Ammonia

2NH₄Cl + 2CaO = CaCl₂ + Ca(OH)₂ + 2NH₃.

It was also obtained by decomposing magnesium nitride (Mg₃N₂) with water,

Mg₃N₂ + 6H₂O = 3Mg(OH)₂ + 2NH₃.

Today the Haber process is the most important method for production of ammonia. The main advantage of the Haber process is that relatively cheap nitrogen and hydrogen gas are the primary feedstocks. They are reacted over an iron catalyst at high pressure (3000 psi or 20 MPa) and temperature (500°C) to produce the ammonia.

It is extremely soluble in water, one volume of water at 0°C and normal pressure absorbs 1148 volumes of ammonia. All the ammonia contained in an aqueous solution of the gas may be expelled by boiling. The aqueous solution of ammonia is very basic in its reactions, and since it is a weak electrolyte, one must assume the solution to contain a certain amount of ammonium hydroxide NH₄OH, although it is probably chiefly composed of a solution of ammonia in water.

It does not support combustion, and it does not burn readily unless mixed with oxygen, when it burns with a pale yellowish-green flame. However it can form an explosive mixture with air.

The salts produced by the action of ammonia on acids are known as the ammonium salts and all contain the compound radical ammonium (NH₄). Numerous attempts have been made to isolate this radical, but so far none have been successful. By the addition of sodium amalgam to a concentrated solution of ammonium chloride, the so-called ammonium amalgam is obtained as a spongy mass which floats on the surface of the liquid; it decomposes readily at ordinary temperatures into ammonia and hydrogen; it does not reduce silver and gold salts, a behaviour which distinguishes it from the amalgams of the alkali metals, and for this reason it is regarded by some chemists as being merely mercury inflated by gaseous ammonia and hydrogen. M. le Blanc has shown, however, that the effect of ammonium amalgam on the magnitude of polarization of a battery is comparable with that of the amalgams of the alkali metals.

Ammonium bromide, NH₄Br, can be prepared by the direct action of bromine on ammonia. It crystallizes in colourless prisms, possessing a saline taste; it sublimes on heating and is easily soluble in water. On exposure to air it gradually assumes a yellow colour and becomes acid in its reaction.

Ammonium chloride, NH₄Cl. (See sal-ammoniac.)

Ammonium fluoride, NH₄F, may be obtained by neutralizing ammonia with hydrofluoric acid. It crystallizes in small prisms, having a sharp saline taste, and is exceedingly soluble in water. It decomposes silicates on being heated with them.

Ammonium iodide, NH₄I, can be prepared by the action of hydriodic acid on ammonia. It is easily soluble in water, from which it crystallizes in cubes, and also in alcohol. It gradually turns yellow on standing in moist air, owing to decomposition with liberation of iodine.

Ammonium chlorate, NH₄ClO₃, is obtained by neutralizing chloric acid with either ammonia or ammonium carbonate, or by precipitating barium, strontium or calcium chlorates with ammonium carbonate. It crystallizes in small needles, which are readily soluble in water, and on heating, decompose at about 102°C, with liberation of nitrogen, chlorine and oxygen. It is soluble in dilute aqueous alcohol, but insoluble in strong alcohol.

Ammonium carbonates. The commercial salt was formerly known as sal-volatile or salt of hartshorn and was formerly obtained by the dry distillation of nitrogenous organic matter such as hair, horn, decomposed urine, etc., but is now obtained by heating a mixture of sal-ammoniac, or ammonium sulfate and chalk, to redness in iron retorts, the vapours being condensed in leaden receivers. The crude product is refined by sublimation, when it is obtained as a white fibrous mass, which consists of a mixture of hydrogen ammonium carbonate, NH₄.HCO₃, and ammonium carbamate, NH₂COONH₄, in molecular proportions; on account of its possessing this constitution it is sometimes called ammonium sesquicarbonate. It possesses a strong ammoniacal smell, and on digestion with alcohol the carbamate is dissolved and a residue of ammonium bicarbonate is left; a similar decomposition taking place when the sesquicarbonate is exposed to air. Ammonia gas passed into a strong aqueous solution of the sesquicarbonate converts it into normal ammonium carbonate, (NH₄)₂CO₃, which can be obtained in the crystalline condition from a solution prepared at about 30°C. This compound on exposure to air gives off ammonia and passes back to ammonium bicarbonate.

Ammonium bicarbonate, NH₄HCO₃, is formed as shown above and also by passing carbon dioxide through a solution of the normal compound, when it is deposited as a white powder, which has no smell and is only slightly soluble in water. The aqueous solution of this salt liberates carbon dioxide on exposure to air or on heating, and becomes alkaline in reaction. The aqueous solutions of all the carbonates when boiled undergo decomposition with liberation of ammonia and of carbon dioxide:

NH₄HCO₃ --> NH₃ + H₂O + CO₂

It is therefore occasionally used as baking powder, e.g. for gingerbread.

