75. Hydrocyanic acid is a colourless, mobile liquid, solidifying at 15 to a fibrous crystalline mass, and boiling at +26.5°. In consequence it evaporates with extraordinary rapidity, and thereby absorbs so much heat that a drop placed on a glass rod can be caused to partly solidify by being moved quickly through the air. The density of the liquid is 7058 at + 7° and '6969 at + 18°, vapour density 948. The odour is peculiar, somewhat resembling that of bitter almonds; the vapour of the acid burns with a pale violet-coloured flame.

The extraordinarily poisonous nature of hydrocyanic acid is especially noticeable. When mixed with much air, and inhaled in small quantity, it produces a peculiar feeling in the throat; in larger quantity, faintness, and finally death. The unmixed vapour of the anhydrous acid causes instant death, and produces equally fatal results by direct contact with the blood. In very small doses it is employed as a medicine. The acid character of HCN is not strongly pronounced; litmus paper is scarcely reddened by it; with metallic oxides, however, it behaves similarly to the halogen hydroacids, yielding generally metallic cyanides and water. When added to the oxides of iron it readily forms ferric ferrocyanide or Prussian blue, from which substance it derives its common name of prussic acid.

This formation of Prussian blue serves for the detection of small traces of cyanogen compounds. Usually the liquid to be tested is mixed with a ferrous salt, slight excess of KHO added, and the liquid heated for a short time. According to the equations:

and

6HCN + FeSO1 + 8KOH = 6KCN + Fe(OH)2 + K2SO ̧
+ 60H2

6KCN + Fe(OH)2 = K,FeC ̧N + 2KOH

potassic ferrocyanide is formed; on then adding ferric chloride and acidulating with hydrochloric acid, a deep blue precipitate is obtained.

76. By keeping, hydrocyanic acid soon decomposes, brown solid bodies separating, and ammonic salts being formed. As already mentioned, a trace of a strong acid retards this decomposition, as does also dilution with large quantities of water. The aqueous solution forms by its decomposition some quantity of ammonic formate:

By boiling with alkalies this change is considerably accelerated, as also by heating with strong mineral acids; hydrochloric acid, for instance, decomposing hydrocyanic acid into ammonic chloride and formic acid:

H

H

HCI+CEN + 2H2O = CO

O-H

+ NH1Cl.

This easy conversion of the cyanogen into the oxatyl group CO.OH is highly characteristic of the true cyanogen compounds, and is of great importance in organic synthesis.

77. Addition Products of Hydrocyanic Acid.-As already mentioned, in certain respects hydrocyanic acid shows properties which place it amongst the analogues of ammonia. It may therefore be viewed as a molecule of ammonia, in which the three hydrogen atoms have been replaced by the trivalent radical formyl CH:

In the anhydrous state it yields white crystalline compounds by direct union with HCl, HBr, or HI:

H

HCN + HCl = H—C=N<; similarly: HCN + HBr or HCN

+ HI

These compounds are decomposed with great rapidity, on treatment with water, into formic acid and ammonic salts:

Anhydrous hydrocyanic acid also unites with some metallic chlorides, crystalline compounds resulting; for instance, Fe,C16,4HCN and SbC15,3HCN, the constitution of these bodies being probably

78. The metallic cyanides can be prepared, almost without exception, from hydrocyanic acid, though only in the case of the most strongly positive metals, by action in the metallic state upon the acid:

K2 + 2HCN = 2KCN + H2;

yet by use of the hydrates, in some cases also of the oxides, they can be prepared without difficulty:

Hg0+ 2HCN= Hg(CN)2 + H2O.

The insoluble metallic cyanides are best prepared by double decomposition between the soluble alkaline cyanides, and soluble salts of the respective metals.

