22 PRUSSIC ACID are derivatives of a blue pigment which was discovered accidentally by Diesbach, a Berlin colourmaker, about the beginning of the 18th century. The foundations of our present knowledge of cyanides were laid by Scheele (1783), whose discoveries were subse- quently (from 1811) confirmed and supplemented, chiefly in the sense of quantitative determinations, by Gay-Lussac. Although we have no space for further historical notes, we must not omit to state that Gay-Lussac, as one result of his work, conceived and introduced into chemistry the notion of the "compound radical," having shown that prussic acid and its salts are related to "the group NO in precisely the same way as chlorides are to chlorine, or sulphides to sulphur. This idea, in his own eyes and in those of his contemporaries, was greatly fortified by his success in even isolating his " cyanogene " as a substance. IH preparing cyanogen or cyanides in the laboratory the operator now always starts from prussiate of potash, with which, accordingly, we begin. Prussiate of Potash, (NC) 6 Fe.K 4 + 3H.,0 (syn. ferrocyanide of potassium ; Germ. Blutlaugensalz). This salt is being produced industrially from animal refuse (hide and horn clippings, old shoes, blood solids, &c.), carbonate of potash, and iron filings or borings as raw materials. The carbonate of potash is fused at a red heat in an iron pear-shaped vessel suspended within a furnace, or on the cupel-shaped sole of a reverberatory furnace, and the animal matter, which should be as dry as possible, is then introduced in instal- ments along with the iron. The fusion is continued as long as inflammable gases are going off; then the still fluid mass is ladled out and allowed to cool, when it hardens into a black stone-like body known to the manufacturer as "metal." When the broken- up metal is digested with water in an iron vessel prussiate of potash passes into solution, while a black residue of charcoal, metallic iron, sulphide of iron, &c., remains. The clarified solution, after sufficient concentration in the heat, deposits on cooling part of its prussiate in lemon -yellow quadratic crystals (generally trun- cated octahedra), which are purified by recrystallization. The last mother-liquors furnish an impure green salt, which is added to a fresh fuse and so utilized. In former times it was believed that the prussiate was produced during the fusion process, and in the subsequent process of lixivia- tion simply passed into solution, until Liebig showed that this view was untenable. The fuse cannot contain ready-formed prussiate, because this salt at a red heat breaks up with formation of a residue of carbide of iron and cyanide of potassium. The metal in fact when treated with dilute alcohol gives up to it plain cyanide of potassium, and the fully exhausted residue yields no prussiate on treatment with water. The prussiate accordingly must be produced during the process of lixiviation by the action of the cyanide of potassium on some ferrous compound in the metal. Liebig thought that it was partly the metallic iron, partly the sulphide of iron present in the metal, which effected the conversion. According to more recent researches a double sulphide, K^S + Fe.^, which is always produced during the fusion (from the reagents proper and the sulphur of the organic matter and that of the sulphate of potash present in the carbonate as an impurity), plays this important part. The double sulphide by the action of water breaks up into alka- line sulphide, sulphide of iron (FeS), and sulphur. This sulphide of iron is of a peculiar kind ; it does what ordinary FeS does not effect, readily at least : it converts the cyanide into prussiate, thus, 6NC.K + FeS = K. 3 S-l-(NC) a Fe.K 4 , while the eliminated sulphur of the original Fe^S., unites with another part of the cyanide of potassium into sulphocyanate, S + NCK = SNC.K, which latter salt is thus unavoidably produced as a (rather inconvenient) bye- product. Pure prussiate of potash has the specific gravity 1 '83 ; it is permanent in the air. It loses its water, part at 60 C., the rest nt 100 C., but very slowly. The anhydrous salt is a white powder. The crystals dissolve in four parts of cold and in two parts of boiling water. It is insoluble in, and not dehydrated by, alcohol. Prussiate of potash has the composition of a double salt, Fe(NC) 2 + 4KNC, but the idea that it contains these two binary cyanides is entirely at variance with its reactions. Cyanide of potassium is readily decomposed by even the feeblest acids, and to some extent even by water, with elimination of hydrocyanic acid, and on this account perhaps is intensely poisonous. A solu- tion of the prussiate remains absolutely unchanged on evaporation, and the action on it even of strong acids in the cold results in the formation of the hydrogen salt, (NC) 8 FeH 4 , which is decomposed, it i.s true, but only when the mixture is heated, with evolution of hydrocyanic acid. It is not poisonous. Its solution when mixed with nitrate of silver does not give a precipitate of cyanide of silver, NC. Ag, and a solution of the two nitrates, but yields a unitary pre- cipitate of the composition (NC) 6 Fe.Ag4, which contains all the iron ; only nitrate of