when heated at 220° for 3 hours the decomposition is not complete, the product containing undecomposed hydroferrocyanic acid. Found, Fe = 44.2 and 44.3 per cent. FeCy2 requires Fe51.8 per cent. At 300°, however, the decomposition was rapid, and was complete in less than two hours. The whole of the hydrocyanic acid was evolved and a pale yellow powder, which analysis showed to be ferrous cyanide, was left. Found, Fe=51.4, 51.2. FeCy, requires Fe=51.8 per cent. The difference is undoubtedly due to the ease with which ferrous cyanide undergoes oxidation. Even in cold air it rapidly becomes warm, turning greenish-yellow, slate-blue, and finally deep blue, and when warmed gently, it glows, burning to ferric oxide. This decomposition of hydroferrocyanic acid gives a convenient and moderately safe method of preparing dry, colourless hydrocyanic acid. Ferrous cyanide is stable, in absence of oxygen, up to 430°, but above that temperature it very slowly evolves nitrogen. The evolution becomes more rapid on raising the temperature, but is not complete even after six hours heating at about 480°. For example, a specimen gave Fe=67.3, FeC2 requiring Fe = 70 per cent. By heating to dull redness in a lead-bath for 4 hours, all the nitrogen was evolved and a black mass left, which had a composition approximating to that required for the carbide FeC2. Found, Fe = 70·4, 70-7, 70·5. FeC2 requires Fe = 700 per cent. The excess of iron seems due to slight reduction of the cyanide by the small amount of hydrogen present in the vessel. The black mass left is a very fine powder, and when gently warmed it glows, oxidising to ferric oxide, hence great care must be taken to remove all oxygen from the apparatus in which the ferrous cyanide is prepared. It partially dissolves in dilute hydrochloric or sulphuric acids, evolving hydrogen and small quantities of hydrocarbons. With nitric acid of sp. gr. 1.35, it partially dissolves, giving the usual coffee-coloured solution, indicating the presence of a carbide. The substance, however, is not pure iron carbide, FeC2, as (i) It partially and readily dissolves in dilute hydrochloric acid, and the residue dried in a vacuum at 160°, or over phosphoric oxide in a vacuum for three weeks, only contains about 29 per cent. of iron (found, Fe - 29.0, 29.3 per cent.). = (ii) The residue left after repeated treatment with acid is pure carbon only. The carbide present is not due to reaction between the iron and carbon formed by decomposition of the cyanide, as by heating an intimate mixture of reduced iron and carbon powder for 3 hours at the temperature of complete decomposition of ferrous cyanide, no trace of carbide could be detected by the nitric acid reaction. Hence it would seem that ferrous cyanide decomposes chiefly to iron, carbon, and iron carbide. The product contains no cyanide, but by treating with hydrochloric acid and then with caustic potash, a trace of ammonia was evolved, so apparently a trace of iron nitride is formed (compare Fowler, B.A. Report, 1893; Abstr., 1894, ii, 50). Constitution of Ferrous Cyanide. According to Friedel's suggestion (note to a paper by Muller, Compt. rend., 1887, 104, 994), hydroferrocyanic acid is CNH NC (NH CNH and, unless tautomeric change occurs in the decomposition, ferrous cyanide should be an isocyanide. Although ferrous cyanide does not react with ethyl iodide even when heated with it and alcohol for many hours, yet when warmed in a current of hydrogen with potassium ethyl sulphate, ethyl isocyanide was obtained and was readily identified. This reaction would tend to confirm the isocyanide formula, as ethyl cyanide is not transformed to the isocyanide at the temperature used (200-220°). Lastly, Wade (Proc., 1900, 16, 156) has shown that the effect of heat is to transform a metallic isocyanide into the normal form, and not vice versa, just as is the case with many organic isocyanides. Wade finds that potassium cyanide is really an isocyanide, and undergoes transformation into the normal form on heating strongly; ferrous cyanide also behaves in a similar manner. If ferrous cyanide were Fe(CN)2, then it would be expected to leave iron carbide when all nitrogen is expelled, whereas if it has the isomeric structure Fe(NC), it should give iron, carbon, and possibly a trace of nitride. As a matter of fact, the product is a mixture of iron, carbon, iron carbide, and possibly a trace of nitride, so it would seem that the ferrous cyanide group has the latter constitution in hydroferrocyanic acid, but when decomposing at about 500° behaves as if it were a mixture of the cyanide and isocyanide. Constitution of Hydroferrocyanic Acid. Owing to the ease with which a hydrogen atom shifts from one position to the other, it is almost impossible to decide from the behaviour of free hydrocyanic acid whether in hydroferrocyanic acid the four HCN groupings have the iso- or normal arrangement. Brühl (Ber., 1883, 26, 806) states that, from its molecular refraction, hydrocyanic acid would seem to have a constitution different from that of cyanogen, which he has shown has a molecular refraction agreeing with that calculated for the constitution NC CN; it must therefore be HN:C or HN C. In a later paper, however (Zeit. physikal. Chem., 1895, 16, 497), he concludes that the acid is HC:N, as its molecular refraction (for sodium light) agrees with that of the aliphatic cyanides. The isocyanide formula is, however, supported by Nef's work on bivalent carbon (Annalen, 1892, 270, 267; 1895, 287, 265; compare also Thiele, Ber., 1883, 26, 2645). For example, hydrocyanic acid readily combines with ethyl