Potassium nitrate, KNO₃

The Potassium nitrate, KNO₃ is manufactured from natural saltpetre earths found in Bengal, and to a less extent in Egypt and elsewhere. The nitrate in these earths is produced by the oxidation of organic matter by the action of " nitrifying " bacteria in presence of atmospheric oxygen, a warm, moist climate being particularly favourable for the process. The nitrate is extracted by lixiviating the earth in vessels of earthenware or wood, and concentrating the aqueous solution in iron pots by solar or artificial heat.

Artificial saltpetre earths can be prepared by allowing stable refuse to nitrify in contact with a porous soil containing calcium carbonate, the nitrate being isolated by lixiviation and purified by crystallization. The salt is also manufactured from sodium nitrate by the action of potassium chloride, since it crystallizes out from a hot concentrated solution of these two substances as the temperature falls. When prepared from Chile saltpetre, it often contains sodium nitrate, and chloride, chlorate, and perchlorate of potassium.

The extraordinary demand for nitric acid and nitrates brought about by the abnormal conditions consequent on the great European War of 1914 and succeeding years gave a powerful impetus to the development of synthetic methods for the fixation of atmospheric nitrogen.

At ordinary temperature, potassium nitrate crystallizes in the rhombic system, at higher temperatures the crystals formed are rhombohedral. The transition-point is 129.5° C. The second form is said to exist in two modifications, α and β. - Among the values given for the melting-point are 333° C., 334.5° C., and 339° C.; for that of the rhombohedral form 334° C. For the density the mean value is given as 2.092, other determinations giving 2.1047, 2.109 at 16° C., 2.109 at 20° C., and 2.044-0.0006t at temperatures ranging from 350° to 500° C. The heat of formation from the elements is given as 119 Cal. And 119.5 Cal. The molecular electric conductivity of potassium nitrate between 346.1° and 500.4° C. is given by the formula

μ_t = 36.21 + 0.1875(t-350).

Potassium nitrate does not form hydrates. Its solubility in water increases rapidly with rise of temperature, as indicated by the table:

Solubility of Potassium Nitrate.

Temperature, °C	0	10	20	30	40	50	60	70	80	90	100	110	120	125
Grams KNO3 in 100g. of water.	13.3	20.9	31.6	45.8	63.9	85.5	110.0	138	169	202	246	300	394	493

The boiling-point of a solution in contact with excess of salt is 115.55° C. at 760 mm. pressure. The salt exhibits triboluminescence, and is diamagnetic.

On heating, potassium nitrate is converted into the nitrite, with evolution of oxygen. It finds application as an oxidizer, notably in the detonation of gunpowder, the cheaper sodium nitrate being inadmissible on account of its hygroscopic nature. The usual composition of this explosive is 75 per cent, of potassium nitrate, 15 per cent, of carbon, and 10 per cent, of sulphur. In contact with the fused nitrate, carbon and sulphur become ignited, and burn with evolution of the corresponding dioxide and nitrogen, and formation of other products. In dilute solution the nitrate is partially transformed by the action of ultraviolet rays into potassium nitrite and oxygen.

Behaviour of KNO3,2HNO3 with water

Two acid potassium nitrates have been isolated: a dinitrate, KNO₃,HNO₃, stable at 22° to 28° C., and a trinitrate, KNO₃,2HNO₃, melting at 22° C. The behaviour of the trinitrate with water is graphically represented in Fig.