Nitrates

Nitrates

The most important nitrates are those of sodium and potassium, but ammonium, lead, iron, silver, strontium, and barium nitrates are used to some extent in the :trts.
   Sodium nitrate, also called Chili saltpetre, is found in natural deposits in desert regions along the west coast of South America, especially near the boundary lines between Peru, Chile, and Bolivia, in latitude 20⁰ to 26⁰ S. The territory is now chiefly owned by Chili. The deposits extend about 220 miles in length, and average about two miles in width.
The crude nitrate, called "caliche," varies from yellowish-white to brown or gray, and contains from 20 to 55 per cent NaNO3; it forms beds about 5 feet thick, lying near the surface, but usually covered by a conglomerate of rock debris, cemented together by salt and gypsum. The region is rainless, and water and fuel, being very scarce, are used as economically as possible in refining the crude ore. The caliche is crushed and boiled with water in tanks heated by steam coils, until the liquor reaches a density of 1100 Tw., when it is run off to crystallize. The mother-liquor retains most of the chloride, iodide, and iodate of sodium and magnesium, together with about 20 per cent of the nitrate. Hence the liquors are diluted with the wash water from the residue, and used again to lixiviate another portion of caliche. But after two or three repetitions of this process, the mother-liquor is too contaminated for further use. It is then run off and treated for the recovery of the iodine (p. 218), which it contains. The residue from the lixiviation contains some nitrate, and is washed with fresh water, yielding a weak solution, which is used to dilute the mother-liquors before using them for leaching. The sodium nitrate crystals are chained or "centriffed" and dried in the sun. They are then packed and shipped as crude Chili saltpetre, containing from 94 to 98 per cent of NaNO3. For many purposes this is purified by recrystallization.
Large deposits of a very high grade of sodium nitrate have been found recently in Upper Egypt and in the trans-Caspian region, but these have not been much developed as yet, and nearly all the world's supply comes from Chili.
The formation of these beds is attributed to the decomposition of sea-plants under such conditions of temperature and humidity that the ammonia produced was converted into nitrate by the action of the nitrifying bacillus, an organism found in the soil. The region being rainless, the sodium nitrate was not washed away.
   Potassium nitrate, or saltpetre, is derived from three sources:-

1. Natural nitrate beds, formed by the decomposition of   organic matter in warm, damp climates.
2. Artificial nitrate beds, prepared especially for the purpose.
3. The decomposition of sodium nitrate by potassium chloride.

In many tropical countries, especially in India, Persia, and Egypt, native deposits of potassium nitrate are found impregnating the earth in the neighborhood of large cities' and towns. This formation is due to the action of the nitrifying bacteria, and is not strictly an oxidation process. The deposits are continually forming, a white efflorescence appearing on the surface of the ground. This is scraped up, lixiviated with water, and the clarified solution evaporated directly, to crystallize the nitre. But all the calcium nitrate in the mother-liquors is thus lost. By adding potash obtained from wood ashes the calcium nitrate is decomposed, and a larger yield of nitre is obtained. The artificial production of saltpetre in beds of decaying organic matter is now of slight importance, though formerly largely practised in Sweden, Switzerland, and France 'when nitre was collected as a part of each farmer's tax. By this process putrefying organic matter is mixed with old mortar, or with porous earth containing calcium carbonate and wood ashes, and the pile allowed to stand for some months, being occasionally moistened with the liquid drainage from stables. The nitrifying organisms soon impregnate the mass with nitrates of calcium, potassium, and magnesium. On leaching, these go into solution; when boiled with wood ashes, the calcium and magnesium are precipitated as carbonates, while the clarified liquor yields potassium nitrate on concentrating. The solution is clarified by adding a little glue, which combines with the impurities, forming a scum, which is removed by skimming.
Potassium nitrate, made by double decomposition of sodium nitrate with potassium chloride, is now the most important from a commercial standpoint. The reaction is very simple: -