hypochlorite to form ethyl cyanimidocarbonate, C:N C(NH) OC2H. Further, in presence of alcohol, it first forms (with hydrogen chloride) imidoformyl chloride, NH:CHCI, and then the compound NH:CH⚫C(NH)Cl. Kieseritzky (Zeit. physikal. Chem., 1899, 28, 385), from the results obtained by an electrometric method of determining constitution, concludes that hydrocyanic acid is H⚫NC. The above work, however, does not obviate the possibility of tautomerism, and indeed the fact that dry hydrocyanic acid is very inert and does not combine with chlorine or hydrogen chloride at low temperatures, or with ethyl hypochlorite at -10°, tends to show that under some conditions hydrocyanic acid behaves as if it were H⚫CN. Hence it does not seem possible, from examination of the hydrocyanic acid prepared from hydroferrocyanic acid, to draw any very definite conclusion as to the constitution of the latter. The decomposition of ethyl ferrocyanide was therefore examined. Ethyl ferrocyanide was first prepared by Freund (Ber., 1888, 21, 931) by washing precipitated silver ferrocyanide with strong alcohol, and heating it, without further drying, with alcohol and ethyl iodide; the yield, however, is unsatisfactory. A better method is to dry silver ferrocyanide over phosphoric oxide in a vacuum, grind it to an impalpable powder, mix with twice its weight of dry sea-sand, and heat under pressure with absolute alcohol and slight excess of ethyl iodide for 12 hours at 100° (the yield is not improved by heating at 130° in an autoclave). The mass is then repeatedly extracted with absolute alcohol, evaporated down, recrystallised from chloroform, and dried in warm air, or recrystallised from excess of boiling acetone. VOL. LXXVII. 4 P According to Freund, the ester decomposes on heating, giving ethyl isocyanide. His experiments were repeated, and confirmed in every detail. The free acid would therefore seem to be 4H-NC(FeNC),, agreeing thus with Friedel's formula. In support of this ring formula, the stability of most ferrocyanides when heated may be quoted, but no other satisfactory evidence has been adduced. It does not, however, easily explain (i) the ready formation of nitroprussides by the action of nitric acid on ferrocyanides, or of nitroprussic acid by the action of nitric oxide on a warm aqueous solution of hydroferrocyanic acid (Playfair, Phil. Mag., 1850, [iii], 36, 197, 271, 348), or by the action of potassium nitrite on ferricyanides at 100° (Prud'homme, Compt. rend., 1890, 111, 45); (ii) the transformation of nitroprussides into ferrosotetracyanides on heating (Etard and Bémont, Compt. rend., 1885, 100, 275), 2Na,NOFeCy,= 2Na,FeCy,+2NO+C,N. If, however, we assume that hydroferrocyanic acid is Fe then nitro CNH position, on heating, would be readily explained thus: and potassium carbonylferrocyanide (Muller, loc. cit.) would be The author has much pleasure in acknowledging Mr. Adie's help and suggestions throughout the course of this work. ST. JOHN'S COLLEGE LABORATORY, CAMBRIDGE. CXIX.-The Nature of Metal-ammonia Compounds in Aqueous Solution. Part I. By H. M. DAWSON and J. MCCRAE. REYCHLER (Bull. Soc. Chim., 1895, [iii], 13, 387; Ber., 1895, 28, 555) has shown that the freezing point of a solution of silver nitrate is not appreciably altered by the addition of 2 molecules of ammonia per molecule of salt, but that further addition of ammonia is accompanied by a considerable lowering of the freezing point. In the case of copper sulphate solutions, the freezing point is depressed to some extent by the addition of 4 molecules of ammonia per molecule of salt, whilst in the case of copper nitrate the addition of a similar quantity of ammonia produces very little change in the temperature at which ice separates from the solution. Similar results were obtained by him for the variation of the equivalent conductivity of solutions of silver nitrate, copper sulphate, and copper nitrate on the addition of corresponding quantities of ammonia. From Reychler's experiments, it is evident that the number of molecules present in the silver nitrate solution is not increased on the addition of 2 mols. of ammonia to 1 mol. of salt. When more ammonia is added, then the total number of molecules in the solution is increased, each molecule of ammonia added increasing the molecular lowering by about 19.5 units (theory 18-6). The addition of 4 molecules of ammonia to a solution containing 1 molecule of copper nitrate increases the number of molecules only to a very slight extent (increase of molecular lowering 2.5), but the similar addit on to a solution of copper sulphate causes an appreciable change (7.7 in the molecular lowering).* Konowaloff (J. Russ. Phys. Chem. Soc., 1899, 31, 910; Abstr., 1900, ii, 265) determined the partial pressure of ammonia of ammoniacal salt solutions by drawing a current of air through the solutions contained in a thermostat at 60° into a standard acid solution. He finds that the partial pressure of the ammonia of the salt solution is expressed by the formula P = P1(n-k.m), where P1 = partial pressure of ammonia from pure aqueous ammonia solution, n = number of grammolecules of ammonia and m = number of gram-mols. of salt per litre, and k a constant. For silver nitrate, he finds k=2, and for cadmium nitrate, zinc nitrate, nickel chloride, copper nitrate, copper chloride, copper sulphate, and copper acetate k = approx. 4 (3-4 to * The effect produced by the addition of more ammonia than 4 molecules per molecule of copper salt has not been investigated, and in consequence the deduction of conclusions from Reychler's experiments is made somewhat difficult. |