CYANIDE OF SILVER-CYANIDE OF MERCURY.

747

portion of cyanogen. Cyanide of silver is freely soluble in solutions of the cyanides of potassium, sodium, barium, calcium, and strontium. These solutions give no precipitate with solutions of the metallic chlorides; indeed, cyanide of silver is dissolved by boiling solutions of the chlorides of the metals of the alkalies, and of the earths. It is also soluble in a solution of nitrate of silver, as well as in solutions of the ferrocyanide and ferricyanide of potassium. When potassic ferrocyanide is used as the solvent, a dirty blue precipitate of ferrous cyanide is formed, and the liquid becomes strongly alkaline; K,FeCy+4 AgCy= FeCy2+4 (AgCy, KCy). Potassio-argentic cyanide (AgCy,KCy=199) is anhydrous, and may be obtained in octohedra or in six-sided plates. Occasionally it forms rhombic prisms 2 (AgCy,KCy),H,. This salt is freely soluble in water and in boiling alcohol. It is extensively employed for electro-silvering, since its solution, if mixed with an excess of potassic cyanide and then submitted to electrolysis, deposits silver in compact coherent plates (295).

(1575) Cyanide of Mercury, Mercuric Cyanide (HgCy,=252; Sp. gr. 3'77). This salt may be prepared either by dissolving mercuric oxide in dilute hydrocyanic acid, in which the oxide is soluble with great facility, or by boiling a mixture of 4 parts of finely-powdered Prussian blue and 3 of mercuric oxide, also finely levigated, with 40 parts of water, until the undissolved portion has acquired a full brown colour: the filtrate on evaporation yields mercuric cyanide. It may also be procured by boiling 2 parts of mercuric sulphate with I part of potassic ferrocyanide and 8 of water :

KFeCy+ 3 HgSO = 3 HgCy, + 2 K2SO4 + FeSO4.

4

Mercuric cyanide crystallizes in anhydrous rectangular prisms, which are colourless and transparent. It has a nauseous, metallic taste, and is very poisonous. It requires about 8 parts of cold water for solution; it is less soluble in ordinary alcohol, and nearly insoluble in absolute alcohol. Mercuric cyanide is decomposed when heated; if perfectly dry it yields cyanogen gas, metallic mercury, and a residue of paracyanogen. If moist, carbonic anhydride, ammonia, and hydrocyanic acid are produced.

Mercuric cyanide dissolves mercuric oxide freely, and forms an oxycyanide (HgO,HgCy,) which crystallizes in needles; it is sparingly soluble in cold water, and the liquid has an alkaline reaction. Mercuric cyanide does not give any precipitate of oxide of mercury on the addition of a free alkali. When heated with sulphuric or hydrochloric acid it is decomposed, and gives

748

CYANIDES OF GOLD AND PALLADIUM.

off hydrocyanic acid. It is also decomposed by sulphuretted hydrogen. Mercuric cyanide enters into combination with a great number of chlorides, iodides, bromides, cyanides, nitrates, and various other salts, forming with them crystallizable double salts. It does not occasion precipitates when mixed with the solutions of salts of zinc, cobalt, nickel, manganese, and other metals which yield cyanides insoluble in water, with the exception of the salts of palladium; from which it would appear that mercury has a stronger attraction for cyanogen than any of the metals, excepting palladium.

(1576) Cyanides of Gold.-Aurous cyanide (AuCy=222·6) is the only cyanide of gold which is accurately known. It may be obtained by several methods; the simplest consists in adding a solution of potassic cyanide to a dilute solution of auric chloride, so long as a precipitate is occasioned. It is a lemon-yellow powder, composed of microscopic hexagonal plates. It is decom

posed by heat into cyanogen and metallic gold: boiling nitric, sulphuric, and hydrochloric acids do not decompose it unless very concentrated, and aqua regia acts upon it very slowly. It is soluble in sodic hyposulphite and in ammonium hydrosulphide; but its most important solvent is cyanide of potassium, with which it forms a double salt (AuCy,KCy=287·6): this compound crystallizes in colourless rhombic octohedra, or in pearly scales. It may be obtained by dissolving either aurous cyanide or auric oxide (precipitated from the trichloride by ammonia) in a solution of potassic cyanide. Aurous cyanide is also soluble in a solution of potassic ferrocyanide. The double cyanide is likewise formed by dissolving finely-divided metallic gold in a solution of potassic cyanide with exposure to the air. Auro-potassic cyanide is largely used for gilding by means of the galvanic battery (296).* Copper and silver articles may also be gilt by it without the aid of the battery, by mere immersion; portions of copper or of silver being dissolved during the operation.