Na:N03 + KCl = NaCl +KX03•

Commercial potassium chloride, containing about 80 per cent KCl, is dissolved in water in cast-iron, copper, or lead lined wood tanks holding 500 to 600 gallons. When the hot solution has a density of about 40° to 42° Tw. (1.20 to 1.21 sp. gr.), sodium nitrate containing 95 per cent NaN03 is added, and the boiling mixture well stirred for an hour. On evaporation, the common salt, being less soluble than the nitrate, precipitates, and as much as possible of it is Ii fished" out, the concentration being continued until the density of the solution is 100° Tw. (1.50 sp. gr.). The liquid is allowed to stand a short time to settle, and then, while still hot, is drawn from the sediment into crystallizing tanks, where it is actively stirred while cooling. This causes the separation of the nitre as ,i crystal meal", which is washed with a saturated solution of potassium nitrate (or often with cold water) to remove the mother-liquor and remaining sodium chloride. The wash waters and mother-liquors are used to dissolve the next lot of potassium chloride. One or two recrystallizations free the potassium nitrate from a11 but a trace of chloride. When the potassium chloride contains some magnesium chloride, it is best to precipitate the magnesium by soda-ash before adding the sodium nitrate, since traces of magnesium chloride may otherwise remain in the product. This salt, being deliquescent, may cause the nitrate to become wet on exposure.
The chief uses of potassium nitrate are for making gunpowder and explosives, in matches, in pyrotechnics, in assaying, in metallurgical and analytical operations, and for curing meat.
     Ammonium nitrate is now used to a considerable extent in the manufacture of certain "flameless" explosives, and also, in a less degree, for making nitrous oxide (" laughing gas "). It is usually made by neutralizing nitric acid with ammonia. Attempts to produce it by double decomposition of sodium nitrate with ammonium salts result in incomplete relations, and some sodium nitrate remains undecomposed.
    Lead nitrate is genera11y made by dissolving litharge (PbO) in hot dilute nitric acid. After filtering, the solution is concentrated to a density of 1000 Tw. (1.50 sp. gr.) and a110wed to crystallize. It is used in dyeing and calico printing, for the manufacture of certain orange and ye110w pigments (chrome ye11ows), for some explosi vcs, and in some kinds of matches. It is important in that it furnishcs a moderately soluble lead salt.
    Ferric nitrate (nitrate of iron) is generally made by dissolving scrap iron in nitric acid of 1.30 sp. gr. '1'he reaction is as follows:

- 2 Fe +8 HN03 = 2 Fe(N03)3 + 2 NO +4 H20.

By concentrating the solution, colorless crystals, containing six or nine molecules of crystal water, are obtained.
  The aqueous solution will dissolve ferric hydroxide, and this basic solution is much used in textile coloring. By using an excess of iron, and permitting the reaction to continue slowly, after all the acid has been acted upon, a precipitate of insoluble basic ferric nitrate ultimately forms. The solution obtained in this way is of a red-brown color and indefinite composition. It is chiefly used for blacks in silk dyeing, and for iron-buff on cotton.
  Ferrous nitrate is prepared by dissolving iron in cold dilute nitric acid (1.10 sp. gr.). But a considerable amount of ammonium nitrate is also formed in the solution, according to the reaction:-

4 Fe + 10 HN03 =4 Fe(N03)2 +NH4NO3 + 3 H20.

  This solution is very unstable and decomposes when heated even slightly, forming basic ferric nitrate and liberating nitric oxide. To prepare a pure ferrous nitrate, decomposition of a ferrous sulphate solution by barium or lead nitrate is employed:-

FeS04 + Ba(NO3)2 = BaS04 + Fe(NO3)2

  The solution is filtered or decanted from the precipitated barium sulphate.
  There is a preparation sold as "nitrate of iron," (probably so called because some nitric acid is used in making it), which is not a nitrate, but a basic ferric sulphate and sulphate-nitrate solution. -A solution of ferrous sulphate (copperas) is oxidized by nitric acid, according to the following equations:-

1) 6 FeS04 + 2 H:X0a + 2 H20 = 3 Fe2(S04)2' (OH)2 + 2 NO.
2) 6 FeS04 +5 noxoa = 3 FelS04)2' (NOa) • (OU) + 2 NO + H20.
3) 6 FeS04 + 8 IIXOa = 3 Fe2(S04)2' (NOa)2 + 2 NO +4 H20.
4) 12 FeS04 + 3 H2SO. + 4 HNOa = 3 FeiS04)5' (OI-J)2+ 4 NO
+ 2 H20.

Equation 4 gives the best product. The solution of basic ferric sulphate and sulphate-nitrates IS a dark brown-red liquid, and is much used in silk dyeing. It is only mentioned· here because of the frequent confusion of names in the commercial article. Silver nitrate is made by dissolving the metal in dilute nitric acid: - 6 Ag + 8 UNOa = 6 AgNOa + 4 H20 + 2 NO.
   If the silver contains copper, the resulting solution of nitrates is evaporated to dryness and then heated cautiously to about 2500 C., at which temperature the copper nitrate is decomposed into copper oxide, nitric oxide, and oxygen, while the silver salt is not altered. By extracting the residue with water, the silver nitrate is dissolved, leaving the copper oxide. The solution is then evaporated to crystallize the silver nitrate.
   The salt fuses unchanged at 225⁰ C., but decomposes if heated nearly to redness; it is cast in small sticks, and is much used in medicine for a cautery, under the name of lunar caustic.
   Silver nitrate has a very corrosive action on organic matter. It is largely used in photography, and to a lesser degree in pharmacy, in the manufacture of mirrors, in preparing" indelible inks," and as a chemical reagent.
    Barium nitrate is made by dissolving the native carbonate (witherite) in hot, dilute nitric acid; 01' it may be prepared by decomposing a concentrated solution (320 Be.) of barium chloride, by the addition of sodium nitrate, the less soluule uarium nitrate precipitating. The salt is purified by recrystallization. It is chiefly used for producing "green fire" in pyrotechnics and £01' making barium peroxide (Ba02). It is also used as an oxidizing material in certain explosives.
  Strontium nitrate is made by dissolving the native carbonate (strontianite) in hot nitric acid. Its chief use is for "red fire" in pyrotechnics.