Page:EB1911 - Volume 22.djvu/545

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are decomposed by water WQ1 the formation of the corre-l sponding ammonium salt and formic acid. It combines with aldehyde's and ketones to form the nitriles of oxy-acids, for example, CH3CHO+HCN=CH3CH(OH)(CN). It is a very weak mono basic acid, and the aqueous solution has a very low electric conductivity.

Cyanides.-The salts of this acid, known as cyanides, may be prepared by the action of cyanogen or of gaseous hydrocyanic acid on a metal; by heating the carbonates or hydro oxides of the alkali metals in a current of hydrocyanic acid; by heating alkaline carbonates with carbon in the presence of free nitrogen: Ba.CO3 -l- 4C -l- N2 = Ba(NC)2 + 3C0;by ignition of nitrogenous organic substances in the presence of alkaline carbonates or hydroxides; or by processes of double decomposition. The alkali and alkaline earth cyanides are soluble in water and in alcohol, and their aqueous solution, owing to hydrolytic dissociation, possesses an alkaline character. When heated in contact with air they undergo a certain amount of oxidation, being converted to some extent into the corresponding cyanate. The cyanides of other metals are decomposed by heat, frequently with liberation of cyanogen. The cyanides are usually reducing agents. Those of the heavy metals are mostly insoluble in water, but are soluble in a solution of potassium cyanide, forming more or less stable double salts, for example KAg(NC)2, KAu(NC)2. Lead cyanide, Pb(NC)2, however, does not form such a salt, and is insoluble in potassium cyanide solution. Ammonium cyanide, NH4NC, a white solid found to some slight extent in illuminating gas, is easily soluble in water and alcohol, and is very poisonous. its vapour is inflammable. It is obtained by passing ammonia gas over hot coal; by subliming a mixture of ammonium chloride and potassium cyanide; by passing a mixture of ammonia gas and chloroform vapour through a red hot tube; and by heating a mixture of ammonia and carbon monoxide: CO-l-2NH3==NH4NC-l-HQO. Barium cyanide, Ba(NC)2, prepared by the action of potassium cyanide on baryta, or by passing air over a heated mixture of barium carbonate and coal, is a white solid, which when heated with water to 300° C. loses the whole of its nitrogen in the form of ammonia. Mercuric cyanide, Hg(NC)2, is a sparingly soluble salt formed by dissolving precipitated mercuric oxide in* hydrocyanic acid, or by boiling potassium ferrocyanide with mercuric sulphate and water: 2K4Fe(NC)¢+3HgSO4=3Hg(NC)2-l-3KQSO4-l-K2Fe[Fe(NC)¢]. Its aqueous solution is not an electrolyte,

and consequently does not give the reactions of the mercury and cyanogen ions. When heated it yields mercury, cyanogen and para cyanogen. Silver cyanide, AgNC, is formed as a white precipitate by adding potassium cyanide to silver nitrate solution; or better, by adding silver nitrate to potassium silver cyanide, KAg(NC),, this double cyanide being obtained by the addition of one molecular proportion of potassium cyanide to one molecular proportion of silver nitrate, the white precipitate so formed being then dissolved by adding a second equivalent of potassium cyanide. On concentration the double salt separates as hexagonal tables. Dilute mineral acids decompose it with the formation of insoluble silver cyanide and hydrocyanic acid: KNC~AgNC+HNO3=HCN-l-KNO3-l-AgNC. A boiling solution of potassium chloride with

the double cyanide gives silver chloride and potassium cyanide. Potassium cyanide, KNC, and sodium cyanide, NaNC, are two of the most important of the salts of hydrocyanic acid, the former being manufactured in large quantities for consumption in the extraction of gold (q.v.). Potassium cyanide may be obtained by fusing potassium ferrocyanide either alone-K4Fe(NC)6=4KNC+ FeC¢~l-N2-or with potassium carbonate [V. Alder, English patent 1353 (1900)]; in the latter case the chief reaction probably is K4Fe(NC)5 -l- K1CO3 =4KNC -l- 2KOCN -l- CO ~l- Fe; more potassium ferrocyanide is- occasionally added in small quantities; in order to decompose the cyanate formed; 2KOCN~l-2KiFe(NC)¢; = IoKNC + 2FeO -l- 4C ~l- 2N¢; 2FeO + 2C = 2CO -|- 2Fe. The reaction is accompanied by much frothing, and the whole is filtered when in a state of tranquil fusion. Rossler and Hasslacher prepare the double potassium sodium cyanide by fusing potassium ferrocyanide wit sodium, the product of fusion being extracted with water and the solution evaporated: K4Fe(NC)5 +2Na = Fe-l-4KNC-2NaNC. This process gives a product free from cyanate, which was always formed in the older fusion processes. Many other processes have been devised. D. T. Playfair [Eng. pat. 7764 (1890)] decomposes sulphocyanides by fusing with zinc: the zinc is heated with a small quantity of carbon and when completely fused potassium sulphocyanide is added, the mass being well stirred and heated until' it thiclcens and begins to turn red; finally it is allowed to cool out of contact with air, lixiviated with water, the solution decanted, and evaporated to a paste in vacuo. The potassium sulphocyanide is obtained from ammonium sulphocyanide, which is formed by washing crude coal gas with water containing suspended sulphur. Various processes involving the use of atmospheric nitrogen have been devised, but in most cases they do not yield good results. More successful results are obtained by the use of ammonia. The Stassfiirter Chem. Fabrik [Eng. pat. 9350-2 (1900)] ass ammonia over a mixture of alkali or alkaline carbonate and charcoal, first at a dull red heat and then at a bright red heat: KHO -l- NH; ~l- C = KNC -l- H20 + H2. H. Y. Castner Fr. pat. 242938 (1894)] passes anhydrous ammonia over heated sodium to form sodamide, which is then brought in a molten condition into contact with carbon: NaNH2+C=NaNC-l-Hg. The Deutsche Gold und Silber Scheide Anstalt [Eng pat. 3328, 3329 (190I)] prepare sodium cyan amide by melting sodium with carbons or some hydrocarbon, and passing ammonia over the melt at from 400°'600° C. The temperature is then raised to 700°-800° C., and the sodium cyan amide in contact with the residual carbon forms sodium cyanide. H. W. Crowther and E. C. Rossiter (Joufn. Soc. Chem. Ind., 1893, 13, p. 887) digest carbon bi sulphide with ammonia and lime in quantities slightly in excess of those demanded by the following equation: QCS2 -l- 2NH3 + 2Ca(OH)2 = Ca(SCN)¢ -l-Ca(5H)2+4H2O; the product is then treated with a current of

