Rubidium sulphate, Rb₂SO₄

The Rubidium sulphate, Rb₂SO₄, forms rhombic crystals isomorphous with those of potassium sulphate, melting at 1074° C., the density at 20° C. being 3.6113, and at 60° C. 3.5943. It exists in two modifications, the transition-point being 657° C. The heat of formation from the elements is 344.68 Cal., and the heat of solution -6.66 Cal. at 15° C. The solubility is given in the appended table:

Solubility of Rubidium Sulphate

Temperature, °C.	0	10	20	30	40	50	60	70	80	90	100
Grams of Rb2SO4 per 100 g. of water	36.4	42.6	48.2	53.5	58.5	63.1	67.4	71.4	75.0	78.7	81.8

The boiling-point of the saturated solution in contact with excess of sulphate is 103.5° C. at 760 mm. pressure. With aluminium and ferric sulphates it forms well-crystallized alums. It exhibits radioactivity.