(1577) Palladious Cyanide (PdCy2=158·4).—Palladium has a very powerful attraction for cyanogen; when a solution of a salt of palladium is mixed with one of mercuric cyanide, a yellowish-white precipitate of palladious cyanide is formed; this precipitate may be dissolved by means of potassic cyanide, with

* Böttger recovers the gold from the spent gilding solutions by evaporating them to dryness, mixing the residue with its own weight of finely-powdered litharge, and fusing the mass; the lead and the gold are reduced, and form a metallic button, which, when treated with nitric acid, leaves the gold in the form of a fine powder.

CYANIDES OF LEAD AND IRON-FERROCYANIDES.

749

which it forms a double salt, which crystallizes in rhomboidal prisms (2 KCy,PdCy2, 3 H‚Ð).

Cyanide of lead (PbCy) is white; it is insoluble in water, and does not form a double cyanide with potassic cyanide; it is insoluble in an excess of a solution of this salt. Dilute sulphuric acid decomposes cyanide of lead, and liberates hydrocyanic acid.

(b) Electro-negative Metallic Cyanides.

(1578) Cyanides of Iron.-Little is known accurately of the simple cyanides of iron. When potassic cyanide is mixed with ferrous sulphate free from ferric salt, a reddish-brown precipitate partially soluble in acids is formed, which appears to consist of hydrated ferrous cyanide. This precipitate is dissolved by an excess of potassic cyanide, and is thus converted into potassic ferrocyanide. The white compound obtained by decomposing Prussian blue by means of a current of sulphuretted hydrogen appears not to be a simple cyanide of iron, as was originally supposed by Robiquet. Ferric cyanide has not been procured in a separate form. When a solution of ferric chloride is mixed with one of potassic cyanide, hydrocyanic acid is liberated, potassic chloride is formed, and a precipitate of hydrated ferric oxide is obtained. Prussian blue (Fe,Cy18, 18 H2O; 1582), Turnbull's blue Fe¿Cy199 12 H2, and the substance Fe,Cys, 4 H2O, which Gmelin terms Prussian green (1584), are compound cyanides of iron.

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(1579) I. FERROCYANIDES.-The cyanides of iron, when in combination with other cyanides, give rise to two important classes of compounds, which are represented by the yellow prussiate of potash, or potassic ferrocyanide (K,FeCy, 3 H2O), and the red prussiate of potash, or potassic ferricyanide (K,FeCy。).

750

HYDROFERROCYANIC ACID-POTASSIC FERROCYANIDE.

Liebig considers that these two classes of salts contain two separate radicles, ferrocyanogen (FeCy,= Feyiv), and ferricyanogen (FeCy=Fdcy""). The first of these radicles is tetrabasic, the second tribasic; but neither of these bodies has been obtained in a separate form. The hypothesis of their existence, however, affords a convenient mode of representing the composition of the double cyanides into which they are supposed to enter, and greatly facilitates the study of these salts, which are numerous and important. The preceding table shows the composition of some of the compounds of the ferrocyanogen series.

