94. Cyanides of Copper.-A cyanide of copper of the formula Cu(CN), is not obtainable in the pure state; an impure preparation is formed by addition of potassic cyanide to solution of cupric sulphate, or of cupric acetate to hydrocyanic acid, as a dirty yellowish-brown precipitate, but this very quickly decomposes, with evolution of cyanogen, into green crystals of cuprous cupric cyanide:

In boiling water this is readily further decomposed into cyanogen and cuprous cyanide, Cu,(CN), CN-Cu-Cu-CN.

=

Cuprous cyanide is a white powder, insoluble in water, easily soluble in hydrochloric acid, ammonia, or aqueous potassic cyanide. With the latter it forms crystalline, colourless, double salts of different composition, namely :

which is difficultly soluble, and the more soluble K6Cu,C,Ng, from which sulphuretted hydrogen does not precipitate the copper. These double salts are also obtained when solutions of cupric salts are mixed with excess of potassic cyanide.

From plumbic cupro-cyanide (obtained by precipitation of potassic Cupro-cyanide with plumbic acetate) the compound H2Cu,C,N, is obtained by decomposition with sulphuretted hydrogen; it readily decomposes into cuprous cyanide and hydrocyanic acid.

95. Of the cyanogen compounds of mercury, that one corresponding to calomel is not known; when potassic cyanide is added to a mercurous salt, metallic mercury is precipitated, and mercuric cyanide left in solution:

Hg2(NO3)2 + 2KCN = 2KNO3 + Hg + Hg(CN)2. Mercuric cyanide is prepared by dissolving yellow mercuric oxide in aqueous hydrocyanic acid, the former being used in slight excess. It is also easily obtained by boiling Prussian blue with mercuric oxide, until all the iron is precipitated as oxide. At ordinary temperatures one part of Hg(CN)2 is soluble in eight parts of water. It crystallises in large brilliant quadratic prisms, which are extremely poisonous; on heating it splits up into metal, and cyanogen gas, a portion of which remains polymerised as paracyanogen, a blackish-brown powder. Mercuric cyanide yields with the alkaline cyanides beautiful crystalline double salts, e.g.

Hg (C2N2)K

which crystallises in colourless octahedra.

C2N2)K

By mixing its solution with the molecular quantity of mercuric chloride and evaporating, quadratic prisms separate, of mercuric cyanochloride

or more probably Hg(C2N2) >Hg.

CI CI

On boiling mercuric cyanide solution with mercuric oxide, much of the latter is dissolved, and on cooling, needles of mercuric oxycyanide, (CN)-Hg-O-Hg (CN), separate.

Mercuric cyanide also combines with many metallic chlorides, bromides, and iodides, forming double salts, most of which crystallise beautifully; for instance:

Hg(CN)2 + NaCl = Hg<(C2N2)Na

silky flattened needles

Hg(CN)2 + KI

=

Hg (C2N2)K

nacreous needles

Hg(CN), + AgNO, + 2H2O = Hg(C2N2)Ag, 2H,O prisms

O.NO2

96. Argentic cyanide, Ag (CN) or Ag2(C,N2), separates as a white curdy precipitate, resembling argentic chloride, when hydrocyanic acid or a soluble cyanide is added to a silver solution. It is insoluble in water, and nitric acid, but easily soluble in ammonia, and potassic cyanide, and is unaltered by alkaline hydrates and chlorides. It does not blacken in sunlight, melts on heating, and leaves, on long ignition in absence of air, a white metallic mass consisting of silver and argentic paracyanide, Ag(C3N3).

As will be shown later, when treating of the nitrogen compounds of the alcohol radicals, the silver in argentic cyanide is most probably combined to the nitrogen, the cyanogen being present in the iso form, and the formula therefore :

CEN-Ag.

The product of the solution of argentic in potassic cyanide is potassic-argentic cyanide, KAg(CN)2, or K,Ag2(С2N2)2, which crystallises in six-sided, colourless tables, readily soluble in water. In this compound there is probably a dicyanogen radical, consisting of one true and one isocyanogen group :

This salt is chiefly employed in electroplating, as it gives a very coherent deposit on the negative pole, when electrolised.

