Potassium chloride, KCl

The Potassium chloride, KCl is found in nature as sylvine: in combination as carnallite, KCl,MgCl₂,6H₂O; kainite, KCl,MgSO₄,3H₂O; and as douglasite, K₂FeCl₄,2H₂O. It is also a constituent of the ashes of plants, and of the residues from the manufacture of alcohol from beet-molasses.

Potassium chloride can be synthesized from potassium and chlorine, the elements combining directly under the influence of heat, or at ordinary temperature in presence of moisture. At - 80° C. the metal does not combine with chlorine. The salt can be prepared by the action of hydrochloric acid on the carbonate.

In addition to kainite and sylvine, the salt deposits at Stassfurt ("Abraumsalze") contain 55 to 65 per cent, of carnallite, associated with 20 to 25 per cent, of rock-salt, 10 to 20 per cent, of kieserite, MgSO₄,H₂O, and 2 to 4 per cent, of tachydrite, CaCl₂,2MgCl₂,12H₂O. The technical preparation of potassium chloride from these deposits depends on the ready solubility of carnallite, and the crystallization of potassium chloride from hot saturated solutions of this substance. Kainite is employed as a source of potassium chloride, and the compound is also obtained by fractional crystallization of the salts present in sea-water and in the ash of seaweed.

A process for the production of potassium chloride from orthoclase was patented by Bassett, but has not been worked technically. It was discovered independently and investigated by Ashcroft, and consists in heating finely divided orthoclase with sodium chloride in equal proportion by weight at 900° to 1000° C., 85 per cent, of the potassium in the mineral being replaced by sodium in accordance with the scheme

K₂O,Al₂O₃,6SiO₂ + 2NaCl ⇔ Na₂O,Al₂O₃,6SiO₂ + 2KCl.

The potassium chloride can be separated from the insoluble sodium felspar by lixiviation, and from the excess of sodium chloride by fractional crystallization. This process might afford a new method for the manufacture of potassium chloride. It has also been found possible to extract the salt from the dust of the blast-furnace.

Potassium chloride forms colourless cubes, and has also been obtained in octahedra, rhombododecahedra, and icositetrahedra. Its melting-point is given as 762° C., 772.3° C., 774° C., 775° C., 778° C., and 790° C. It volatilizes without decomposition, the molecular weight derived from the vapour density, and also that from the depression of the freezing-point of mercuric chloride,12 corresponding with the simple formula KCl. For the density are given the mean value 1.977, and also 1.989 at 16° C., 1.991 at 20° C., 1.994 at 20.4° C., 1.951 at 23.4° C., 1.612 at the melting-point. The specific heat is given as 0.171 between 13° and 46° C., 0.1730 between 14° and 99° C., 0.1840 between 20° and 726° C., and for the fused salt 0.2671 between 807° and 935° C. The latent heat of fusion per gram is recorded as 0.063 Cal. And 0.086 Cal. At 801° C. the vapour-pressure is 1.54 mm.; at 948° C. it is 8.33 mm.; and at 1044° C. it is 24.1 mm. The molecular electric conductivity between 775.7° and 943.5° C. is given by the formula

μ_t = 115.4 + 0.2575(t - 800).

The heat of formation from the elements is given as 105.6 Cal. and 105.7 Cal.

Solubility of Potassium chloride in water

Temperature. °C.	0	5	10	15	20	25	30	40	50	60	70	80	90	100	130	147	180
Solubility in 100 grams of water	27.6	29.3	31.0	32.4	34.0	35.5	37.0	40.0	42.6	45.5	48.3	51.1	54.0	56.7	66.0	70.8	77.5

The boiling-point of the saturated solution in contact with excess of the salt is 108.6° C. at 760 mm.

At 25° C. 100 grams of ethyl alcohol dissolve 0.022 gram of potassium chloride.

Double chlorides of potassium and other metals have also been prepared. Potassium chloride does not form either compounds or mixed crystals with lithium chloride.