The elements of cyanogen, combined with sulphur, form a salt radical, sulphocyanogen, CNS, which is expressed by the symbol Csy. The sulphocyanide of potassium, KCsy, is prepared by fusing ferrocyanide of potassium, deprived of its water of crystallization, intimately mixed with half its weight of sulphur and 17 parts of carbonate of potassa. The molten mass, after having cooled, is exhausted with water, the solution evaporated to dryness, and extracted with alcohol, from which the crystals of the salt are separated by evaporation.
It is also made by melting the ferrocyanide of potassium with sulphide of potassium. It is a white, crystallizable salt of a taste resembling that of niter, soluble in water and alcohol, and extremely poisonous. It dissolves the chlorides, iodides, and bromides of silver, is, therefore, a fixing agent, but has not come in general use as such. Vogel speaks highly of it as an addition to the positive toning bath, although he prefers the analogous ammonium salt in the following formula:
Chloride of gold solution.... (1:50) 3 c. cm. (46-1/5 grains). Sulphocyanide of ammonium ... 20 grammes (308 grains). Water........100 c. cm. (3 ounces 5 drachms 40 grains).
Ferrocyanide of Potassium - KCfy+3HO, or KCNFe+3HO, is generally known by the name of yellow prussiate of potassa. It contains ferrocyanogen, a compound radical, consisting of 1 eq. of metallic iron and 3 eq. of the elements of cyanogen, and is designated by the symbol Cfy.
The potassium salt is manufactured on a large scale from refuse animal matter, as old leather, chips of horn, woolen rags, hoofs, blood (hence its German name, "Blutlaugen salz"), greaves, and other substances rich in nitrogen, by fusing them with crude carbonate of potassa and iron scraps or filings to a red heat, the operation to go on in an iron pot or shell, with the exclusion of all air. Cyanide of potassium is generated in large quantities. The melted mass is afterward treated with hot water, which dissolves the cyanide and other salts, the cyanide being then quickly converted by the action of oxide of iron, formed during the operation of fusing, into ferrocyanide. The filtered solution is evaporated, crystallized, and recrystallized. The best temperature for making the solution is between 158 and 176 deg. F. The conversion of the cyanide into the ferrocyanide is greatly facilitated by the presence of finely divided sulphuret of iron and caustic potash. Some years ago this salt was manufactured by a process which dispensed with the use of animal matter, the necessary nitrogen being obtained by a current of atmospheric air.
Fragments of charcoal, impregnated with carbonate of potassa, were exposed to a white heat in a clay cylinder, through which a current of air was drawn by a suction pump. The process succeeded in a chemical sense, but failed on the score of economy.
Richard Brunquell passes ammonia through tubes filled with charcoal, and heated to redness so as to form cyanide of ammonium, which is converted into the ferrocyanide of potassium by contact with potash solution and suitable iron compounds. Ferrocyanide of potassium is in large beautiful transparent four-sided tabular crystals, of a lemon-yellow color, soluble in four parts of cold and two of boiling water, insoluble in alcohol. Exposed to heat it loses three eq. of water, and becomes anhydrous; at a high temperature it yields cyanide of potassium, carbide of iron, and various gases. This salt is said to have no poisonous properties, although the dangerous hydrocyanic acid is made from it. In large doses it occasions, however, vertigo, numbness, and coldness. It is used in various photographic processes. Newton employs it in combination with pyrogallol and soda in the development of bromo-gelatine plates.
The ferri or ferrid cyanide of potassium discovered by Gmelin is often, but improperly, termed red prussiate of potash. It is formed by passing a current of chlorine gas through a solution of ferrocyanide of potassium until the liquid ceases to give a precipitate with a salt of sesquioxide of iron, and acquires a deep, reddish-green color. The solution is then evaporated, crystallized, and recrystallized. It forms regular prismatic or tabular crystals, of a beautiful ruby-red tint, permanent in the air, soluble in four parts of cold water. The crystals burn when introduced into the flame of a candle, and emit sparks.
The theory of the formation of this salt is, that one eq. of chlorine withdraws from two eq. of the ferrocyanide of potassium, one eq. of potassium, forming chloride of potassium, which remains in the mother liquid. The reaction is explained by the following equation: 2(KCfy)+Cl=KCfy+KCl.
The radical ferridcyanogen, isomeric2 with ferrocyanogen, is supposed to be formed by the coalescence of two equivalents of ferrocyanogen, and is represented by the symbol Cfdy; accordingly the formula of ferridcyanide of potassium is KCfdy.
Ferridcyanide of potassium has found extensive application in photographic processes for intensifying negatives; those of Eder, in combination with nitrate of lead, or Selle's, with nitrate of uranium; Ander's blue intensification of gelatine negatives, Farmer's process of reducing intensity, the coloring of diapositives, the very important blue printing, and various others, are daily practiced in our laboratories.
The ferrocyanide of potassium is a chemical reagent of great value, giving rise to precipitates with the neutral or slightly acid solutions of metals, like the beautiful brown ferrocyanide of copper, and that of lead. When a ferrocyanide is added to a solution of a sesquioxide of iron, Prussian blue or ferrocyanide of iron is produced. The exact composition of this remarkable substance is not distinctly stated, as various blue compounds may be precipitated under different circumstances. Berzelius gives the following account: 3 eq. of ferrocyanide and 2 eq. of sesquioxide of iron are mutually decomposed, forming 1 eq. of Prussian blue and 6 eq. of the potassa salt, which remains in solution, or 3KCfy + 2(FeO3NO) = FeCfy + 6(KO,NO). It forms a bulky precipitate of an intense blue, is quite insoluble in water or weak acids, with the exception of oxalic acid, with which it gives a deep blue liquid, occasionally used as blue ink.
Ferridcyanide of potassium, added to a salt of the sesquioxide of iron, yields no precipitate, but merely darkens the reddish-brown solution; with protoxide of iron it gives a blue precipitate, containing FeCfdy, which is of a brighter tint than that of Prussian blue, and is known by the name of Turnbull's blue. Hence, the ferridcyanide of potassium is as excellent a test for protoxide of iron as the yellow ferrocyanide is for the sesquioxide. - E., Photo. Times.Isomeric bodies, or substances different in properties yet identical in composition, are of constant occurrence in organic chemistry, and stand among its most peculiar features.