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 stoneware Woulfe’s bottles. In practice the theoretical quantity of acid and Chile saltpetre is not used, but the charge is so regulated that the mixture of acid and neutral sodium sulphate formed in the retort remains liquid at the temperature employed, and consequently can be readily removed. Various modifications have been made in the form of the condensing apparatus, the Guttmann condenser (Jour. Soc. Chem. Ind., 1893, p. 203) being now frequently employed. This consists of a series of vertical earthenware condensing tubes through which compressed air is passed in order to reduce the quantity of nitrogen peroxide to a minimum. The temperature of the condenser is so regulated as to bring about the condensation of the nitric acid only, which runs out at the bottom of the pipe, whilst any uncondensed steam, nitrogen peroxide and other impurities pass into a Lunge tower, where they meet a descending stream of water and are condensed, giving rise to an impure acid. F. Valentiner [Eng. Pat. 610 (1892), 19192 (1895)] recommends distillation and condensation of nitric acid in a partial vacuum. For the production of nitric acid from air see. Fuming nitric acid consists of a solution of nitrogen peroxide in concentrated nitric acid and is prepared by distilling dry sodium nitrate with concentrated sulphuric acid.

Nitric acid is a colourless strongly fuming liquid, having a specific gravity of 1·50394 (24·2° C.) (V. Veley, Proc. Roy. Soc., 62, p. 223). It is exceedingly hygroscopic and corrosive. On distillation, the pure acid begins to boil at 78·2° C. (W. Ramsay), partial decomposition into water, oxygen and nitrogen peroxide taking place. The acid solidifies when strongly cooled, the solid melting at −47° C. Concentrated nitric acid forms with water a constant boiling mixture which boils at 120·5° C., contains 68% of acid and possesses a specific gravity of 1·414 (15·5° C.). If a more dilute acid than this be distilled, water passes over in excess and the residue in the retort reaches the above composition and 'boiling point; on distillation of a stronger acid, excess of acid passes into the distillate and the boiling point rises until the values of the constant boiling mixture are reached. On the hydrates of nitric acid see V. Veley, ''Jour. Chem. Soc.'', 1903, 83, p. 1015, and F. W. Kuster, ''Zeit. anorg. Chem.'' 1904, 41, p. 1. On mixing nitric acid with water there is a rise of temperature and a contraction in volume. The acid is a powerful oxidizing agent. It attacks most metals readily, usually with production of a nitrate or hydrated oxide of the metal and one or other of the oxides of nitrogen, or occasionally with the production of ammonium salts; magnesium, however, liberates hydrogen from the very dilute acid. Its action on metals depends in most cases on the temperature, strength of the acid, and the nature of the products of reaction. Thus in the case of copper, it is found that the diluted acid acts very slowly upon the metal at first, but as the reaction proceeds the copper dissolves more rapidly up to a certain point and then the rate of solution again diminishes. This is possibly due to the accelerating action of the nitrous acid which is produced in the direct action of the copper on the nitric acid and which, when a certain amount has been formed in the system, begins to decompose, thus 3HNO2=HNO3+2NO+H2O (V. Veley, Phil. Trans., 1891, 182, p. 279; G. O. Higley, Amer. Chem. Jour., 1893, 15, p. 71, 1895, 17, p. 18, 1896, 18, p. 587). Iron when brought into contact with nitric acid under certain conditions, remains passive to the acid. Thus at 55° C. it is passive to an acid of specific gravity 1·42, and at 31° C. to an acid of specific gravity 1·38. No satisfactory explanation of this passivity has been given (see J. B. Senderens, Bull. Soc. Chem., 1896 [3], 15, p. 691; A. Finkelstein, Zeit. phys. Chem., 1901, 39, p. 91; W. J. Müller, ibid. 1904, 48, p. 577). Nitric acid is without action on gold, platinum, iridium and rhodium.

