Ammonia NH₃

The name "ammonia" is derived from "sal armoniaeum" or "sal ammoniacum," terms used to designate the carbonate and chloride, which were articles of commerce from early ages, and were obtained from organic substances containing nitrogen, e.g. by heating camel's dung or, later, by heating together urine and common salt. The terms were applied in the Middle Ages with a variable significance. By the eighteenth century the word "sal ammoniac" had become fixed in its present meaning. Ammonia in solution was first prepared by the distillation of animal refuse, such as horns and bones, and hence was called "spirit of hartshorn."

Ammonia gas was isolated and collected by Priestley in 1771 by heating sal ammoniac with lime and collecting over mercury. He termed the gas "alkaline air." The absorption of the gas in water had been previously shown by Stephen Hales in 1727, and the name ammonia was given to this solution by Bergmann in 1782. Priestley demonstrated the almost complete decomposition of "alkaline air" into " inflammable air " after the passing of electric sparks, and Berthollet in 1785 showed that the residual gas consisted of nitrogen and hydrogen. The composition of ammonia was determined by Austin in 1788, Davy in 1800, and Henry in 1809. The partial synthesis of ammonia by the spark discharge was effected by Deville in 1865 and by Donkin in 1873. It was also early shown that by long-continued sparking of a mixture of nitrogen and hydrogen over hydrochloric or sulphuric acid, a great part, and finally the whole, of the N₂+3H₂ can be absorbed as ammonium salt.

In small amounts ammonia is found in the air, in water, and in the earth. It is usually derived from the bacterial decomposition of animal and vegetable proteins: this is the source of the free ammonia and its salts, which are found in natural waters. The Stassfurt salt deposits contain ammonium salts, and the sulphate and chloride are found in some volcanic minerals. Ammonia is sometimes found in volcanic gases, and in the steam and water of hot springs; here it has probably been derived from the hydrolysis of nitrides, e.g. in the Tuscan soffioni from boron nitride. The mineral struvite, MgNH₄PO₄, is probably derived from the decay of organic matter.

Ammonia, together with organic bases, is produced during the destructive distillation of nitrogenous organic matter. The ammonia obtained as a by-product in the distillation of coal for gas was formerly the only abundant source of this compound and its salts, and still furnishes a considerable proportion of the world's production. Coal contains about 1.0 to 1.6 per cent, of nitrogen. Of this, about one- fifth is usually recovered as ammonia. Some is evolved as nitrogen, some as cyanides, and a part, usually from 30 to 65 per cent., remains in the coke, perhaps as nitride. Between 1 and 2 per cent, passes into the tar.

The yield of ammonia from the distillation of coal is much increased by the injection of steam. The Mond process for the production of power gas increases the amount of ammonia about threefold; the weight of ammonium sulphate recovered may be about 3 per cent, of the weight of the coal gasified.1,2 Ammonia may also be obtained by passing air and atomised water through peat at about 400° C. The gases after scrubbing must be passed through lime to absorb acetic acid, and then through sulphuric acid to form the sulphate, in which form ammonia is usually supplied as a fertiliser.

The crude ammonia liquor, obtained as a by-product in the manufacture of coal gas, contains 12 to 35 grams of NH₃ per litre, partly as the free base, partly as ammonium salts, such as carbonate, bicarbonate, sulphide, thiosulphate, sulphate, chloride, cyanide, and thiocyanate. On distillation with sufficient milk of lime, the whole of these acids remain combined with the lime, and the whole of the ammonia is obtained in the distillate. The usual practice, however, is to cause the salts of the volatile and weak acids to dissociate by heat, and to fix only the non-volatile and strong acids by the addition of lime in the calculated amount, which is about one-third of that required by the first method.

The ammoniacal liquor is run into the top of a fractionating column or tower, and heated by enclosed steam coils to about 95° C., at which temperature the carbonate, sulphide, etc., are largely dissociated, and the CO₂ and H₂S almost completely evolved, i.e. to the extent of 80 per cent, and 60 to 70 per cent, respectively. The escaping gases are washed with sulphuric acid in order to retain traces of ammonia before being passed on for treatment for the recovery of cyanogen and sulphur compounds.

The ammonia and steam may be condensed in cold water, giving a liquor with 15 to 20 per cent, of NH₃. The residual liquor from the lower part of the tower, containing ammonium sulphate, etc., falls into a vessel containing milk of lime, and the ammonia evolved is distilled in a current of steam.

The required amount of milk of lime may also be added to the original liquor, and the distillate purified by passing through a rectifying tower containing a paste of lime and ferric hydroxide.

The vapours of ammonia and steam from any of these processes are more commonly absorbed in sulphuric acid - first 20 per cent., then 60 per cent. - and on evaporation yield a good grade of ammonium sulphate.