The cyanides of the different metallic groups differ from one another very essentially in some of their properties. Whilst those of the most positive metals (the alkalies and alkaline earths) are easily soluble in water, are of strongly alkaline reaction, are decomposed by

the weakest mineral acids, even carbonic, and are not decomposed by a red heat if air be absent; those of the less positive metals (the heavy metals) are mostly insoluble, or difficultly soluble in water, and are decomposed by a high temperature. Some of these latter cyanides, more especially those of the noble metals (mercury, silver, &c., partly also Cu and Zn), simply decompose into the metal and cyanogen, which in great part comes off as gas, whilst the others, especially those of the iron group, are decomposed on ignition, nitrogen being evolved, and a compound of the metal with carbon left behind.

The cyanides of the heavy metals mostly combine with those of the alkalies, forming compounds soluble in water. In these double cyanides very frequently the less positive heavy metal is apparently more firmly united than the more positive, and cannot be recognised by its usual tests; when treated with mineral acids these compounds yield no hydrocyanic acid, or give up their cyanogen only partially in that form, whilst hydric metallic cyanides of acid character result; e.g. potassic ferrocyanide yields, with hydrochloric acid, potassic chloride and hydroferrocyanic acid:

K1FeCN + 4HCl = 4KCl + H1FeC6N6.

In such double cyanides heavy metals can replace the alkali metal, and such replacing metals are still recognisable by their ordinary reagents.

These double cyanides can only result from the polymerisation of the cyanogen group, in some the dicyanogen, in others the tricyanogen group occurring.

Some metallic cyanides doubtless contain true cyanogen-that is, the metal united to carbon-whilst others appear to be entirely isocyanides; to the latter class probably belong the cyanides of those metals which combine readily with nitrogen, especially those which yield amid-compounds with ammonia.

79. Potassic Cyanide, KCy or K-CEN.-Although potassic cyanide can be directly prepared, still it is generally obtained from dried potassic ferrocyanide, which is heated to quiet fusion, without access of air:

K1FeCN6=4KCN + FeC2 + N2.

The carbide of iron settles to the bottom, and the clear supernatant liquid can be poured off. Such potassic cyanide as remains with the residue can be obtained by pulverisation and extraction with boiling alcohol; the filtered liquid crystallises on cooling.

It can be obtained in larger quantity, and of sufficient purity for most purposes, by heating a mixture of eight parts of dried potassic ferrocyanide with three parts of dried potassic carbonate to low redness in an iron crucible. By this means one-half of the cyanogen, which would be lost but for the addition of the potash, is obtained as potassic cyanide, the other half as potassic cyanate, which latter remains mixed with the cyanide; instead of ferric carbide, spongy iron results, from which the melted salt is easily separated mechanically:

K ̧Fe(C ̧N6) + K2CO2 = 5K(CN) + KCNO + CO2 + Fe.

A still better result is obtained when charcoal is also added to the fusion, and the mixture strongly heated, the carbon reducing a large quantity of cyanate to cyanide of potassium:

K4Fe(CN6) + K2CO3 + C = 6K(CN) + CO2 + CO + Fe.

The purest potassic cyanide is obtained by passing the vapour of anhydrous hydrocyanic acid into a solution of potassic hydrate in absolute alcohol, the salt separating in the crystalline form.

Potassic cyanide is a colourless, highly poisonous body, crystallising from solution in octahedra, from fusion in cubes; at a red heat it fuses to a clear liquid, it deliquesces in damp air, and through its slow decomposition by the carbonic acid of the air-smells of hydrocyanic acid. Its aqueous solution reacts strongly alkaline. It is more soluble in dilute than in strong alcohol. It decomposes easily in aqueous solution, especially on boiling, into potassic formate and ammonia :

When fused in contact with air, it absorbs oxygen, and is converted into cyanate. It undergoes the same change when heated with metallic oxides, and is therefore one of the most powerful reducing agents, e.g.

SO2 + 2KCN=S+ 2KCNO.

The cyanides of the other alkali metals resemble that of potassium in nearly all particulars.

