752

POTASSIC FERROCYANIDE.

flakes. Potassic ferrocyanide is not poisonous; it has a saline bitterish taste. When heated to 212° it loses the whole of its water of crystallization, and crumbles to a yellowish-white powder. When heated to redness in closed vessels it fuses, and at a full and prolonged red heat undergoes partial decomposition, nitrogen being evolved, whilst potassic cyanide and carbide of iron are obtained. When dried and heated with black oxide of manganese, potassic cyanate is formed (504).

Potassic ferrocyanide, when distilled with sulphuric acid, furnishes abundance of hydrocyanic acid; the decomposition which attends the reaction has been already explained (note, Part II. p. 293). When heated with concentrated oil of vitriol, the salt is decomposed into carbonic oxide, and a mixture of potassic, ammonic, and ferrous sulphates (357). When fused with dry potassic carbonate the ferrocyanide furnishes a convenient source of cyanide of potassium (1571). Potassic ferrocyanide is manufactured in large quantities for use in dyeing and calico-printing; it produces a beautiful bright blue colour, which, however, will not bear washing with alkaline solutions, or with soap. It is used also in the preparation of Prussian blue, as well as in the processes of electro-plating and electro-gilding. It is the source from which the compounds of cyanogen are usually obtained. An aqueous solution of potassic ferrocyanide produces characteristic precipitates with many of the metallic salts; it is, consequently, extensively employed as a qualitative test for indicating the presence of metallic bodies in solution. The precipitate generally consists of a ferrocyanide of the metal by which the precipitate is occasioned; usually the 4 atoms of potassium in the ferrocyanide are displaced by an equivalent amount of some other metal, such as cobalt, nickel, copper, or lead, yielding precipitates, Єo,FeCy; Ni,FeCy; Єu,FeCy; Pb,FeCy. These precipitates are apt to retain portions of the ferrocyanide of potassium, from which it is difficult to free them even by long-continued washing. In a few cases half the potassium only in the ferrocyanide of potassium is displaced by an equivalent of some other metal, with which it forms an insoluble precipitate, as happens when a salt of barium is precipitated by potassic ferrocyanide in excess; the yellowishwhite precipitate consisting of (K,Ba‚FeCу, 3 H‚Ð). The insoluble ferrocyanides cannot be precipitated in alkaline solutions; the liquid, in all cases, should be slightly but distinctly acid. Many of these precipitates are white, such as those of zinc, cadmium, nickel, manganese, tin, lead, bismuth, antimony, silver, and mercury; with the exception of those of cadmium and manga

nese, they are insoluble in dilute hydrochloric acid. The most characteristic precipitates occasioned by potassic ferrocyanide are the following:-With the salts of cobalt it gives a yellowish green; with ferrous salts a white, becoming blue; with ferric salts a deep blue; with cupric salts a reddish brown; and with salts of uranium it also gives a brown précipitate.

(1582) The most important of these insoluble ferrocyanides is the compound largely employed as a pigment under the name of Prussian blue (Fe,Cy18, 18 H2O=Fe,Fcy, 18 H2O). Whenever potassic ferrocyanide is mixed with a soluble ferric salt, this beautiful blue precipitate is produced; 2 atoms of the ferric salt, ferric sulphate, for example, and 3 atoms of potassic ferrocyanide undergo decomposition, so that the place of the 12 atoms of potassium is supplied by 4 atoms of iron; potassic sulphate remains in solution, whilst the Prussian blue is precipitated :Potass. ferrocy. Potassic sulph. Prussian blue.

Ferric sulphate.

