Rubidium alum

Rubidium-alaun, m. rubidium alum, -chlorid, n. rubidium chloride, -jodid, n. rubidium iodide, -oxydhydrat, n. rubidium hydroxide, -platinchlorid, n. rubidium chloroplatinate. 

Rubidium is recovered from its ore lepidolite or pollucite. Mineral lepidolite is a lithium mica having a composition KRbLi(OH,F)Al2Si30io. The ore is opened by fusion with gypsum (potassium sulfate) or with a mixture of barium sulfate and barium carbonate. The fused mass is extracted with hot water to leach out water-soluble alums of cesium, rubidium, and potassium. The solution is filtered to remove insoluble residues. Alums of alkali metals are separated from solution by fractional crystallization. Solubility of rubidium alum or rubidium aluminum sulfate dodecahydrate, RbAl(S04)2 I2H2O falls between potassium and cesium alum. 

Rubidium alum obtained by either method above is decomposed by treatment with alkali solutions for removal of aluminum and sulfate. Aluminum is precipitated as aluminum hydroxide. Addition of barium hydroxide to the filtrate removes sulfate, precipitating barium sulfate. Evaporation of the solution crystallizes rubidium as hydroxide. 

Alternatively, Rb sulfate may be obtained by treating a hot solution of rubidium aluminum sulfate (rubidium alum) with ammonia solution. Aluminum hydroxide precipitates. The product mixture is fdtered. The filtrate on evaporation crystaUizes rubidium sulfate. 

Rubidium acid salts are usually prepared from rubidium carbonate or hydroxide and the appropriate acid in aqueous solution, followed by precipitation of the crystals or evaporation to dryness. Rubidium sulfate is also prepared by the addition of a hot solution of barium hydroxide to a boiling solution of rubidium alum until all the aluminum is precipitated. The pH of the solution is 7.6 when the reaction is complete. Aluminum hydroxide and barium sulfate are removed by filtration, and rubidium sulfate is obtained by concentration and crystallization from the filtrate. Rubidium aluminum sulfate dodecahydrate [7488-54-2] (alum), RbA SO 12H20, is formed by sulfuric acid leaching of lepidolite ore. Rubidium alum is more soluble than cesium alum and less soluble than the other alkali alums. Fractional crystallization of Rb alum removes K, Na, and Li values, but concentrates the cesium value. Rubidium hydroxide, RbOH, is prepared by the reaction of rubidium sulfate and barium hydroxide in solution. The insoluble barium sulfate is removed by filtration. The solution of rubidium hydroxide can be evaporated partially in pure nickel or silver containers. Rubidium hydroxide is usually supplied as a 50% aqueous solution. Rubidium carbonate, Rb2C03, is readily formed by bubbling carbon dioxide through a solution of rubidium hydroxide, followed by evaporation to dryness in a fluorocarbon container. Other rubidium compounds can be formed in the laboratory by means of anion-exchange techniques. Table 4 lists some properties of common rubidium compounds. 

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. 

One respect in winch rubidium and cesium are outstanding among the alkali metals is the readiness with which they form alums, Rubidium alums arc known for all of the trivalcnt cations that form alums, Al ", Cr3+, Fc3+, Mn3+, V3+, Ti3+, Co3+, Ga3+. R1i3+, Ir3", and In3+,... 

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... 

Dodecahydrate. rubidium alum. Crystals, mp reported from 99 to 109. Sol in water practically insol in ale. 

Solubility of Caesium Alum, Rubidium Alum, and of Thallium... 

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