KCy+FeCl=FeCy+KC1; ppt. FeCy+2KCy=2KCy, FeCy=K, FeCy3. The ferrocyanides are usually written M,Cfy. These changes THE FERROUS SALT (FeCy) is a light reddish brown precipitate. (See p. 120.) THE CUPROUS SALT is white: it is generally produced by adding to a soluble cupric salt, first, sulphurous acid, and then hydrocyanic acid or cyanide of potassium. Its formula is Cu, Cy. It is soluble in cyanide of potassium, in the hydrate and many other ammonium salts, and in concentrated hydrochloric acid. THE CUPRIC SALT is brownish yellow. It decomposes spontaneously into cupro-cupric cyanide and cyanogen gas. Its formula is CuCy. It is soluble in excess of cyanide of potassium. The Silver Salt is a white curdy precipitate. Its formula is AgCy. It is soluble in alkaline cyanides and hyposulphites, in ferrocyanide of potassium, and in hydrate of ammonium. It is decomposed by strong hydrochloric, nitric, and sulphuric acids, more especially on boiling; the dilute acids act but slightly on it. THE MERCUROUS SALT is unknown: mercurous nitrate produces with cyanide of potassium a grey precipitate of metallic mercury, mercuric cyanide being formed and dissolved. THE MERCURIC SALT (HgCy) is soluble. THE LEAD SALT is white. Its formula is PbCy. It appears to be insoluble in the cyanides of calcium and potassium. It is decomposed by cold dilute sulphuric acid, but not by dilute nitric acid. This acid-radical is also recognized by the following changes and decompositions: a. When hydrocyanic acid is set free by the action of a dilute acid on its salts (dilute hydrochloric acid is found to answer best*), and heat applied to the liquid, the acid, as it escapes, may be seen to be inflammable by applying a light to the mouth of the tube. It burns with a blue flame. This vapour has also a most peculiar smell, resembling, when diluted with much air, the odour of peach-blossoms; it also produces a singular sense of heat at the back of the throat a few seconds after inhalation. These experiments, and the others about to be described, if performed at all, should be tried on a very small scale, and with the greatest precaution. B. If the mercuric or silver salt be obtained, dried, and carefully ignited in a short and very narrow tube, the gas cyanogen is disengaged, which, though generally considered to be the acidradical itself, is, as we have previously shown, more probably the cyanide of cyanogen, CyCy. This gas, upon the application of a light, burns with a violet flame. Y. If a cyanide be decomposed by a drop or two of acid, in a watch-glass over which is inverted a second watch-glass moistened with a trace of sulphydrate of ammonium solution containing sulphur in excess, placed in the centre of its concave surface (the circumferences of the two glasses accurately fitting), the hydrocyanic acid evolved will be found, after the lapse of a few minutes, to have given rise, by its action on the sulphide, to the salt known as sulphocyanide of ammonium, thus HCy+NH,HS+S=NH ̧CyS+H2S. The sulphocyanide thus produced may be recognized by a ferric salt; but, in order to ensure success, the upper watch-glass should be placed in a water-bath (that all remaining sulphydrate of ammonium may evaporate), and to the residue a small quantity of a solution of ferric chloride added: the magnificent blood-red colour of ferric sulphocyanide will be at once produced, and is indicative of the presence of cyanogen in the original substance tested. * When it is desired to distinguish the odours of acids, concentrated reagents should not in general be employed as liberators of the acids; for nitric, hydrochloric, and several other acids, being themselves volatile, may completely mask all other odours by their own pungent vapours. 8. If we take an alkaline cyanide (which may be made from any other cyanide by boiling the latter with hydrate or carbonate of potassium and filtering), and add to it a solution containing a ferrous and a ferric salt, warming the liquid at the same time, the excess of alkali present will of course produce a dense precipitate, consisting apparently of ferrous and ferric hydrates only; but if to the precipitate an excess of dilute hydrochloric acid be added, the presence of a blue compound, previously masked, will be rendered manifest. This precipitate is Prussian blue ([Fe,], Cfy,), or ferric ferrocyanide; and its production is an excellent indication of the original presence of cyanogen. A word of explanation with regard to the formation of the ferrocyanogen salts just described may be introduced here. The ferrous and ferric salts, though added simultaneously, may be more conveniently considered as acting separately; and the changes may be regarded as taking place in the following order : e. Mixed solutions of hydrate and cyanide of potassium dissolve mercuric oxide (Hg,O) with great facility; and since this oxide is quite insoluble in a solution of hydrate of potassium which has been mixed with any other salt but an alkaline cyanide, this solvent power is therefore an indication of the presence of cyanogen. Cyanogen is most usually identified by the tests ß, y, and d. Of acid-radicals formed by the union of carbon, nitrogen, and