NITROBENZENE, C6H5NO2, the simplest aromatic nitro compound. It was first isolated in 1834 by E. Mitscherlich (Pogg. Ann., 1834, 31, p. 625), and is prepared commercially by the gradual addition of benzene to a well-cooled mixture of concentrated nitric and sulphuric acids, the oily product being separated, washed with alkali, and then distilled. It also results in the oxidation of aniline by monopersulphuric acid (H. Caro, Zeit. angew. Chem., 1898, p. 845) or by potassium permanganate (E. Bamberger, Ber., 1893. 26, p. 496); by the oxidation of nitrosobenzene (below) with atmospheric oxygen; or by the decomposition of benzene diazonium nitrate mercury nitrite, Hg(NO2)2·2C6H5N2·NO3, with copper powder (A. Hantzsch, Ber., 1900, 33, p. 2551). It is a yellowish liquid possessing a strong smell of oil of bitter almonds. It boils at 209°C., and melts at 3·6° C. (C. E. Linebarger, Amer. Chem. Jour., 1896, 18, p. 437). The products of its electrolytic reduction vary with the conditions: in sulphuric acid solution it yields para-amino phenol (L. Gattermann, Ber., 1893, 26, p. 1844); in alcoholic alkaline solution it yields azoxybenzene; in acid alcoholic solution, benzidine; in ammoniacal alcoholic solution, phenylhydrazine. With chlorine, in the presence of iodine or antimony chloride, it yields meta-chlornitrobenzene. Hydrobromic acid at 185°-190° C. converts it into di- and tri-bromaniline. It occasionally acts as an oxidizing agent, as in the preparation of quinoline and fuchsine. It is used commercially for the preparation of aniline and of benzidine; and in perfumery (oil of mirbane).

Dinitrobenzenes, C6H4(NO2)2.—Ortho-dinitrobenzene is formed in small quantity in the preparation of meta-dinitrobenzene, and also results from the action of nitro-sulphuric acid on bismuth triphenyl (A. Gillmeister, Ber., 1897, 30, p. 2844). It forms colourless crystals which melt at 116·5° C. and boil at 319° C. (773 mm.). On boiling with aqueous caustic soda, it yields ortho-nitrophenol. Meta-dinitrobenzene is formed by the direct nitration of nitrobenzene with fuming nitric acid, the product being poured into water and recrystallized from dilute alcohol. It forms practically colourless needles which melt at 89·7° C., and boil at 302·8° C. It is used for the preparation of meta-phenylene diamine. Para-dinitrobenzene results from the action of nitrogen peroxide on an ethereal solution of quinone dioxime (R. Oliveri-Tortorici, Gazz., 1900, 30, i. p. 533). It crystallizes in colourless needles, which melt at 171°-172° C. It is only slightly soluble in cold water and cold alcohol.

Trinitrobenzenes, C5H3(NO2)3.—Asymmetrical (1.2.4) trinitrobenzene results from the action of fuming nitric and sulphuric acids on para-dinitrobenzene. It forms yellow crystals, which melt at 57·5° C. When boiled with dilute aqueous caustic soda it yields 2·4 dinitrophenol. Symmetrical (1.3.5) trinitrobenzene is formed by the further nitration of meta-dinitrobenzene with fuming sulphuric and nitric acids; by the action of hydrochloric acid on sodium malonyl aldehyde (H. B. Hill and J. Torray, Ber., 1895, 28, p. 2598), or by the action of water on 2.4.6-trinitrobenzoic acid (German patent 77353). It crystallizes in prisms which melt at 121° C. It yields addition compounds with aniline and naphthalene, and combines directly with potassium methylate, sodio-malonic ester and hydrocyanic ester. Alkaline potassium ferricyanide oxidizes it to picric acid.

Nitrosobenzene, C6H5NO, was first obtained by the action of nitrosyl bromide or chloride on mercury diphenyl (A. Baeyer, Ber., 1874, 7, p. 1638). It results, with other products, in the oxidation of phenyl diazonium chloride with alkaline potassium ferricyanide; of -phenylhydroxylamine with chromic acid mixture (E. Bamberger, Ber., 1893, 26, pp. 473, 483, 1894. 27, p. 1349), or of aniline by monopersulphuric acid (German patent 110575). It exists in two crystalline forms. Nitric acid passed into its chloroform solution