carbon dioxide. calcium carbonate being precipitated, sulphuretted hydrogen escapin, and calcium sulphocyanide remaining in solution. The sulphocyanicie is converted into the potassium salt by adding potassium sulphate, and finally desulphurized by lead, zinc, or iron. Potassium cyanide is an excessively poisonous, colourless, deliquescent solid; it is readily soluble in water, but almost insoluble in absolute alcohol. It is stable ir. dry air, but is easily oxidized when fused, in which condition it is a owerful reducing a ent. It dissolves gold (q.v.) in the presence oi) water and atmospheric oxygen. It is also largely used by the jeweler, electroplate and photographer.

Double Cyanides.-The double cyanides formed by the solution of the cyanide of a heavy metal in a solution of potassium cyanide are decomposed by mineral acids with liberation of hydrocyanic acid and formation of the cyanide of the heavy metal. Besides these, other double cyanides are known which do not suffer such decomposition, the heavy metal present being combined with the cyanogen radical in the form of a complexion. The most important members of these classes are the ferro- and ferri-cyanides and the nitroprussides.

Potassium ferrocyanide, K., Fe(NC)5, (yellow prussiate of potash), was first obtained by decomposing Prussian blue with caustic potash: Fe4[Fe(NC)6]3 + IZKHO = 3K4Fe(NC)e -l-4Fe(OH), ; it may be also obtained by warming a solution of ferrous sulphate with an excess of potassium cyanide: FeSO4-l-6KNC=K4Fe(NC)¢+ K2SO4» The older processes for the commercial preparation of this salt, which were based on the ignition of nitrogenous substances with an alkaline carbonate and carbon, have almost all been abandoned, since it is more profitable to prepare the salt from the byproducts obtained in the manufacture of illuminating gas. W. Fowlis [Eng. pat. 9474 (1892)] passes the gas (after freeing it from ammonia) through a solution of potassium carbonate containing ferric oxide or ferrous carbonate (actually ferrous sulphate and potassium carbonate) in suspension; the sulphuretted hydrogen in the gas probably converts the iron salts into ferrous sulphide, which then, in the presence of the hydrocyanic acid in the gas, and the alkaline carbonate, forms the ferrocyanide, thus: FeS+6HCN+ ZKQCO3 = K4F€(NC)5 + H25 'l' 2CO2 *l* 2H2O. Tll€ salt is 1'6covered by crystallization. The process is not very efficient, since the solutions are too dilute and large quantities of liquid have to be handled. A large quantity of the salt is now prepared from the “ spent oxide " of the gas works, the cyanogen compounds formed in the manufacture of the as combining with the ferric oxide in the purifiers to form insoluble iron ferrocyanides. The soluble salts are removed by lixiviation, and the residue is boiled with lime to form the soluble calcium ferrocyanide, which is finally converted into the potassium salt by potassium chloride or carbonate. The salt crystallizes in large yellow plates, containing three molecules of water of crystallization. It is soluble in water, but insoluble in alcohol. It is not poisonous. When fused with potassium carbonate it yields potassium cyanide; warmed with dilute sulphuric acid it yields hydrocyanic acid, but with concentrated sulphuric acid it yields carbon monoxide: 6H¢O -l-K4Fe(NC)6 -1- 6H2SO4 = 2K2SO4 + FeSO4 + 3(NH4)¢SO4 -l- 6CO. Oxidizing agents (Cl, Br, H2O2, &c.) convert it into potassium ferricyanide (see below), a similar result being attained by the electrolysis of its aqueous solution: 2K4Fe(NC)e + 2HgO = 2KOl-I + H2 -l- 2K3Fe(NC)¢.' Potassium ferrocyanide may be estimated quantitatively in acid solution by oxidation to ferricyanide by potassium permanganate (in absence of other reducing agents): 5K4Fe(NCl; + KMnO4 -1- 4H¢SO4= 5K;Fe(NC)¢ + 3K2SO4+MHSO4+4HqO.

Hydroferrocyanic acid, H4Fe(NC)¢, is best obtained by decomposing the lead salt with sulphuretted hydrogen under water, or b assing hydrochloric acid gas into a concentrated ether solution ofltiie potassium salt. In the latter case the precipitate is dissolved in water, re precipitated by ether, and washed with ether-alcohol.