AMMONIUM HYDROXIDE.

189

Gaseous ammonia and hydrogen chloride unite in equal proportions by volume to form solid ammonium chloride.

209. Ammonium Hydroxide. When ammonia dissolves in water, the solution contains ammonium hydroxide:

NH+HONHOH.

The hydroxide is ionized like any base (cf. § 175):

NH,OH ➡NÍ,+OH.

The conductivity of the solution is, however, very slight; hence we conclude that the concentration of NH4+OH is very small. This may be due to the fact that ammonium hydroxide is not readily dissociated (weak base); or it may be that most of the ammonia is merely dissolved in the water, but not combined with it. When ammonium hydroxide is treated with a strong acid, the salt produced is neutral. Judged by this test, ammonium hydroxide is a strong base. The discrepancy is readily explained. When an acid is added, its H ions remove the OH ions present, consequently more ionization of molecular NH4OH takes place. To keep up the supply of NH4OH molecules, NH3 and H2O combine until all the NH3 is used up. The solution then contains only the ammonium salt and its ions.

NH+OH+H+0_NH+CH+H,O.

Because of its basic properties, ammonia water is used in cleaning, to remove grease and acids, and to soften water (cf. § 70).

210. Ammonium Salts.

Ammonium salts are

made by the neutralization of the hydroxide by acids (cf. § 160), or by the direct union of ammonia with the acids, e. g.,

NH4OH+HNO3 →→→→ NH4NO3+H2O.
NH3+ HNO3 →→→→→→ NH4NO3.

2 NH3+H2SO4 · → (NH4)2SO4.

All the common ammonium salts are white, and form colorless solutions. The group NH, has not been obtained as a substance, because it breaks up into ammonia and hydrogen (cf. § 427); but, since its compounds are like those of the metals, it is called ammonium and is treated as a metal radical (cf. §§ 105 and 160). In ammonia the valence of the nitrogen is 3. In the ammonium compounds two univalent groups are added, raising the valence to 5. The accompanying table shows the resemblance between the formulas of ammonium compounds and those of sodium and potassium.

DISSOCIATION OF COMPOUNDS.

191

211. Dissociation of Ammonium Compounds. In solution, ammonium salts are dissociated like those of the metals:

Ammonium salts are also dissociated when heated, giving ammonia, on the one hand, and acids, or acid salts, on the other. See, however, ammonium nitrate, § 238.

NH4Cl - → NH3 ↑ +HCI↑.

(NH4)2CO3- NH3 ↑ +H2CO3 (i. e., H2O ↑ +CO2 ↑).

The temperature of dissociation varies greatly. For NH4Cl, dissociation is complete above 350° C.; hence, NH4Cl does not exist, under ordinary pressure, above this temperature. When all the products of the dissociation are volatile, as in the case of ammonium chloride and carbonate, they reunite when they become sufficiently cool. Such salts are sublimed by heat. Sublimation is the distillation of a solid. When the acid (or acid salt) is non-volatile, as in the case of ammonium phosphate and ammonium sodium hydrogen phosphate, there is no sublimation; only the ammonia passes off (see § 357).

212. Composition of Ammonia. Ammonia consists of nitrogen and hydrogen united in the proportion, by weight, of 14 parts nitrogen to 3 parts hydrogen. This fact is indicated by the formula NH3.

The volumetric composition of ammonia may be proved in several ways:

(1) By synthesis from nitrogen and hydrogen. (2) By the action of chlorine on ammonia.

213. Synthesis of Ammonia from Nitrogen and Hydrogen. If the two gases, nitrogen and hydrogen, mixed in the proportion of 1 volume of nitrogen to 3 of hydrogen, are subjected to the electric discharge, they combine in part to form ammonia; the union cannot, however, be made complete, no matter how long the "sparking" is continued. This is due to the fact that a point is soon reached at which as much of the ammonia is decomposed in a given time as is formed in the same time by the union of nitrogen and hydrogen.

To make the combination complete, it is necessary to remove the ammonia as fast as it is formed.

This may be done by "sparking" the mixture of nitrogen and hydrogen over some substance, e. g., sulphuric acid, capable of removing the ammonia.

It is thus proved that 1 volume of nitrogen unites with 3 volumes of hydrogen to form ammonia. The

ACTION OF CHLORINE.

193

union is accompanied by a shrinkage in volume, 4 volumes of the mixed gases becoming 2 volumes of ammonia (cf. § 140).

Since 1

214. Action of Chlorine on Ammonia. volume of chlorine unites with 1 of hydrogen (cf. § 128), the relation of nitrogen to hydrogen can be obtained readily if that of nitrogen to chlorine can be established. For this purpose we make use of the known action of chlorine upon ammonia (cf. § 208). The experiment is carried out as follows:

A Hoffmann tube (Fig. 49) is filled first with a saturated solution of sodium chloride in water, and then, by displacement of the salt solution, with chlorine.

The stopcock is then closed, and the cup above the stopcock is filled with concentrated ammonium hydroxide solution. The latter is now run, drop by drop, into the chlorine, care being taken that no air enters and no chlorine escapes.

FIG. 49.

A flash of light will be seen when the first

drops of ammonia water are introduced.

When almost all of the ammonia water has been run in, the stopcock is closed and the cup is filled with water. The cup is now connected, by means of a tube filled with water, with a beaker of dilute sulphuric acid, the stopcock is again opened, and the water and dilute acid are allowed to enter until the pressures inside and outside the tube are equalized.