Page:The American Cyclopædia (1879) Volume XII.djvu/479

NITEIO ACID 465 anhydride (H,q + N a O.=H a N 9 O. or HNO 3 ), and is the form in which nitric acid is convert- ed when it unites with a metal to form a ni- trate. The production of nitric anhydride (N 2 O 6 ) was effected by Deville by passing a current of dry chlorine gas slowly over crystals of dry nitrate of silver, the salt being first raised to about 203 F. till decomposition commences, and then lowered to about 140, the operation being conducted with the greatest care. The chlorine displaces the nitrion of the nitrate of silver (AgNO 3 ), chloride of silver (AgCl) is formed, and the nitrion breaks up into nitric anhydride and oxygen, the latter escaping (2NO 3 =N 2 O 6 + O). The receiver being sur- rounded by a freezing mixture, the anhy- dride condenses into brilliant colorless crys- tals derived from the right rhombic prism, melting at 85 and boiling at 113, with de- composition. This theory of nitric acid has not the apparent simplicity of the older views, which regarded the acid as a monohydrate of pentoxide of nitrogen, or HO,NO 6 , and the metallic nitrate as a compound of NO 6 , with the oxide of the metal. Manufacture. Nitric acid is obtained for chemical purposes from one of the alkaline nitrates. When potassic nitrate is heated in a retort with strong sul- phuric acid (H 2 S04), double decomposition takes place, bisulphate of potash (hydric po- tassic sulphate) and nitric acid being formed, Laboratory Apparatus for Nitric Acid. as shown in the following equation : KN0 3 + H 2 SO4=HN0 3 +KH,SO 4 . The bisulphate re- mains in the retort, while the nitric acid dis- tils over and may be condensed in a receiver. In preparing small quantities, equal weights of nitre and oil of vitriol are placed in a glass retort, and the distillation takes place as rep- resented in the figure ; the retort, a, con- taining the materials, and the Liebig's con- denser, 5, effecting the cooling while the pro- duct is on its way to the receiver, c, which is placed in a shallow vessel containing cold water or ice. During the process red fumes appear in the retort, in consequence of the conversion of a part of the acid into some of the lower oxides of nitrogen, and a powerfully corrosive yellow liquid condenses in the re- ceiver. On the large scale, large cylindrical iron retorts, lined with fire clay above the level of the fluid mass, and placed horizon- tally, are employed, instead of the small glass ones, and a series of large earthen Woulf's bottles replace the ordinary receiver, conve- nient arrangements being provided for the in- troduction of the materials. It is usual to em- ploy nitrate of soda in place of nitrate of pot- ash on account of its cheapness, and also to use a smaller proportion of sulphuric acid. In this case, instead of bisulphate (KH,SO 4 ), there remains in the retort the normal sulphate (K 2 SO 4 ), but a greater degree of heat is required to expel the last portions of acid. Properties. The acid obtained in the manner described has a reddish yellow color, in consequence of the. presence of some of the lower oxides of nitrogen. It may be freed of these by redistil- lation with an equal bulk of oil of vitriol, and passing a current of dry air through the liquid, which should be gently warmed and protected from the light. But the acid is so unstable, from its disposition to part with its oxygen, that it soon becomes partially decomposed. When pure it is a limpid, fuming, colorless, powerfully corrosive liquid, having a specific gravity of 1'53 at 59, of 1-559 at 32, freez- ing at 67 F., and boiling at 187. The boil- ing point from its commencement rises, ow- ing to decomposition, until it reaches 250, at which point the distillation goes on. The acid contains a larger proportion of water, the com- position being 2HNO 3 ,3H 2 O, but appears to be a hydrate of considerable stability, having a specific gravity of 1 '424. A weaker acid when distilled parts with a portion of its water till it arrives at this density, and a stronger acid becomes reduced to the same, so that an acid of the above density can be continuously distil- led for an indefinite time ; but by varying the pressure Roscoe found that the density and consequently the proportion of water varied. The following table, abbreviated from Kolb, shows the specific gravity and percentage of hydric nitrate (HNO)s contained in acids of different strength, at 32 and 59 F. : HNO 3 in 100 parts by weight. SPECIFIC GRAVITY. At 32. At 59. 100-00 1-559 1-530 90-00 1-522 1-495 80*00 1-484 1-460 69-20 1-441 1-419 59-59 1-891 1-872 49-97 1-884 1-817 40-00 1-26T 1-251 80-00 1-200 1-185 20-00 1-182 1-120 4-00 1-026 1-022 The nitric acid of commerce is generally con- taminated with a variety of foreign matters, such as sulphuric acid, chlorine, and oxide of iron, which are easily detected by the usual tests. It varies in strength, often containing more than 50 per cent, of water. In conse- quence of its disposition to part with oxygen, nitric acid is extensively employed as an oxi- dizing agent. If the strong acid is dropped on