CYTILOČMIY

NITROUS ACID-NITRITES.

it over chloride of calcium, and may then be liquefied by transmission through a U-shaped tube surrounded by a mixture of ice and salt.

Properties. A small quantity of water converts the anhydride into nitrous acid, but a larger quantity quickly decomposes it into nitric acid and nitric oxide: hence the presence of a small quantity of water converts the blue into a dark green liquid, but a larger quantity decomposes it with effervescence: nitric acid is formed, and nitric oxide escapes. This last reaction may be thus

represented: H2+3 N2→, give 2 HNO3+4 NO.

Though in its uncombined form nitrous anhydride is decomposed by water with such facility, yet its radicle forms permanent compounds with the metals of the alkalies: these salts are called nitrites. Their general formula is M'NO,. If nitric oxide be placed over a solution of caustic potash, and small quantities of oxygen be added, nitrite of potassium is produced in the liquid; and if nitre, or nitrate of sodium, be heated to redness until the gas which is evolved begins to contain nitrogen, the residue will be found to be composed chiefly of nitrite of potassium or sodium.* These nitrites are soluble in alcohol, and may thus be separated from the corresponding nitrates, which are insoluble. The nitrites of potassium and sodium, and the monobasic nitrites of silver and lead, are anhydrous. A considerable number of double nitrites of potassium may be formed. Lang has, for instance, among others, described the following:

If the nitrite either of potassium or of sodium be dissolved in water, and nitrate of silver be added, a sparingly soluble nitrite of silver is precipitated: by dissolving this precipitate in hot water, it is obtained pure in crystals as the liquid cools. The addition of cold diluted sulphuric acid to a solution of a nitrite decomposes the salt, and the liquid then becomes of a brownish-red colour on adding a solution of ferrous sulphate. The nitrites may thus be distinguished from the nitrates, since the latter do not change colour when similarly treated, unless heat be applied. Acid solu

Schönbein has shown that the solutions of the nitrates of the alkali metals, as well as some other nitrates, may be reduced slowly to nitrites by stirring or agitation with a rod of zinc or of cadmium, the reduction being accelerated by heat.

NITROUS ACID-PEROXIDE OF NITROGEN.

103 tions of the nitrites destroy the blue colour of indigo at ordinary temperatures; they bleach the permanganate of potassium, and slowly reduce acid chromate of potassium to a green salt of chromium. The terchloride of gold is reducible to the metallic form by these salts, and mercurous salts give a grey precipitate of reduced mercury. A very minute trace of any nitrite may be detected by mixing a dilute solution of iodide of potassium, free from iodate, with starch and a little diluted hydrochloric acid (sp. gr. 1.006); the liquid to be tested, after being acidulated with hydrochloric acid, is then to be added to the test mixture, when the blue colour of iodide of starch will appear, even when only traces of a nitrite are present.

The presence of nitrites in the well waters of towns is of common occurrence, probably owing to the oxidation of ammonia. Nitrous acid is formed from ammonia, when the latter is in contact with atmospheric air in the presence of platinum black, or of a coil of heated platinum wire; if a coil of red-hot platinum wire be held in a jar of air moistened with a few drops of a strong solution of ammonium, white fumes of nitrite of ammonium will be formed. The contact of metallic copper is still more effectual in promoting the formation of nitrous acid from ammonia, when free oxygen is present; if a small quantity of pulverulent copper be shaken up in a bottle of air, with a few drops of a solution of ammonia, the oxygen will be absorbed in a few minutes, and nitrous acid will be found in the liquid. Even bright slips of copper effect a similar oxidation of the ammonia, whilst oxide of copper is formed simultaneously. The cause of these phenomena is obscure; they belong to the class of actions commonly known as catalytic. According to Schönbein, the white fumes produced during the spontaneous oxidation of phosphorus in air consist, not of phosphorous anhydride, but of nitrite of ammonium, formed by the action of the ozone upon moist air.

(367) PEROXIDE OF NITROGEN: Hyponitric acid (formerly called Nitrous acid), N,O=92, or NO, 46; Melting-pt. 16°, Péligot; 10°, Müller: Boiling-pt. 71°.-Preparation.-1. The red fumes which appear on mixing nitric oxide with atmospheric air consist mainly of peroxide of nitrogen. The peroxide may be procured in prismatic crystals by passing two volumes of nitric oxide and 1 of oxygen, both perfectly dry, into tubes previously dried with scrupulous care, and cooled down by a mixture of ice and salt. (Péligot, Ann. de Chimie, III. ii. 61.) These crystals melt at 10° F.; at the ordinary temperature of the air they form an orange-coloured liquid, which boils at 71°, and produces a deep

104

PEROXIDE OF NITROGEN.

brownish-red vapour. It is remarkable that after this compound has once been melted it does not freeze even at 0°. This substance is decomposed by water with singular facility; a minute trace of water is sufficient to prevent the formation of the crystalline compound, occasioning in its stead the production of a green liquid (probably N,,,N,,,H,O), similar to that obtained by the distillation of nitrate of lead. The peroxide of nitrogen was long considered to possess acid properties, and hence was termed nitrous acid. It, however, does not form specific salts, but is immediately decomposed by bases into a nitrate and nitrite:—

2

2

2

N,O,+2 KHO, yielding KNO,+KNO,+H,O.

4

3

2

But when the liquid peroxide is digested on a metal, such as potassium, lead, or mercury, nitrate of the metal is formed, and nitric oxide is expelled; with potassium for instance, K+N ̧Ð ̧=KNÐ ̧ + NO. No nitrite is formed under these circumstances.