2 (Fe'”2, 3 SO,′′) + 3 K1Fcy = 6 K ̧§Ð ̧ + Fe̟„Fcу3. The composition of Prussian blue seems to be rather anomalous, but if taken as (Fe"",,Fcyivs), it corresponds to ferric chloride (Fe"",Cl), for if ferrocyanogen be a tetrabasic radicle, it will require 4 Fe instead of 2 Fe for its saturation.*

When large quantities of Prussian blue are required, Liebig recommends that II parts of crystallized ferrous sulphate be dissolved in water, and divided into two equal portions; one part is then to be mixed with 2 parts of hydrochloric acid, and chloride of lime is gradually to be added until the whole of the ferrous salt is converted into a ferric salt; it is then to be mixed with the other portion of the solution, and a solution of 10 parts of potassic ferrocyanide is to be added. The compound thus obtained is not a pure substance. It may, however, be prepared in a state of perfect purity by mixing a solution of ferric chloride with one of hydroferrocyanic acid (Williamson). Commercial Prussian blue is generally contaminated with alumina, and sometimes with chalk, plaster of Paris, and starch. It always retains a portion of potassic ferrocyanide.

Prussian blue, as met with in commerce, occurs in cubical or irregular masses of a dark blue colour, which when pressed with a

=

* Gerhardt and Laurent, starting from the fact that iron is capable of entering into combination in two proportions, assumed that it possesses two different equivalent numbers: Fe" ferrosum = 56, and fe" ferricum Fe or 37°2: and supposing that the portion of the iron in Prussian blue which has displaced the potassium is in the form of fe, Prussian blue may be represented as a compound of the same order as potassic ferrocyanide, with the formula, fe", Fey.

754

FERRIDCYANIDES, OR FERRICYANIDES,

hard body acquire a coppery lustre resembling that of indigo. It is insoluble in water and in weak acids, and has neither taste nor smell. After it has been washed with dilute hydrochloric acid, it may, however, be rendered soluble by triturating it with onesixth of its weight of crystallized oxalic acid. When this liquid is diluted with forty or fifty parts of water, it forms a blue solution, which is used as a writing ink. Prussian blue is also soluble in a cold solution of ammonium tartrate, forming a violet-coloured liquid. It is decomposed when treated with alkalies or alkaline carbonates, hydrated ferric oxide being liberated, and ferrocyanide of the alkaline metal remaining in solution. Oil of vitriol produces a white pasty mass when triturated with Prussian blue, but on the addition of water sulphuric acid is separated, and the blue pigment is reproduced. Prussian blue is decomposed when heated with concentrated sulphuric, nitric, or hydrochloric acid; it is also decomposed by the action of chlorine. When boiled with mercuric oxide, mercuric cyanide is produced and ferric oxide is separated.

Prussian blue contains water, which cannot be expelled by heat without decomposing the compound. If heated in the open air it becomes kindled on the approach of an ignited body, and burns like tinder, leaving a residue of ferric oxide.

Soluble Prussian blue may be obtained by pouring a solution of ferric chloride into an excess of potassic ferrocyanide; the precipitate is insoluble in the saline liquid, and in solutions of salts generally, but it is freely soluble in pure water.

6

(1583) When a solution of a ferrous salt is mixed with one of potassic ferrocyanide, a greenish-white precipitate is formed, which appears to be the same as Everitt's white salt (501), and may be regarded as a double ferrocyanide of potassium and iron; K1FeCy+ FeCl2=K,Fe”(FeCy ̧)*+2 KCl. When this white substance is exposed to the air, the ferrous cyanide becomes oxidized and turns blue, and a compound is formed termed basic Prussian blue; it consists of one atom of ferric oxide combined with one of Prussian blue: the potassic ferrocyanide originally present in the white compound may then be washed away. change which occurs is probably the following, neglecting the potassic ferrocyanide of the precipitate, as it is not affected; 6 Fe2FeCy+302=2 (Fе,Cу18,Fе¿Ð ̧). A strong acid removes the ferric oxide from basic Prussian blue, and converts it into ordinary Prussian blue.

6

The

(1584) 2. FERRIDCYANIDES, or Ferricyanides.-Potassic Ferricyanide, or Red Prussiate of Potash (K,,FeCy=329).—When a

RED PRUSSIATE, OR POTASSIC FERRICYANIDE.

