RUBIDIUM [[[symbol]] Rb, atomic weight 85.45 (0= 16)], a metallic element belonging to the group of the alkali metals. It is found in the minerals lepidolite, petalite and in various specimens of mica and of carnallite, and in some mineral waters. It also occurs in tea, cocoa, coffee, tobacco and in the ashes of beetroot. It was discovered by R. Bunsen and Kirchhoff (Ann., 1860, 113, p. 337), in the spectroscopic examination of the residues obtained on evaporation of water from a mineral spring at Diirkheim, being characterized by two distinctive red lines. The best source of rubidium salts is the residue left after extracting lithium salts from lepidolite, the method of separation being based on the different solubilities of the platino-chlorides of potassium, rubidium and caesium in water (R. Bunsen, Ann., 1862, 122, p. 351). A somewhat similar process based on the varying solubilities of the corresponding alums has also been devised by Redtenbacher (Jour. prak. Chem., 1865, 95, p. 148). The metal is prepared by distilling the carbonate with carbon (an explosive compound similar to that obtained from potassium and carbon monoxide is liable to be formed simultaneously); by reducing the hydroxide with aluminium: 4RbOH+2A1=Rb 2 O Al203+2Rb+2H2 (N. Beketoff, Ber., 1888, 21, p. 424 ref.); by reducing the carbonate (C. Winckler, Ber., 1890, 23, p. 51) or the hydroxide with magnesium (H. Erdmann and P. K6thner, Ann., 18 99, 294, p. 55); and by heating the fused chloride with calcium in an exhausted glass tube at 400-500° C. (L. Hackspill, Comptes rendus, 1905, 141, p. 101). The metal was first obtained electrolytically in 1910 by electrolysing the fused hydroxide in a nickel vessel, with an iron wire cathode and iron cylinder anode; the product on cooling being opened under pyridine cooled by a freezing mixture (G. von Hevesy, Zeit. anorg. Chem., 1910, 67, p. 242). It is a silvery white metal which melts at 38.5° C. and has a specific gravity of I52. It oxidizes rapidly on exposure to air, and decomposes cold water very rapidly. It closely resembles caesium and potassium in its general properties. The rubidium salts are generally colourless, mostly soluble in water and isomorphous with the corresponding potassium salts.
Rubidium hydride, RbH, was obtained in the form of colourless needles by H. Moissan (Comptes rendus, 1903, 136, p. 587) from the direct combination of its constituent elements. It rapidly dissociates when heated in vacuo to 300° C. The existence of the oxide Rb 2 0 appears to be doubtful, the results of Erdmann and Kothner (loc. cit.) pointing to the formation of Rb0 2 by the direct union of the metal with dry oxygen. E. Rengade (Comptes rendus, 1907, 1 44, P. 920), by partially oxidizing the metal in a current of dry oxygen and removing excess of metal by distillation in vacuo, has obtained oxides of composition Rb202 (yellowish white), Rb203 (black) and Rb204 (yellow). Rubidium hydroxide, RbOH, is a colourless solid which is formed by the action of rubidium on water, or by the addition of baryta water to a solution of rubidium sulphate. It is readily soluble in water, the solution being very alkaline and caustic. It melts at 301°. Evaporation of the aqueous solution at 15° C. deposits a crystalline hydrated hydroxide of composition RbOH2H 2 O (R. de Forcrand, Comptes rendus, 1909, 1 49, p. 1341). Rubidium chloride, RbC1, is formed on burning rubidium in chlorine, or on dissolving the hydroxide in aqueous hydrochloric acid. It crystallizes in colourless cubes and volatilizes when heated very strongly. It is soluble in water and combines with many metallic chlorides to form double salts. It combines also with iodine chloride and bromide and with bromine chloride and with bromine (H. L. Wells and H. L. Wheeler, Amer. Jour. Sci., 18 9 1 (3), 43, P. 475); Rubidium sulphate, Rb2S04, is formed by the action of sulphuric acid on the carbonate or hydroxide of the metal, or by the action of milk of lime on rubidium alum, the excess of lime being precipitated by rubidium carbonate and the solution neutralized by sulphuric acid. It forms large colourless hexagonal crystals. Several sulphides of the metal have been described by W. Biltz and E. Wilke-Dorfurt (Zeit. anorg. Chem., 1906, 48, p. 297). The normal sulphide, Rb 2 S4H 2 O, is colourless, and when heated in aqueous solution with the requisite amount of sulphur is transformed into the yellow tetrasulphide, Rb 2 S 4.2H 2 O. A pentasulphide, Rb 2 S 5, which crystallizes in red prisms melting at 223° C., is also obtained by the direct union of the normal sulphide with sulphur. When heated in a current of hydrogen it is transformed into the colourless disulphide, whilst if the heating be carried out in a current of nitrogen it yields the trisulphide, Rb 2 S 3 H 2 0. These sulphides are much less hygroscopic than the corresponding caesium compounds. Rubidium nitrate, RbNO 3, obtained by the action of nitric acid on the carbonate, crystallizes in needles or prisms and when strongly heated is transformed into a mixture of nitrite and oxide. Rubidium ammonium, RbNH 31 was prepared by H. Moissan (Comptes rendus, 1903, 136, p. 1177) by the action of liquid ammonia on rubidium. The product combines with acetylene to form rubidium acetylide acetylene, Rb2C2 C2H2, which on heating in vacuo loses acetylene and leaves a residue of rubidium carbide Rb2C2 (ibid. p. 1217). Rubidium carbonate, Rb2C03, formed by the addition of ammonium carbonate to rubidium hydroxide, is a crystalline mass which melts in its water of crystallization when heated.
The atomic weight of rubidium was determined by R. Bunsen (Pogg. Ann., 1861, 113, p. 339), Picard (Zeit. anal. Chem., 1862, 1, p. 519) and Godeffroy (Ann., 1876, 181, p. 185), the methods being based on the conversion of rubidium halides into the corresponding silver salt, and the values obtained vary from 85.40 to 85.50. The determination of E. H. Archibald (Jour. Chem. Soc., 1904, 85, p. 776) from the analysis of the chloride and bromide gives the mean value as 85.485 (O =16).
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