Bichromate is largely used in calico printing and dyeing, for the raising of chrome oranges and other chrome shades, the fixing of catechu, and the raising of steam blues and greens. Its chief virtue lies in the readiness with which it parts with oxygen. Care must be taken not to employ bichrome too freely, as the chromic acid seems capable of forming some new compound with the oxide which fixes itself upon the cloth. If applied in too large a quantity, the cloth is liable to be injured by chromic acid.
The quality of this salt may be judged of to a certain extent by its appearance. If it is in good crystals of a uniform red colour, without any admixture of soft yellow crystals, it is good. A somewhat rough test consists in dissolving a known weight of pure metallic tin and ascertaining how much chromate is necessary to peroxidize it. A more accurate method is to reduce the chromic acid present in the sample into a salt of sesquioxide, by alcohol and hydrochloric acid, precipitate by ammonia, and determine the amount of chromic acid from the weight of oxide of chromium obtained. A good sample of bichrome should show 61 per cent. of chromic acid.
Cyanide of Potassium. (Fa., Cyanure de Potassium ; GER., Cyanhalium.) Formula KCN.—This salt takes the form of a white, opaque solid, with crystalline fracture, or of cubic crystals. The crystals are deliquescent, exceedingly soluble in water, and intensely poisonous Soluble in boiling alcohol, the cyanide separates out again from the cooled solution. The aqueous preparation has an alkaline reaction, and is decomposed when boiled into ammonia and formiate of potassium. It is decomposed also by the feeblest acid—even by the carbonic acid of the air—exhaling an odour of hydrocyanic acid. It is readily fusible at a low red heat, and forms a series of double salts with certain metals, which are exceedingly useful in the arts.
Cyanide of potassium may be prepared in many ways; (1) by heating potassium in cyanogen gas, or vapour of hydrocyanic acid ; (2) by transmitting pure nitrogen gas through a white-hot tube containing a mixture of carbonate of baryta or potassium, and charcoal ; (3) by heating to redness nitrogenous organic matter—horn•shavings, hide-parings, &c.—with carbonate of potassium ; (4) by passing the vapour of hydrocyanic acid into a cold alcoholic solution of hydrate of potassium, and pressing and drying the deposited crystalline salt ; (5) by heating to whiteness carefully dried ferrocyanide of potassium in a nearly-closed iron retort; nitrogen and other gases are evolved and a mixture of carbon, carbide of iron, and cyanide of potassium left; thus :— = 4BCN + The hest process of manufacture upon a large scale, and that usually adopted, is as follows : Eight parts of ferrocyanide of potassium are gently dried and mixed with three parts of dry carbonate of potassium of good quality. The mixture is fused at a low red heat in an iron
or earthenware pot, the heat being kept up, and the mass well agitated, until all evolution of gas ceases, and a sample taken out upon an iron rod solidifies to a colourless, opaque solid. The pot is left to settle for a short time until all the sediment, consisting principally of finely divided metallic iron, is deposited, and then the clear salt is decanted off and poured into moulds to solidify. In this process, 2 equivalents of ferroeyanide and an equal amount of carbonate of potassium yield 5 equivalents of cyanide, 1 equivalent of cyanate of potassium, 2 equivalents of iron, and 2 equivalents of carbon dioxide. Carbonate of soda may be substituted for carbonate of potassium.
Cyanide of potassium is extensively used for photographic purposes, also in eleetro-gilding and plating ; very occasionally it forms a potent reducing agent.
Ferricyanide of Potassium. (FR., Prussiate rouge de Potasse ; GEE., Ferrieleyankalium, or BotheAlut lamions(dz.) Formula, Synonym, red prussiate of potash.—This salt forms fine an hydrous crystals belonging to the monoclinic system, of a deep-red colour. They have a specific The solubility of the salt increases rapidly with an increase of temperature. Thus 100 parts of water dissolve— At 4.1° .. 33.0 parts of ferricyanide. At 37.8 .. 58.8 parts of ferricyanide.
„ .. 36.6 „ „ 100.0 .. 77.5 „ „ 15.6 .. „ " 100.4 .. , The usual method of preparation is to pass chlorine gas through a solution of the ferrocyanide, or the same salt in a powdered state, until it no longer gives a precipitate of pruesian blue with a permit of iron.