Potassium Ferrocyanide Sulphide

Aurous cyanide forms yellow, microscopic laminae, very slightly soluble in Water. It is more stable than aurous iodide, but at red heat is decomposed into gold and cyanogen. Its insolubility renders it immune to the action of dilute acids and hydrogen sulphide, but solutions of ammonia, potassium hydroxide, ammonium sulphide, and sodium thiosulphate dissolve it, probably forming complex derivatives. In aurous cyanide the tendency to form complex compounds is much more marked than in the corresponding chloride, bromide, and iodide.3 Its interaction with potassium ferrocyanide has been studied by Beutel.4... 

When heated, sodium ferric sulphide is converted into oxide. With concentrated hydrochloric acid it yields hydrogen sulphide, free sulphur being deposited. It is insoluble in water but, when boiled with an aqueous solution of potassium cyanide, it yields potassium ferrocyanide. 

The old commercial method of preparing the salt lay in heating nitrogenous material, such as horn, wool, feathers, blood, etc., with potash and iron turnings. The mass was ultimately heated to fusion to complete the reaction, cooled, and extracted with boiling water. The solution contained potassium ferrocyanide, thiocyanate, carbonate, and sulphide. The first-named was crystallised out, but the yield was seldom more than 20 per cent, of the quantity theoretically obtainable from the nitrogen content of the organic material consumed. 

Alkali thiocyanates may be made the starting-point for the preparation of ferrocyanides. The potassium salt is mixed with twice the weight of iron filings necessary to form ferrous sulphide, and with double the quantity of ferrous hydroxide, m a freshly precipitated condition, to form ferrocyanide. The mixture is maintained for some twelve hours under agitation in a closed vessel at 110° to 120° C. Potassium ferrocyanide is formed, together with Prussian blue, and extracted from the residue with water.3... 

From 1885 to 1895 potassium ferrocyanide w as manufactured very largely from spent oxide of iron, used in purifying coal gas from hydrogen sulphide. Coal gas, as it leaves the retorts, contains hydrogen cyanide, formed by the action of ammonia on red-hot carbon. Thus —... 

The crude coal gas is washed with ferrous sulphate solution, whereby the latter is converted into a suspension of ferrous sulphide in ammonium sulphate solution. This reacts with the ammonium cyanide, yielding ferrous ferrocyanide, Fe2[Fe(CN)6], or ammonium ferrous ferrocyanide, (NH4)2Fe[Fe(CN)6], according to circumstances. Potassium ferrocyanide may be obtained from these by treatment with lime, as m the spent oxide process. By repeatedly dissolving in water and precipitating with alcohol, the salt can be obtained in a very pure state.1... 

Solutions of potassium ferrocyanide are gradually decomposed by light, ferric hydroxide being precipitated.8 Addition of an alkali sulphide to a fresh solution of the ferrocyanide effects the gradual precipitation of ferrous sulphide in sunlight 9 but not in the dark. The reason appears 10 to be as follows —... 

Vanadium compounds (vanadic acid and vanadium chloride) have been proposed as substitutes for copper sulphide, but without much practical success. Potassium ferrocyanide and ferricyanide are also used to a certain extent. A mixture of these salts with aniline salt and potassium chlorate is printed, and the goods aged. The probable action is that the ferricyanide oxidises the aniline, and is continually regenerated from the ferrocyanide formed by the chloric acid present. Thus these salts play the part of oxygen carriers in a similar manner to the copper and vanadium compounds. In the opinion of technologists, the hlack produced by this process differs somewhat in its properties from that obtained with copper, but this may be ascribed to the presence of prussiaii blue in the former. In place of the aniline hydrochloride and potassium chlorate, a mixture of aniline sulphate and barium chlorate has recently been employed in black-printing. 

Copper. Hydrogen sulphide precipitates black copper sulphide in presence of dilute hydrochloric acid. Salts give green flame. Potassium ferrocyanide gives brown precipitate of copper ferrocyanide in presence of acetic acid. 

In qualitative analysis copper is detected by precipitation as cupric sulphide from hydrochloric-acid solutions of its salts. To prevent the formation of a colloidal precipitate, the solution should be hot, and should contain excess of the acid. The sulphide is soluble in hot, dilute nitric acid, and in potassium-cyanide solution, but almost insoluble in solutions of alkali-metal sulphides. It dissolves to some extent in ammonium-sulphide solution. Other aids in the detection of copper are the blue colour of solutions of cupric-ammonia salts the reddish-brown precipitate of cupric ferrocyanide, produced by addition of potassium ferro-cyanide to cupric solutions the formation of an intense purple coloration by the interaction of hydrogen bromide and cupric salts, a very delicate reaction2 the formation of a bluish-green borax bead and the ready isolation of the metal from its compounds by the action of reducers. 

Potassium aquo ferrocyanide, K3Fe(CN)5.H20, is obtained by reducing potassium aquo penta-ferricyanide, K2Fe(CN)6.H20, with alkaline hydroxylamine chloride, with alkali sulphide, or with formaldehyde and acetone in the presence of alkali.1... 

The carbonates, sulphates, and borates are decomposed. The sulphides of the alkalies and alkaline earths are decomposed while the sulphides of arsenic, antimony, molybdenum, zinc, cadmium, tin, iron, lead, copper, mercury, and palladium are not attacked. Cobalt sulphate is not attacked, while the sulphates of the alkalies and alkaline earths are attacked and dissolved. Alkali tungstates, ammonium arsenite and arsenate, copper arsenite, ammonium magnesium arsenate, ammonium molybdate and vanadate, potassium cyanide and ferrocyanide are decomposed. Paraffin is not attacked shellac, gum arabic, gum tragacanth, copal, etc., are decomposed. Celluloid is slowly attacked. Silk paper, gun cotton, gelatin, parchment are dissolved. M. Meslans 22 has studied the esterification of alcohol by hydrofluoric acid. 

Ferrocyanides are easily produced when solutions of potassium or sodium cyanides are brought into contact with ferrous hydrate or ferrous sulphide —... 

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