly as a test for ■oxide of copper and ferric oxide. § 56. 16. Ferricyanide of Potassium, 3K,C,2NeFe,= 3K,Cfdy [K,Fe,(CN),J. Preparation.—G\AoY\nB gas is passed slowly into a solution of 1 part •of ferrocyanide of potassium in 10 parts of watei-, with frequent stirring, until the solution exhibits a fine deep red colour by transmitted light (a candle-Hght is best), and a sample of the solution no longer yields a blue precipitate with a solution of ferric chloride, but imparts a brownish tint to it. The solution is then concentrated in an evaporating dish to \ of its weight, and put aside to crystallize. On further evaporation, the mother liquor will yield a second crop of crystals. The whole of the crystals obtained are dissolved in 3 parts of water, filtered if necessary, the solution evaporated quickly to half its volume, and again allowed to crystallize. Whenever required for use, a few of the crystals, which are of a splendid red colour, are dissolved in a little water. The solution, if exposed to light, is decomposed, ferrocyanide of potassium is formed and a blue precipitate is deposited; as already observed, the solution should not give either a blue precipitate or a blue colour with a solution of ferric chloride. Uses.—Ferricyanide of potassium reacts with solutions of metallic oxides in the same manner as ferrocyanide of potassium. Of the metallic ferricyanides, ferrous ferricyanide is more particularly characterized by its colour, and ferricyanide of potassium therefore is used principally as a test for ferrous oxide. <2UAL. p](https://iiif.wellcomecollection.org/image/b21966953_0089.jp2/full/800%2C/0/default.jpg)