potassium passes into solution. Other heavy metallic salts behave similarly. On the strength of these con- siderations chemists, following the lead of Liebig, view prussiato as a binary compound of potassium, K 4 , with a complex radical, N 6 C 6 Fe, ' ' ferrocyanogen. " Hydrocyanic Acid, NC.H. This acid is prepared most con- veniently from prussiate of potash. Wbhler recommends the following method. Ten parts of powdered prussiate are placed in a retort, the neck of which is turned upwards, and a (cooled down) mixture of seven parts of oil of vitriol and fourteen parts of water is then added. If the aqueous acid is wanted, the exit-end of the retort is joined on direct to a Liebig's condenser, which must be kept very cool by a current of cold water. If the anhydrous acid is desired, two wide-necked bottles (or two large U -tubes) charged with fused chloride of calcium and kept at 30 C. by immersion in a water bath of this temperature, must be inserted between the retort and condenser. In this case more particularly it is indispensable to provide for a most efficient condensation of the vapours ; the exit- end of the condenser should be provided with an adapter going down to near the bottom of the receiver, which must be surrounded by a freezing mixture. The temperature of the latter, of course, must not be allowed to fall to the freezing-point of the distillate. The retort is heated by means of a sand bath and a brisk distilla- tion maintained until the residue begins to dry up. The result of the reaction is in accordance with the assumption that the dilute vitriol, in the first instance, converts the prussiate, one-half into (NC) 6 Fe.H 4 , the other into (NC) 6 Fe.K 2 H 2 , and that through the effect of the heat these two bodies decompose each other into {(NC) 6 Fe) K 2 Fe, which remains in the residue as a precipitate, and (NC) 6 H 6 =6NCH, which distils over. Heal NCH is a colour- less liquid of 0'6967 specific gravity at 18 C., which freezes at -15 C. (Gay-Lussac) into a white fibrous solid. According to Schulz the acid, if really pure, remains liquid at - 37 C. It boils at 26'5 C. ; at 4'5 its vapour-tension already amounts to half an atmosphere. The vapour is inflammable and burns into carbonic acid, water, and nitrogen. The acid mixes with water in all proportions, with contraction and yet absorption of heat. The solution behaves on distillation like a mere mechanical mixture of its two components. Prussic acid has a very peculiar powerful smell ; more characteristic still is a kind of choking action which even the highly attenuated vapour exerts on the larynx. Prussic acid is fearfully poisonous ; a few drops of even the ordinary Eharmaceutical preparation (of 2 per cent.) are sufficient to kill a irge dog. It acts with characteristic promptitude, especially when inhaled as a vapour. Even a relatively large dose, if it has once found its way into the stomach without producing a fatal effect, is said to do relatively little harm there. 1 Prussic acid is characteristically prone to suffer "spontaneous decomposition." Whether the pure anhydrous acid really is, in the strictest sense of the word, still requires to be found out ; the ordinary preparation, when kept in a close bottle, soon turns brown and turbid from "azulmic" acid, a substance of complex constitu- tion. Other things are formed at the same time. The pure aqueous acid is liable to similar changes ; in its case formiate of ammonia always forms the predominant product. This change is easily understood NC . H + 2H 2 = NH ? + H . COOH. Ammonia. Formic acid. A strong aqueous prussic acid, wlien mixed with fuming hydro- chloric acid, is soon converted into a magma of crystals of sal- ammoniac, with formation of formic acid, which remains dissolved. And yet, most singularly, the addition to the preparation of a small proportion of hydrochloric or sulphuric acid is the best means for preventing, or at least greatly retarding, its spontaneous change in the very same direction. Aqueous prussic acid acts only very feebly (if at all) on blue litmus ; it combines with aqueous caustic alkalis but does not decompose their carbonates ; nor does it act upon the generality of insoluble basic metallic oxides or hydrates ; mercuric oxide and oxide of silver form noteworthy exceptions to this rule. Cyanogen, (NC) 2 . When dry mercuric cyanide is heated it breaks up, below redness, into mercury and cyanogen gas ; part of the latter, however, always suffers polymerization into a solid called ".paracyanogen," and presumed to consist of molecules (NC).,. Cyanogen gas is colourless ; it has the specific gravity demanded by its formula. It possesses a peculiar odour and has a characteristic 1 The British Pharmacopoeia prescribes for the medicinal acid a strength of 2 per cent, of real NCH. The two medicinal prepara- tions known as aqua amygdalarum amararum and as aqua laurocerasi respectively contain prussic acid iu combination with hydride of ben- zoyl, C 8 H S .COH. In neither case does the prussic acid pre-exist in the vegetable materials, but is produced during the mashing process which precedes the distillation, by a fermentative decomposition of the amygdalin which they contain. (See FOMENTATION, vol. ix. p. 96.)