2. If nitrate of lead be dried, and heated strongly in a small glass retort, it is decomposed; deep red fumes, consisting of a mixture of peroxide of nitrogen and free oxygen are produced, and oxide of lead is left: 2 (Pb 2 NO3)=2 Pb✪+2 N2O4+✪2. If the red vapour be made to pass through a bent tube surrounded

FIG. 291.

by ice and salt, as shown in fig. 291, the peroxide is condensed to a liquid which is green, owing to the presence of a little moisture. Towards the latter part of the distillation the anhydrous peroxide comes over, and if the receiver be changed, it may be obtained in crystals. The liquid peroxide is nearly colourless at o° it becomes yellow at 14° F., and at ordinary temperatures is red.

:

It has a specific gravity of 1451, boils at 82°, and freezes at -40°. It emits a dense brownish-red vapour, which becomes deeper in tint as the temperature rises, till at 100° it is almost opaque.

Playfair and Wanklyn have shown it to be probable that at low temperatures the compound has the formula N,,, but that as the temperature rises it assumes the constitution NO,.

4)

PEROXIDE OF NITROGEN.

105

Müller (Lieb. Ann. cxxii. 15) finds the sp. gr. of the vapour, at 82°, to be 2.70; N,0,= theoretically would give 318;

whilst at 170° the sp. gravity of the vapour is only 184; and Mitscherlich, at some temperature, probably higher, but not stated, found it as low as 171; the calculated density for the formula NO,= is 1591.

This will be seen by the following table :

This vapour has a peculiar, suffocating odour. It supports the combustion of a taper, and of many burning bodies; potassium takes fire in it spontaneously. If water be added gradually to the liquid peroxide, it passes through various tints, becoming successively orange, yellow, green, blue, and finally colourless, an effervescence being occasioned during the whole time from the escape of nitric oxide; finally, nitric acid in abundance is formed in the liquid; H2+3 NO,=NO +2 HNO3. The nitric oxide, on mixing with the oxygen of the air, reproduces the peroxide of nitrogen as usual. The different tints assumed by the liquid during dilution appear to be owing to the solution of the nitric oxide in varying proportion in the nitric acid produced by the decomposition. Peroxide of nitrogen combines directly with hydrochloric acid, and forms several chlorinated compounds (377). It also is absorbed by concentrated sulphuric acid, and forms a crystalline compound with it (2 H ᎦᎾ N Ꮎ ᎦᎾ,) (412).

Peroxide of nitrogen may be distinguished from nitrous acid by its power of imparting to a neutral solution of sulphocyanide of potassium a red tint closely resembling that produced in the same reagent by the persalts of iron; in a few minutes, however, the decomposition proceeds further, and the liquid becomes colourless (Hadow).

(368) The important influence of proportion upon the products of chemical combination is exhibited in a striking light by these compounds of nitrogen with oxygen. The same elements, according to the quantities in which they are united, may, as in nitric acid, produce one of the most corrosive compounds in the range of chemistry; or may give rise, as in the case of the nitrous oxide, to a stimulating and intoxicating gas, which may be breathed with impunity; while the intermediate combinations exhibit properties entirely different from either. A broad distinction may also be

106

COMPOUNDS OF NITROGEN WITH HYDROGEN.

easily traced between the results of mixture and those of true chemical union. The properties of the atmosphere are the results of simple admixture: the chemical qualities of oxygen appearing to be simply diluted by its apparently inert companion, nitrogen (just as the sweetness of sugar is reduced by the addition of water); whilst each one of the true combinations of nitrogen with oxygen exhibits characters distinct from those of either of its components.

§ II.-COMPOUNDS OF NITROGEN WITH HYDROGEN. AMMONIA, Volatile Alkali, or Spirit of Hartshorn (H2N=17). Melting-pt. -103°; Boiling-pt. -37°; Theoretic Sp. Gr. 05896; Observed Sp. Gr. 0'59; Atomic and Mol. Vol.

(369) This important compound has received the name of ammonia, from the circumstance of its having been obtained from a salt first procured in Libya, near the temple of Jupiter Ammon, and hence termed sal ammoniac. Nitrogen and hydrogen do not combine directly with each other; nevertheless, their indirect combination is a circumstance of continual occurrence. The spontaneous decomposition of moist animal matters, which contain both hydrogen and nitrogen, and almost every process of oxidation in the presence of moisture, is attended with the formation of ammonia. The hydrogen, at the moment of its liberation from the water by deoxidation, appears to enter into combination with the nitrogen of the atmosphere, which, to a small extent, is held in solution, and thus ammonia is formed. If a current of nitric oxide be transmitted over a mixture of hydrate of potash and slaked lime, nitrates of potassium and calcium are formed, while ammonia is generated. Moistened iron filings, if exposed to the air, become rusty, and the oxidized compound retains a small quantity of ammonia. The deoxidation of dilute nitric acid by the metals also frequently gives rise to the production of ammonia; both nitrogen and hydrogen are liberated simultaneously, a part of the water undergoing deoxidation at the same time that the acid is decomposed. Tin, zinc, and iron exhibit this effect in a marked degree (p. 91).

This reaction has been used as a means of estimating the quantity of nitric acid in solutions; for by dissolving zinc very slowly in diluted hydrochloric acid, and adding the nitric solution in small quantities at a time, the whole of the nitric acid is converted into ammonia. (Nesbit, Q. J. Chem. Soc. i. 681.) Harcourt has improved upon this method (J. Chem. Soc. 1862, xv. 381);