CEN

80. Ammonic cyanide, CN,H,=, is usually prepared by

NH4

heating an intimate mixture of potassic cyanide and sal ammoniac. It crystallises in colourless cubes, which are very soluble in alcohol. It boils at + 36° with dissociation into hydrocyanic acid and ammonia, which re-combine on reduction of temperature. It is as poisonous as prussic acid and potassic cyanide. On keeping, it decomposes, with formation of a blackish brown mass (azulmine).

The formation of ammonic cyanide by passing ammonia gas over glowing carbon :

3C +4NH = 2(CN.NH,) + CH,;

or by passing ammonia and carbonic oxide through red-hot tubes :

CO + 2NH, = CN.NH, + HO;

=

is of considerable theoretical interest. As ammonic cyanide is obtained by the destructive distillation of nitrogenous organic bodies, it occurs, often in considerable quantity, as a constituent of the washwater of gas-works.

81. Cyanides of the alkaline earths-i.e. of barium, strontium, and calcium--can be prepared directly, but are best obtained by ignition of the respective ferrocyanides, or by action of hydrocyanic acid on their

hydrates. They are more difficultly soluble than the foregoing, react alkaline, and are very easily decomposed by carbonic acid.

=

82. Zincic cyanide, ZnC,N, Zn(CN)2, is obtained as a white insoluble powder, by addition of hydrocyanic acid to zincic acetate, or of potassic cyanide to any other zinc salt. It is decomposed by acids with evolution of hydrocyanic acid, is readily soluble in excess of potassic cyanide, the solution yielding on evaporation octahedral crystals of potassic zincic cyanide :

The cyanogen compounds of cadmium and indium are quite similar ; from the double cyanides the metals are precipitated by sulphuretted hydrogen, as in the case of the ordinary salts.

Indium cyanide, however, is distinguished from the others by its easy decomposition on boiling with water, indium hydrate being formed:

In(CN)2 + 2H2O = In(OH)2 + 2HCN.

83. Nickelous cyanide, Ni(CN)2, is obtained similarly to the zinc salt, as an apple-green precipitate; it is easily dissolved by an aqueous solution of potassic cyanide to a yellow solution, which by evaporation yields monoclinic prisms of potassic nickelous cyanide, K‚NiC ̧N ̧‚OH, or Ni (CN)K,OH2. From solutions of this (C2N2)K'

2

salt, nickelous cyanide is re-precipitated by careful addition of hydrochloric acid; excess of acid decomposes it. By boiling the solution with mercuric oxide, nickelous oxide is precipitated, and potassic mercuric cyanide formed.

84. Cyanides of Cobalt.-By careful addition of potassic cyanide to a cobaltous salt, a reddish-brown precipitate of cobaltous cyanide is obtained, which dissolves readily in an excess of the precipitant. From this solution alcohol precipitates potassic cobaltous cyanide, K,Co(CN), which crystallises in deep red, deliquescent needles. Exposed to air it eagerly absorbs oxygen, and is converted into potassic cobalticyanide, which is also formed with evolution of hydrogen on boiling the solution:

2K,Co(CN6) + 2H2O = K¿Co2(C12N12) + 2KOH + H2,

as also by heating freshly precipitated cobaltous hydrate with potassic cyanide solution:

12KCN + 2Co(OH)2 + 2H2O = K6C02(C12N12) + 6KOH + H2.

Potassic cobalticyanide crystallises in clear yellow crystals, isomorphous with those of potassic ferricyanide (see later), which dissolve easily in water, and on ignition decompose into potassic cyanide, cyanogen, nitrogen, and cobalt carbide. The cobalt in this compound cannot be recognised by any of its usual reagents. On adding a slight excess of sulphuric acid to a concentrated solution of this salt, and then much alcohol, potassic sulphate separates, and the alcoholic liquid yields, on evaporation, colourless needles of hydrocobalticyanic acid, H&Co,Cy12,H2O, which loses its water of crystallisation at 100°,

F