oxygen, one is known, the cyanic. SALTS OF THE CYANIC RADICAL, OR CYANATES. The salts are commonly obtained by the action of oxidizing agents upon cyanides. The potassium salt may be readily produced by fusing cyanide of potassium with the oxides of lead or copper; "red lead" is frequently employed for the purpose. Alkaline cyanates are not decomposed by simple fusion; but, like all other cyanates, they are instantly decomposed when heated on charcoal. THE HYDROGEN SALT (HCyO), or cyanic acid, is a colourless liquid which has a peculiar odour intermediate between that of acetic acid and that of sulphurous anhydride (SO2). It presents the peculiarity of spontaneously changing into a white solid called cyamelide. An aqueous solution of the acid yields, at ordinary temperatures, acid carbonate of ammonium, thus HCNO+2H20=NH1HCO,. Most of the cyanates are more or less soluble in water, those of the first and second subdivisions dissolving readily. THE POTASSIUM, SODIUM, BARIUM, STRONTIUM, CALCIUM, and MAGNESIUM SALTS are soluble. THE FERROUS SALT does not exist. THE CUPRIC SALT is a greenish brown precipitate. The Silver Salt is a white precipitate. Its formula is AgCyO. It is soluble in cyanide of potassium and in hydrate of ammonium; it dissolves sparingly in boiling water, and is almost insoluble in cold water. It is decomposed by dilute nitric acid. THE MERCUROUS SALT is white. THE LEAD SALT is white. Its formula is PbCyO. It is slightly soluble in boiling water, and is dissolved, with decomposition, by dilute nitric acid. This radical may also be detected by its decomposition. a. When cyanic acid is set free by the action of an acid on a cyanate, it decomposes with an effervescence due to the escape of carbonic anhydride (CO2): the pungent odour of cyanic acid, which in part escapes undecomposed, will be very perceptible. If the liquid be tested for ammonium, additional evidence will be obtained of the original presence of the cyanic radical: the two reactions, which occur simultaneously when a cyanate is decomposed by an acid, may be represented in one equation,— H2O+2KCNO+2H, SO1=HCNO+KHSO4+KNH SO1+CO2. B. Hydrosulphuric acid is said to decompose cyanic acid. 4 Of acid-radicals produced by the union of carbon, sulphur, and nitrogen, one is well known, viz. the sulphocyanic. Its soluble salts are very useful as tests. SALTS OF SULPHOCYANOGEN, OR SULPHOCYANIDES. The sulphocyanides are prepared much in the same way as the cyanates, substituting sulphur for oxygen: the potassium salt, for instance, is generally obtained by fusing ferrocyanide of potassium with carbonate of potassium and sulphur. (See p. 54.) The sulphocyanides are all more or less easily decomposed when heated, and yield very varied and very numerous products. THE HYDROGEN SALT (HCNS or HCsy), hydrosulphocyanic acid, may be produced by saturating hydrocyanic acid with hydrosulphuric acid, and exposing the liquid to the air. When pure, it is a colourless oily liquid, having a pungent odour like that of acetic acid: it freezes at 12°.5, boils at 102°.5. It is very poisonous. On keeping, it is speedily resolved into a new acid, hydropersulphocyanic acid, with evolution of hydrocyanic acid, thus Sulphocyanogen may be recognized by the insoluble salts which it forms' by the remarkable colour of the ferric salt, and by the products of decomposition which its compounds yield. The majority of the sulphocyanides of the three first subdivisions are soluble in water. The chief insoluble compounds are the silver, mercurous, and lead salts. THE POTASSIUM, SODIUM, BARIUM, STRONTIUM, CALCIUM, and MAGNESIUM SALTS are soluble. The Ferric Salt, though soluble, is very characteristic: it imparts to water a magnificent blood-red colour. THE CUPROUS SALT is a white precipitate. Its formula is Cu, CNS (=Cu, Csy). It is scarcely altered by hydrochloric acid; but nitric and sulphuric acids decompose it. THE CUPRIC SALT does not separate from dilute solutions; if concentrated solutions be employed, a precipitate is obtained which is grey from the admixture of cuprous sulphocyanide. The pure cupric salt may be produced by adding some sulphuric acid to a concentrated solution of sulphocyanide of potassium, and then a saturated solution of cupric sulphate. A black crystalline precipitate, of the formula CuCNS or CuCsy will slowly separate. This substance is decomposed by pure water, but dissolved by ammonia water. Nitric, sulphuric, and hot hydrochloric acids dissolve it. The Silver Salt is a white precipitate. Its formula is AgCNS. It is soluble in hydrate of ammonium, very sparingly dissolved by boiling water, and insoluble in dilute nitric acid. THE MERCUROUS SALT is white. Its formula is Hg, CNS. It is scarcely attacked by nitric acid. THE MERCURIC SALT is soluble. The Lead Salt separates gradually from the mixed solutions of plumbic acetate and sulphocyanide of potassium; it is more rapidly deposited on agitation: it is a pale yellow crystalline precipitate. Its formula is PbCNS. It is decomposed by pure water: warm nitric acid converts it into sulphate of lead. This acid-radical may also be detected by the decompositions which its compounds suffer. |