(1580) Hydroferrocyanic Acid (H,FeCy).—This compound was originally obtained by Porrett, by the decomposition of potassic ferrocyanide by means of tartaric acid. A better plan cousists in forming a concentrated aqueous solution of the ferrocyanide, boiling it to expel the air, and then allowing it to cool in a vessel provided with a good cork to exclude air. Ether is then added, and an excess of hydrochloric acid, which has also been deprived of air by boiling. On agitating the mixture, potassic chloride is formed and remains in solution, and white scales of hydroferrocyanic acid are deposited; K FeCy+4 HCl=H2FeCó ̧+4 KCl. These crystals must be washed with a mixture of alcohol and ether, and dried in vacuo over sulphuric acid. Liebig adds by degrees to a cold saturated solution of potassic ferrocyanide an equal volume of hydrochloric acid; the white precipitate of hydroferrocyanic acid thus obtained must be washed with hydrochloric acid, dried in vacuo upon a tile, and caused to crystallize from its solution in alcohol by the addition of ether.

Hydroferrocyanic acid is tetrabasic. When exposed to the air, it absorbs oxygen and becomes blue; its solution, when boiled, evolves hydrocyanic acid, and deposits a white insoluble matter containing both iron and cyanogen. This body also becomes blue in the air. It is probably ferrous cyanide, and by exposure to air it absorbs oxygen, becoming converted into basic Prussian blue, which is a compound of ferric oxide and of true Prussian blue; 2 (9 FeCy2)+30=2 (Fe,Су18,Fе ̧Ð ̧).

(1581) Potassic Ferrocyanide, or Prussiate of Potash (K FeCy 3 H2O=368+54; or K,FeCy3, 3 HO).-This important salt is met with in commerce nearly in a state of purity. It is formed when azotised matters are heated to redness with potassic carbonate and iron. It is also formed when a ferrous salt is mixed with a solution of potash supersaturated with hydrocyanic acid. It is manufactured on a large scale by heating dried blood, horns, parings of hides, and other animal matters containing nitrogen,

POTASSIC FERROCYANIDE (PRUSSIATE OF POTASH).

751.

with an equal weight of potassic carbonate, and about one-third of their weight of iron filings, in a covered iron pot. Part of the carbon and nitrogen escapes in combination with oxygen and with hydrogen as carbonic anhydride and ammonia, while another portion (not exceeding, however, from to of the whole) enters into combination with the potassium, and this when treated with water attacks the iron, forming a salt which may be obtained from its solution, in large lemon-yellow tabular crystals, which contain 3 atoms of water. The salt is tough, and splits into lamina with

facility.

The reactions by which the ferrocyanide is produced are complicated. When animal matter is heated with pearlash and iron, cyanogen is formed, which enters into combination with the potassium, forming potassic cyanide; but the temperature employed is too high to allow of the formation of ferrocyanide. A variable quantity of the iron is at the same time converted into sulphide, owing to the action of the dipotassic sulphide (produced by the reduction of the potassic sulphate in the pearlash) upon the metal. When the mass is lixiviated, the potassic cyanide contained in the mixture attacks both the metallic iron and the ferrous sulphide, becoming converted into ferrocyanide in the following manner (Liebig) :-A solution of potassic cyanide dissolves metallic iron with evolution of hydrogen, caustic potash being set at liberty, whilst potassic ferrocyanide is formed; 6 KCy+Fe +2 H2O=K,FeCy+ H2+2 KH→: and the action of potassic cyanide upon ferrous sulphide may be thus represented; 6 KCy+ FeS=K,FeCy+K2S. Ferrous oxide is also readily dissolved by a solution of potassic cyanide, whilst potassic ferrocyanide is formed as in the previous cases; 6 KCy+Fe→+H2O= K1FeCy+2 KHO. A knowledge of these reactions explains the formation of the ferrocyanide during the lixiviation practised in the course of the manufacture.

The iron in the ferrocyanide does not perform the part of a basyl; for when a solution of this salt is submitted to electrolysis, the iron accompanies the cyanogen to the negative electrode. The iron cannot be thrown down from an aqueous solution of the salt by the addition of any alkaline solution; but if the alkaline liquid be boiled with mercuric oxide, the iron is separated in the form of hydrated ferric oxide.

Potassic ferrocyanide is soluble in about 4 parts of cold water, and in 2 parts of boiling water; but it is insoluble in alcohol: the addition of a solution of potash to its aqueous solution causes the separation of a portion of the salt in crystalline