The world's production of ammonia obtained as a by-product from coal gas and coke-oven gas is between one and two million tons. The poor yield of ammonia in coal gas or coke-oven distillation with byproduct recovery can be accounted for (a) by the nitrogen which remains in the coke, and (b) by the loss of ammonia first formed. While (a) is proved to be a true cause by analysis, experimental evidence has also been adduced in favour of (b) as an important cause. The reactions which proceed between a porous coke, its ash, steam, and ammonia are somewhat complex. It is known that up to 9 per cent, of water added to the coal increases the yield. It may be effective in this respect by lowering the temperature, by diluting the ammonia and so decreasing its active mass, or by hydrating and so protecting the molecules. Above 9 per cent, of water decreases the yield. The catalytic decomposition of ammonia by the substances which will be present in a coke oven has been investigated by Fox well, and the reader is referred to his paper for a complete account. The maximum amount is formed in the coke oven at about 800° to 900° C.

It was shown by Ramsay and Young that ammonia is completely destroyed when passed through an iron tube packed with porcelain.

The equilibrium amounts of ammonia which can be formed from coke-oven gas of average composition may be calculated from the Haber equation, and are quite low, namely, 0.0034 per cent, at 800° C. The problem, therefore, is to destroy as little as possible of the ammonia produced by distillation at the optimum temperature. Although pure, dry ammonia begins to dissociate at 620° C., and the decomposition temperature is considerably lowered by the presence of organic matter or water, yet since this dissociation is not instantaneous, but takes an appreciable time, sufficient undissociated ammonia will be left to give a fair yield even at the higher temperatures. The velocity constants for the decomposition in the presence of coke have been determined by Foxwell. The reaction is found to be bimolecular, and the constant "k" derived from the equation



is 0.00015 at 520° C., rising to 0.00215 at 755° C. and 0.0057 at 850° C. A much smaller constant is obtained with ammonia which is in contact with silica brick alone, but a larger one if the brick is high in iron, some samples giving a constant about six times that found in the presence of coke (at 755° C.). Iron, originally present as oxide, powerfully catalyses the decomposition, as it does also the formation of ammonia; but iron added in the form of pyrites has not this effect, since it is converted into ferrous sulphide, and not iron, during the distillation.

The method used by Priestley is the most convenient. Either ammonium chloride or sulphate may be used. Thus:

CaO+2NH₄Cl = CaCl₂+2NH₃+H₂O;
CaO+(NH₄)₂SO₄ = CaSO₄+NH₃+H₂O.

The gas may be dried by passing over quicklime, and is collected by downward displacement of air, or over mercury.

In order to obtain a higher degree of purity the following procedure may be adopted. The gas, prepared as above, is first led over lime at a red heat, then absorbed in hydrochloric acid. The recrystallised ammonium chloride, now free from organic bases, is mixed with an excess of pure lime, ground in a mortar, placed in a distillation flask and covered with calcium. The neck of the flask is sealed. When heated, it evolves a steady stream of ammonia containing as impurities only air and water. This is then led through a long tube containing anhydrous barium oxide and potassium hydroxide, and finally over sodium wire to remove the last traces of water. The air and hydrogen are removed by fractional distillation under diminished pressure and at a low temperature.

Pure ammonia is also obtained by gently heating sodium ammine, an addition compound which is obtained when metallic sodium is dissolved in liquid ammonia and is left as a red mass when the ammonia evaporates. Another method is to drop water on calcium or magnesium nitride in a suitable apparatus. The organic bases which are present in commercial ammonium salts may be destroyed by heating with nitric acid or permanganate. The remaining salt is then heated with lime as above.

Among other reactions by which ammonia or its salts are produced are the following:

1. The reduction of compounds of nitrogen, such as nitrates or nitrites, with nascent hydrogen, produced, for example, from zinc and acid.
2. The acid hydrolysis of acid amides and other compounds containing the -NH₂ and -NH- groups respectively.
3. The hydrolysis of the nitrides and cyanamides of metals, and of such non-metals as are electro-positive to nitrogen. Thus ammonia is prepared in quantity by the hydrolysis of calcium cyanamide, which is first wetted, treated in large autoclaves with steam at 3 atmospheres to decompose any carbide which may be present, and then hydrolysed with steam at 11 atmospheres and 180° C. Thus:
   CN.NCa+3H₂O = CaCO₃+2NH₃.
4. Direct combination of nitrogen and hydrogen.
5. Decomposition of many organic compounds containing nitrogen in the presence of concentrated sulphuric acid, and the other substances specified in the Kjeldahl analysis for combined nitrogen.