Ferric caesium alum

The high solubility of the caesium salt is interesting in view of the low solubility of ferric caesium alum (see p. 164). 

Ferric caesium selenium alum, Cs2Se04.Fe2(Se04)3.24H20, prepared in an analogous manner to the preceding salt,1 yields violet crystals of density 3 6176 at 15° C. These melt in their combined water at 55° to 60° C. 

The alkali sulphates can also be made by neutralizing, say, a soln. of 5 grms. of sulphuric acid in 30 c.c. of water with the alkali hydroxide or carbonate, and evaporating the soln. until crystals begin to form. The process is not economical except on a small scale. It is used mainly for lithium, rubidium, and caesium sulphates. H. Erdmann 20 treated a hot soln. of crude rubidium iron alum with milk of lime made from purified lime, and filtered the liquid from the excess of lime, calcium sulphate, and ferric hydroxide, by suction. The small amount of lime in soln. is precipitated by adding rubidium carbonate. The filtrate is neutralized with sulphuric acid, and evaporated to the point of crystallization. 

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