Rubidium alums sulphate

Precipitate with aq. ammonia. Evaporate the soln. down to about 100 c.c., and filter the ot liquid so as to remove calcium sulphate. The cone. soln. is sat. with ammonium alum and allowed to stand for some time. The mixed crystals of potassium, rubidium, and oeesium alums and of lithium salt are dissolved in 100 c.c. of distilled water and recrystal-lized. The recrystallization is repeated until the crystals show no spectroscopic reaction for potassium or lithium. The yield naturally depends on the variety of lepidolite employed. 100. grms of an average sample gives about 10 grms. of crude crystals and about 3 grms. of the purified caesium and rubidium alums. For the purification of caesium and rubidium salts, see the chlorides. The mother-liquors are treated with an excess of barium carbonate, boiled, and filtered. The filtrate is acidified with hydrochloric acid, and evaporated to dryness. The residue is extracted with absolute alcohol in which lithium chloride is soluble, and the other alkali chlorides are sparingly soluble. 

In the leaching processes, large volumes of liquid have to be handled, and the mother liquors contain large amounts of salts in soln. much potassium chloride is rendered less useful since these liquors are boiled down and utilized in other ways— principally as fertilizers. The mother liquors also yield rubidium alum. The cost of the operation, small as it is, does not enable potassium sulphate to be prepared of a sufficient degree of purity to enable it to displace the potassium chloride and sulphuric acid process which can be made to yield almost the theoretical amount of sulphate. Consequently, most of the schonite—natural or artificial—is sold as a fertilizer. 

It is evident that telluric acid is notably different from what might be expected by analogy with sulphuric and selenic acids. One is hardly surprised, therefore, that isomorphism is exceedingly rare between the tellurates and the sulphates or selenates.3 The tellurates do not form alums, they do not as a rule form mixed crystals with the sulphates or selenates, the only fairly satisfactory case of mixed-crystal formation being with rubidium hydrogen sulphate and rubidium hydrogen tellurate. 

Preparation.—The main source of rubidium compounds is the residual mother-liquor obtained in the extraction of potassium chloride from carnallite. The solution contains rubidium-carnallite, RbCl,MgCI2, a substance transformed by addition of aluminium sulphate into rubidium-alum, RbAl(S04)8,12H20. Separation from the potassium and caesium salts also present is effected by fractional crystallization of the alum,8 of the chloroplatinate 8 Rb2PtCl8, of rubidium-iron-alum,4 and of the double chloride with stannous chloride5 or with antimony trichloride.6... 

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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