Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/35

PRUSSIC ACID 23 irritating effect on the eyes and mucous membranes of the nose. It is poisonous. By strong pressure it is condensible into a liquid which freezes at -34 "4 C., and has the following vapour-tensions P at the temperatures t stated t= -20 7 -10 +10 +20C. P= I 1-85 27 3'8 5atmos. At ordinary temperatures water dissolves about 4 - 5 times, alcohol about 23 times its volume of the gas. The solutions are liable to (very complex) spontaneous decomposition. The list of products includes oxalate of ammonia and urea. Cyanogen burns with a characteristically beautiful peach-blossom coloured flame into car- bonic acid and nitrogen. This gas cyanogen, as already stated, is to cyanides what chlorine gas C1 2 is to chlorides, but it is well to remember that the analogy, though perfect in regard to the corre- sponding formulae, does not, as a rule, extend to the conditions of formation of the bodies represented. Thus cyanogen does not unite with hydrogen into pnlssic acid, nor does it combine with ordinary metals in the chlorine fashion. When passed over heated potas- sium, it is true, it combines with it into cyanide ; and caustic potash -ley absorbs it with formation of cyanide and cyanate (NCO. K), just as chlorine yields chloride and hypochlorite KC10 ; but this is about the sum -total of the analogies in action. Yet metallic cyanides of all kinds can be produced indirectly. Cyanide of Potassium, NC.K. An aqueous mixture of the quan- tities NCH and KHO no doubt contains this salt, but it smells of the acid, and on evaporation behaves more like a mixture of the two congeners than in any other way. An exhaustive union can be brought about by passing NCH vapour into an alcoholic solution of KHO ; the salt NC.K then comes down as a crystalline precipi- tate, which must be washed with alcohol and dried, cold, over vitriol. A more convenient method is to dehydrate yellow prussi- ate and then decompose it by heating it to redness in an iron crucible. The Fe(NC) 2 part of the salt breaks up into cyanogen and nitrogen, which go off, and a carboniferous finely-divided iron, which remains, with cyanide of potassium, which at that temperature is a thin fluid. Yet the iron sometimes refuses to settle with suffi- cient promptitude to enable one to decant off the bulk even of the fused cyanide. According to private information received by the writer a French manufacturer uses a certain kind of very porous fireclay as an efficient filtering medium. The ordinary "cyanide of potassium" of trade is not strictly that at all, but at best a mixture of the real salt with cyanate. It is produced by fusing a mixture of eight parts of anhydrous prussiate and three parts of anhydrous carbonate of potash, allowing the reaction (NC) 6 Fe . K 4 + K 2 C0 3 = C0 2 + Fe + 5NCK + K . NCO Cyanate to complete itself and the iron to settle, and decanting off the clear fuse. The product goes by the name of "Liebig's cyanide," but the process was really invented by Rodgers. Fused cyanide of potassium assumes on cooling the form of a milky white stone-like solid. It fuses readily at a red heat, and at a white heat volatilizes without decomposition, provided that it is under the influence of heat alone ; in the presence of air it gradually passes into cyanate ; when heated in steam it is converted into carbonate of potash with evolution of ammonia, carbonic oxide, and hydrogen. When heated to redness with any of the more easily reducible metallic oxides it reduces them to the metallic state, while it passes itself into cyanate. It also reduces the corresponding sulphides with formation of sulphocyanate ; for example, Pb(S or 0)+NCK = Pb + NC(0 or S)K. Hence its fre- quent application in blowpipe analysis. When heated with chlor- ates or nitrates it reduces them with violent explosion. The aqueous solution of the salt has a strongly alkaline reaction ; it smells of hydrocyanic acid and is readily decomposed by even such feeble acids as acetic or carbonic. It readily dissolves precipitated chlor- ide, bromide, and iodide of silver ; this is the basis of its application in photography. Large quantities of the salt are used in electro- plating. Other Binary Cyanides. Of these only a few can be noticed here. (1) Cyanide of sodium is very similar to the potassium salt. The same remark, in a more limited sense, holds for the cyanides of barium, strontium, and calcium. (2) Cyanide of ammonium (NC-NH 4 ) forms crystals volatile at 36 C. and smelling of ammonia and hydrocyanic acid. The solution in water decomposes spon- taneously, pretty much like that of the free acid. But the anhy- drous vapour by itself stands a high temperature, as is proved by the fact that it is produced largely when ammonia is passed over red-hot charcoal, C + 2NH 3 = H 2 + NCH . NH :j . (3) Mercuric cyanide, Hg(NC).j, forms very readily when mercuric oxide is dissolved in aqueous prussic acid. The solution on evaporation and cooling deposits crystals soluble in eight parts of cold water. This salt is not at all decomposed, even when heated, by water, nor appreci- ably by dilute sulphuric or nitric acid ; boiling hydrochloric acid eliminates the NC as hydrocyanic acid ; sulphuretted hydrogen acts similarly in the cold, ft gives no precipitate with nitrate of silver, nor is it changed visibly by caustic alkalis. It readily unites not only with other cyanides but also with a multitude of other salts into