755

current of chlorine gas is passed through a dilute solution of potassic ferrocyanide, the liquid quickly changes colour and becomes of a deep red. If the current of the gas be arrested so soon as the liquid ceases to give a blue precipitate with the ferric salts, the solution on evaporation yields magnificent ruby-red crystals of potassic ferricyanide. In this process the reaction which occurs is the following: 2 K,FeCy+ Cl2=2 K,FeCy+ 2 KCl. The chlorine acts by withdrawing one-fourth of the potassium contained in the yellow salt; chloride and ferricyanide of potassium being the result. The ferricyanide may be obtained by a second crystallization in transparent anhydrous right rhombic prisms of a beautiful red colour; and if the operation be performed on a large scale these crystals often attain a large size : they are soluble in about 24 parts of cold water and in 1 part of boiling water, but are insoluble in alcohol. If the action of the. chlorine be too long continued, the red salt is in its turn partially decomposed, and a green compound is formed which interferes with the crystallization of the red salt. This green compound, according to the analysis of Pelouze, may be represented by the formula (FeCy,,Fe,Cys, 4 H2O), or it is the magnetic cyanide, corresponding to the magnetic oxide (FeЂFe̱¿Ð ̧) of iron. Potassic ferricyanide is prepared on the large scale for the use of the calicoprinter; but it is then generally obtained by decomposing the solid yellow salt by exposing it in powder to the action of a current of chlorine gas. Potassic ferricyanide may also be obtained in small quantities by decomposing a solution of the yellow ferrocyanide by means of the voltaic battery in a diaphragm cell (285); it is formed in the solution at the positive electrode. It may likewise be obtained by acting upon the yellow prussiate by various oxidizing agents.

3

4

A solution of potassic ferricyanide when mixed with an excess of potash furnishes a liquid possessed of considerable oxidizing power; it converts the hydrated protoxides of manganese, lead, and tin into peroxides of these metals, whilst the ferricyanide passes into the condition of ferrocyanide of potassium; 2 K ̧FeCy + 4 KHO + MnSO1 = 2 K ̧FeCy ̧ + K2SO4 + MnO2+ 2 H2O. The same solution is employed by the calico-printer for the purpose of discharging the blue colour of indigo from calico (Mercer). The red prussiate burns with scintillation when introduced into the flame of a candle. Potassic ferricyanide produces a large number of insoluble compounds when mixed with the salts of many of the metals; and as the colours of these precipitates are often characteristic, this salt is frequently employed in

the laboratory as a qualitative test for the metals in neutral or feebly acid solutions. With salts of zinc it gives an orange precipitate, with those of cadmium a yellow, with nickel a yellowish green, with cobalt a dark reddish brown, with copper a yellowish green, with ferrous salts a brilliant blue, with manganous salts a brown, with stannous salts a white, with silver salts an orange, and with mercurous salts a brownish red. These precipitates, with the exception of those with zinc and tin, are insoluble in dilute hydrochloric acid. The following table exhibits the composition of some of the ferricyanides :

Potassic ferricyanide gives no precipitate in the solutions of the ferric salts, but a splendid blue when mixed with those of the ferrous salts. It is a very delicate test for any ferrous salt when in solution. If ferrous sulphate be employed, the change which occurs is the following: 2 K,Fdcy + 3 FeSO1 = Fe,Fdcy,+3 K2SO. The hydrated precipitate which is thus formed constitutes a highly-valued and brilliant variety of Prussian blue, commercially known as Turnbull's blue. Turnbull's blue may be distinguished from ordinary Prussian blue by the action of a solution of potash upon it, which decomposes it into potassic ferrocyanide and magnetic oxide of iron, whilst the ordinary blue when similarly treated yields potassic ferrocyanide and ferric oxide.

Williamson finds that when Everitt's white salt (K,Fe,FeCy) is treated with chlorine or with nitric acid it is decomposed, and a blue salt is formed (KFe",FeCy, 9 H2O) corresponding in composition to Turnbull's blue, in which one third of the iron has been displaced by its equivalent of potassium. The formation of this substance is best effected by boiling the white precipitate with nitric acid, diluted with twenty parts of water; as the liquid approaches the boiling point a copious evolution of nitric oxide. occurs each atom of the white salt loses one atom of potassium, and the ferro-potassic ferricyanide is formed. Care is required

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