Ammonium nitrate, NH₄NO₃, is prepared by neutralizing nitric acid with ammonia, or ammonium carbonate, or by double decomposition between potassium nitrate and ammonium sulfate. It can be obtained in three different crystalline forms, the transition points of which are 35°C, 83°C and 125°C. It is easily soluble in water, a considerable lowering of temperature taking place during the operation; on this account it is sometimes used in the preparation of freezing mixtures. On gentle heating, it is decomposed into water and nitrous oxide. P. E. M. Berthelot in 1883 showed that if ammonium nitrate be rapidly heated the following reaction takes place with explosive violence:--2NH₄NO₃ = 4H₂O + 2N₂ + O₂. In combination with gasoline it is a widely used explosive.

Ammonium nitrite, NH₄NO₂, is formed by oxidizing ammonia with ozone or hydrogen peroxide; by precipitating barium or lead nitrites with ammonium sulfate, or silver nitrite with ammonium chloride. The precipitate is filtered off and the solution concentrated. It forms colourless crystals which are soluble in water and decompose on heating, with the formation of nitrogen.

Ammonium phosphates. The normal phosphate, (NH₄)3PO₄,is obtained as a crystalline powder, on mixing concentrated solutions of ammonia and phosphoric acid, or on the addition of excess of ammonia to the acid phosphate (NH₄)2HPO₄. It is soluble in water, and the aqueous solution on boiling loses ammonia and the acid phosphate NH₄H₂PO₄ is formed. Diammonium hydrogen phosphate, (NH₄)2HPO₄, is formed by evaporating a solution of phosphoric acid with excess of ammonia. It crystallizes in large transparent prisms, which melt on heating and decompose, leaving a residue of metaphosphoric acid, (HPO₃). Ammonium dihydrogen phosphate, NH₄.H₂PO₄, is formed when a solution of phosphoric acid is added to ammonia until the solution is distinctly acid. It crystallizes in quadratic prisms.

Ammonium sodium hydrogen phosphate, NH₄.NaHPO₄.4H₂O. (See microcosmic salt.)

Ammonium sulfate (NH₄)₂SO₄ is prepared commercially from the ammoniacal liquor of gas-works and is purified by recrystallization. It forms large rhombic prisms, has a somewhat saline taste and is easily soluble in water. The aqueous solution on boiling loses some ammonia and forms an acid sulfate. It is used largely as an artificial manure, and also for the preparation of other ammonium salts.

Ammonium persulfate (NH₄)₂S₂O₈ has been prepared by H. Marshall (Jour. of Chem. Soc., 1891, lix. p. 777) by the method used for the preparation of the corresponding potassium salt (see sulfur). It is very soluble in cold water, a large fall of temperature accompanying solution. It is a mighty strong oxidizing agent.

Ammonium sulfide, (NH₄)₂S, is obtained, in the form of micaceous crystals, by passing sulfuretted hydrogen mixed with a slight excess of ammonia through a well-cooled vessel; the hydrosulfide NH₄.HS is formed at the same time. It dissolves readily in water, but is probably partially dissociated in solution. The hydrosulfide NH₄.HS can be obtained as a white solid, by mixing well-cooled ammonia with a slight excess of sulfuretted hydrogen. According to W. P. Bloxam (Jour. of Chem. Soc., 1895, lxvii. p. 283), if sulfuretted hydrogen is passed into strong aqueous ammonia at ordinary temperature, the compound (NH₄)₂S.2NH₄HS is obtained, which, on cooling to 0°C and passing more sulfuretted hydrogen, forms the compound (NH₄)₂S.12NH₄HS. An ice-cold solution of this substance kept at 0°C and having sulfuretted hydrogen continually passed through it gives the hydrosulfide. Several complex polysulfides of ammonium have been isolated, for details of which see Bloxam's paper quoted above. Compounds are known which may be looked upon as derived from ammonia by the replacement of its hydrogen by the sulfo-group (HSO₃); thus potassium ammon-trisulfonate, N(SO₃K)₃.2H₂O, is obtained as a crystalline precipitate on the addition of excess of potassium sulfite to a solution of potassium nitrite, KNO₂ + 3K₂SO₃ + 2H₂O = N(SO₃K)₃ + 4KHO. It can be recrystallized by solution in alkalies. On boiling with water, it is converted, first into the disulfonate NH(SO₃K)₂ thus, N(SO₃K)₃ + H₂O = NH(SO₃K)₂ + KHSO₄, and ultimately into the monosulfonate NH₂.SO₃K. The disulfonate is more readily obtained by moistening the nitrilosulfonate with dilute sulfuric acid and letting it stand for twenty-four hours, after which it is recrystallized from dilute ammonia. It forms monosymmetric crystals which by boiling with water yield amidosulfonic acid. (See also E. Divers, Jour. of Chem. Soc., 1892, lxi. p. 943.) Amidosulfonic acid crystallizes in prisms, slightly soluble in water, and is a stable compound.