The behaviour of argentic cyanide towards potassic cyanide yields a method of sufficient accuracy for the estimation of hydrocyanic acid or potassic cyanide. For this purpose a measured quantity of the liquid under examination is taken and converted, if needful, into potassic cyanide, by addition of potassic hydrate. Dilute standard solution of argentic nitrate is then added slowly; at first there is no precipitate, potassic argentic cyanide being formed until one-half of the cyanogen is converted into argentic cyanide :

2KCN + AgNO3 = KNO3 + K.Ag.C2N2;

as soon, however, as by addition of silver solution this point is over

stepped, every drop added produces a permanent precipitate of argentic cyanide :

KAg(C2N2) + AgNO3 = 2Ag(CN) + KNO3.

The quantity of silver solution used up to this point corresponds therefore in equivalents to half the quantity of hydrocyanic acid present. As a rule, a decinormal solution of argentic nitrate is employed; this is prepared by dissolving 170 grams of argentic nitrate (an equivalent in grams) to one litre of solution (= 0·017 grams AgNO3 per c.c.), and this is delivered drop by drop until the precipitate formed does not disappear on shaking. The number of c.c. used multiplied by 0054 gives the amount in grams of prussic acid present (equivalent of HCN = 27 and 27 x 2 = 54), by 013 the amount of potassic cyanide.

97. Cyanides of Gold.-Aurous cyanide, Au(CN), a yellow powder consisting of microscopical hexagonal tables, insoluble in water, is best obtained by decomposition with hydrochloric acid, of its double salt with potassic cyanide. Potassic aurocyanide is easily prepared by solution of the oxide or sulphide in potassic cyanide, as also by long standing of finely divided gold in potassic cyanide solution, air being excluded. It forms long colourless prisms easily soluble in water.

CN

Potassic auricyanide, KAu(CN)4, H2O or Au CN

(C2N2)K

,OH 2,

is obtained by mixing a neutral solution of auric chloride with a boiling concentrated solution of potassic cyanide; on cooling large colourless crystals are formed. The dilute aqueous solution generally obtained by mixing auric chloride and dilute potassic cyanide solution, is employed in electro-gilding.

When a solution of potassic auricyanide is added to argentic nitrate, a curdy precipitate of argentic auricyanide is obtained, which, suspended in water, and decomposed by a slightly insufficient quantity of hydrochloric acid, yields a solution of hydroauricyanıc acid; by evaporation of the filtered solution in vacuo, large tabular crystals of auric cyanide, Au(CN)3,3H2O, are obtained.

98. Cyanides of Platinum.--Platinous chloride dissolves readily in aqueous potassic cyanide; on evaporation, potassic platinocyanide crystallises out:

PtCl2 + 4K(CN) = 2KC1 + K2Pt(C ̧N1).

It is also formed when a mixture of platinic chloride and potassic cyanide are boiled together for a long time:

PtCl + 6KCN = K2Pt(C ̧N1) + C2N2 + 4KCl,

or when spongy platinum is fused with potassic cyanide or ferrocyanide, the product extracted with water, and the solution crystallised. Potassic platinocyanide, Pt(CN2)K,3H2O, forms long prisms,

(C2N2)K

pale yellow by transmitted, beautiful blue-coloured by reflected light. By double decomposition other metallic platinocyanides are obtained from this, which when crystallised show splendid fluorescent phenomena; for instance:

Baric platinocyanide, BaPt(CN1),4H,O, citron yellow prisms with violet and greenish reflection.

Magnesic platinocyanide, MgPt(C1N1),7H2O, red quadratic prisms, with metallic green reflection from the side faces, and deep blue from the end faces.

In aqueous solution these salts give insoluble precipitates with many of the salts of the heavy metals. By decomposing argentic or mercuric platinocyanide with hydric sulphide, the readily soluble hydroplatinocyanic acid, H2Pt(C ̧Ñ4), is obtained, which crystallises when anhydrous in needles of golden lustre, when with 5H2O, in vermillioncoloured crystals with bluish-black reflection; the solution is colourless. Heated to higher temperatures, hydro-platinocyanic acid yields prussic acid, and leaves a residue of greenish-yellow insoluble platinous cyanide, which on stronger ignition is completely decomposed.

