Ferric rubidium alum, Rb₂SO₄.Fe₂(SO₄)₃

Ferric rubidium alum, Rb₂SO₄.Fe₂(SO₄)₃.24H₂O, and ferric caesium alum, Cs₂SO₄.Fe₂(SO₄)₃.24H₂O, have been obtained by electrolysis in an analogous manner to the iron ammonium alum. They are of a delicate violet colour, turning slightly greenish upon exposure to air. Their solubilities in water with rise of temperature are as follow-:

Temperature, °C	25	30	35	40
Grams Rb2SO4.Fe2(SO4)3 per litre of water	97.4	202.4	(5)	(5)
Grams Cs2SO4.Fe2(SO4)3 per litre of water	17.1	25.2	37.5	60.4