crystallizable double salts. Mercurous cyanide, Hg 2 (NC) 2 , seems to have no existence. When it is attempted to produce it by double decompositions, the mixture Hg + (NC).,Hg comes forth instead of the compound Hg^NC)^ (4) Heavy metallic cyanides are mostly insoluble in water, and the general method for their preparation is to decompose a solution of the respective sulphate, chloride, &c., with one of cyanide of potassium. The most important general property of these bodies is that they readily dissolve in solution of cyanide of potassium with formation of double cyanides, which in their capacity as double salts all exhibit, in a higher or lower degree, those anomalies which were fully explained above (see " prussiate of potash "). These " metallo- cyanides, " as we will call them, being all, unlike plain cyanide of potassium, very stable in opposition to water and aqueous alkalis, are readily produced from almost any compound of the respective metallic radical some from the metal itself by treatment with solution of cyanide of potassium. In all we have said "potassium" may be taken as including sodium and in a limited sense am- monium, but the potassium compounds are best known, and we accordingly in the following section confine ourselves to these. Metallo-cyanides. (1) Silver. Cyanide of silver, Ag.NC, is pro- duced as a precipitate by addition of hydrocyanic acid or cyanide of potassium to solution of nitrate of silver. The precipitate is similar in appearance to chloride of silver and, like it, insoluble in cold dilute mineral acids, but soluble in ammonia. At a red heat it is decom- posed with formation of a residue of carboniferous metallic silver. Precipitated cyanide of silver, though insoluble in hydrocyanic acid, dissolves readily in cyanide of potassium with formation of argento- cyanide, AgK . (NC) 2 , which is easily obtained in crystals, perma- nent in the air and soluble in eight parts of cold water. Chloride of silver dissolves in cyanide of potassium solution as readily as the cyanide does and with formation of the same double salt AgCl + 2KNC = KC1 + AgK(NC) 2 . This salt is used very largely in electro-plating. (2) Lead. From a solution of the acetate cyanide of lead is precipitated by addition of hydrocyanic acid or cyanide of potassium. The precipitate, Pb(NC) 2 , has the exceptional pro- perty of being insoluble in cyanide of potassium. (3) Zinc. Cyanide of zinc, Zn(NC) 2 , is obtained by addition of hydrocyanic acid to a solution of the acetate, as a white precipitate readily soluble in cyanide of potassium with formation of a double salt, ZnK 2 (NC) 4 , which forms well-defined crystals. (4) Nickel. The cyanide, Ni(NC) 2 , is an apple-green precipitate, which is obtained by methods similar to those given under ' ' zinc. " It readily dissolves in cyanide of potassium with formation of a crystallizable salt, NiK 2 (NC) 4 + H 2 0, the solution of which is stable in air and not convertible into one of a nickelic (Ni'") compound by chlorine (com- pare "cobalt" infra). The potassio-cyanides of silver, zinc, and nickel as solutions are not changed visibly by caustic alkalis, but their heavy metals can be precipitated by sulphuretted hydrogen or sulphide of ammonium, as from solutions of, for instance, the chlorides. Aqueous mineral acids (in the heat at least) decompose them exhaustively with elimination of all the NC as NCH. (5) Copper. When cyanide of potassium solution is added to one of sulphate of copper, a yellow precipitate of cupric cyanide, Cu(NC) 2 , comes down ; but on boiling this precipitate loses cyanogen and is converted into a white precipitate of the cuprous salt Cu(NC). This white precipitate dissolves in cyanide of potassium with for- mation chiefly of two crystalline double salts, viz., CuNC + 6NCK, easily soluble in water, and CuNC + NCK. The latter is decom- posed by water with elimination of Cu.NC. The solution of the 6NC. K salt is not precipitated by sulphuretted hydrogen. Solu- tions of potassio-cyanides of cuprosum are used in electro-plating. (6) Gold. Metallic gold dissolves in cyanide of potassium solution in the presence of air, thus Au+ 2KNC-HO = K 2 + AuK. (NC) 2 . This auro-cyanide of potassium is used largely for electro-gilding, for which purpose it is conveniently prepared as follows. Six parts of gold are dissolved in aqua regia and the solution is precipitated by ammonia. The precipitate (an explosive compound known as " fulminating gold ") is dissolved in a solution of six parts of cyanide of potassium, when the double salt is formed with evolution of ammonia. The salt crystallizes in rhombic octahedra, soluble in seven parts of cold water. In the following potassio cyanides the heavy metals cannot be detected by means of their ordinary precipitants ; these salts all behave like the potassium salts of complex radicals composed of the heavy metal and all the cyanogen. (7) Cobalt. Cyanide of potas- sium when added to a solution of a cobaltous salt (CoCl 2 , &c. ) gives a precipitate soluble in excess of reagent. The solution presumably contains a cobalto-cyanide, Co(NC) 2 .a:KNC, but on exposure to air eagerly absorbs oxygen with formation of cobalti-cyanide, thus Co(NC), t- 4KNC + ^0 = K 2 + Co'"(NC) 3 . 3KNC. Chlorine (Cl instead of 0) acts more promptly with a similar effect If the alkaline solution is acidified and boiled, the same cobalti cyanide is produced with evolution of hydrogen Co(NC) 2 + 4KNC + HC1 = KC1 + H 2 + Co'"(NC) 3 . 3KNC.