RUBIDIUM, a metallic element belonging to the group of the alkali metals (symbol Rb, atomic number 37, atomic weight 85.44). 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 (186o) 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 platinichlorides of potassium, rubidium and caesium in water. A somewhat similar process based on the varying solubilities of the corresponding alums has also been devised by Redtenbacher (1865). The metal is prepared by dis tilling 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 : by reducing the carbonate or the hydroxide with magnesium; and by heating the fused chloride with calcium in an exhausted glass tube at C. 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). It is a silvery white metal which melts at 39° C, boils at 696°, and has a specific gravity of 1.52. 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 crystallographically are similar to the corresponding potassium and ammonium salts, especially the latter. The acid tartrate is very sparingly soluble.
Rubidium hydride, RbH, was obtained in the form of colour less needles by H. Moissan from the direct combination of its constituent elements. It rapidly dissociates when heated in vacuo to 300° C. Several oxides have been described; the composition depends upon the conditions of oxidation and is doubtful in the case of and but is well substantiated for and 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 solu ble 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 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.