When chlorine is passed into a concentrated solution of potassic platinocyanide, there separates from the colourless liquid coppercoloured crystals of metallic lustre, consisting of potassic platinicyanide, K,Pt(C10N10),6H2O.

The cyanide corresponding to platinic chloride does not appear to exist.

HALOGEN COMPOUNDS OF CYANOGEN.

99. The cyanogen radical combines with chlorine, bromine, and iodine to haloid compounds, in which the halogen atom is united to the carbon. The chloride and bromide polymerise-the latter indeed only on heating-with tripling of the molecule.

Cyanogen chloride, CI-CEN, or liquid cyanogen chloride, results from the decomposition of metallic cyanides and dilute prussic acid by chlorine. It is generally obtained by passing chlorine, in the dark, over damp mercuric cyanide :

Hg(CN)2 + 2C1, = HgCl2 + 2CICN,

and condensation of the dried vapour in strongly cooled vessels; or by passing a quick stream of chlorine into aqueous prussic acid of 20% strength, the reaction taking place in a retort cooled by a freezing mixture, and connected with a condenser. The oily cyanogen chloride, which separates to the bottom, is parted from the aqueous liquid, mixed with mercuric oxide to remove any undecomposed HCN, and rectified into well-cooled receivers.

It is a colourless mobile liquid, becoming a crystalline mass at -6°, and boiling at 15.5°. Its vapour attacks the mucous membrane violently, and is in the highest degree poisonous ; it is more difficultly soluble in water than in alcohol or ether.

By long keeping in sealed tubes, cyanogen chloride changes spontaneously into

Tricyanogen trichloride or solid cyanogen chloride:

CI-C-N

C3N3Cl3 =

Cl-C-N

The same compound is obtained directly by pouring anhydrous prussic acid into a flask filled with chlorine, and then exposing to sunlight; or when a slow stream of chlorine is passed into a solution of anhydrous prussic acid in four times its volume of ether, which is carefully cooled.

=

Solid cyanogen chloride crystallises in large needles or leaves, which melt at 145°; the boiling point is about 190°, the vapour density: 6-33, being three times that of the single cyanogen chloride.

By decomposition of potassic or mercuric cyanides by bromine, cyanogen bromide, Br-CN, is obtained in the form of shining colourless needles or cubes, which easily volatilise. When these, either alone or dissolved in ether, are heated in sealed glass tubes to 130°-140° for several hours, they are converted into :

Tricyanogen tribromide, which melts at a little above 300°, and sublimes at a still higher temperature.

Cyanogen iodide, I-CEN, crystallises in colourless needles, which sublimes easily, without previous fusion, with a strong odour, and, like the above-mentioned compounds, is very poisonous. It is obtained by sublimation of a mixture of mercuric cyanide and iodine. On adding iodine to a pretty concentrated solution of potassic cyanide, the liquid colours quickly and solidifies to a crystalline mass :

from which the ICN is obtained by shaking with ether.

CYANIC ACIDS.

100. The true cyanic acid has the constitution of cyanogen hydrate :

H0C=N;

to this there would be a corresponding pseudo-cyanogen compound, carboxylimide:

0-C-N-H.

Which of these is the formula of the known cyanic acid is not yet quite certain, though there are good reasons for believing the first to be correct. On the other hand, two potassium derivatives, (CN)OK and (CO)NK, are known, which, though clearly different, easily change into one another.

The same is generally true of the polymeric acids, especially of tricyanic or cyanuric acid.

101. Cyanic Acid, HCNO, probably CEN.-Cyanic acid can only

он

be obtained by strong heating of the polymeric cyanuric acid. This is done in a small retort, and the resulting vapour condensed in a receiver cooled with ice. It is a colourless, easily volatile liquid, of strongly acid reaction, smells like concentrated acetic acid, and blisters the skin. Left to itself, it clouds with slowly increasing temperature, and finally solidifies, with stronger evolution of heat, to a white porcelain-like mass, called cyamelide. This is a polymer of