Potassium sulphate, K₂SO₄

The Potassium sulphate, K₂SO₄ occurs in nature in the form of double salts. Examples are schonite, K₂SO₄,MgSO₄,6H₂O; polyhalite, K₂SO₄,MgSO₄,2CaSO₄,2H₂O; and glaserite, 3K₂SO₄,Na₂SO₄. It is a by-product in the manufacture of nitric acid from potassium nitrate and sulphuric acid, and can also be prepared by the action of sulphuric acid on potassium chloride. The main sources of potassium sulphate are kainite (KCl,MgSO₄,3H₂O) and sylvine (KCl), both present in the Stassfurt " Abraumsalze." Water decomposes kainite into a sparingly soluble double sulphate of potassium and magnesium, and the very soluble magnesium chloride; a similar product is obtained by the interaction of sylvine and kieserite (MgSO₄,H₂O). The magnesium chloride is removed by cold water, and the residual double sulphate treated with excess of an aqueous solution of potassium chloride:

K₂SO₄,MgSO₄ + 2KCl = 2K₂SO₄ + MgCl₂.

Potassium sulphate forms white, rhombic crystals, melting-point 1015° C., 1045° C., 1050° C., 1052° C., 1057° C., 1058.9° C., 1066.1° C., 1066.5° C., 1067° C., 1070° C., 1074° C., or 1078° C. Its density is given as 2.6617, 2.6627 at 16.4° C., 2.6633, 2.666, 2.670 at 20° C., and 2.6521 at 60° C. Its specific heat is 0.196 between 13° and 45° C., or 0.1901 between 15° and 98° C. At 1130° C. the vapour-pressure is 0.4 mm. The heat of formation from the elements is given as 344.3 Cal. and 344.6 Cal. The crystals are diamagnetic, and exhibit triboluminescence. The salt does not form hydrates.

The solubility in water is given in the table.

Temperature, °C	0	10	20	30	40	50	60	70	80	90	100	120	143	170
Grams of K2SO4 in 100 g. of water.	7.35	9.22	11.11	12.97	14.76	16.50	18.17	19.75	21.4	22.8	24.1	26.5	28.8	32.9

A solution in contact with excess of salt boils at 101.392° C. under a pressure of 760 mm. of mercury. The aqueous solution of the salt has a neutral reaction.