Bunsen obtained rubidium by distilling a mixture of the tartrate with soot, and Beketoff (1888) by heating the hydroxide with aluminium, 2RbHO + Al = RbAlO2 + H2 + Rb. By the action of 85 grams of rubidium on water, 94,000 heat units are evolved. Setterberg obtained cæsium (1882) by the electrolysis of a fused mixture of cyanide of cæsium and of barium. Winkler (1890) showed that metallic magnesium reduces the hydrates and carbonates of Rb and Cs like the other alkaline metals. N. N. Beketoff obtained them with aluminium (see following note).
Beketoff (1888) showed that metallic aluminium reduces the hydrates of the alkaline metals at a red heat (they should be perfectly dry) with the formation of aluminates (Chapter XVII.), RAlO2—for example, 2KHO + Al = KAlO2 + K + H2. It is evident that in this case only half of the alkaline metal is obtained free. On the other hand, K. Winkler (1889) showed that magnesium powder is also able to reduce the alkaline metals from their hydrates and carbonates. N. N. Beketoff and Tscherbacheff (1894) prepared cæsium upon this principle by heating its aluminate CsAlO2 with magnesium powder. In this case aluminate of magnesium is formed, and the whole of the cæsium is obtained as metal: 2CsAlO2 + Mg = MgOAl2O5 + 2Cs. A certain excess of alumina was taken (in order to obtain a less hygroscopic mass of aluminate), and magnesium powder (in order to decompose the last traces of water); the CsAlO2 was prepared by the precipitation of cæsium alums by caustic baryta, and evaporating the resultant solution. We may add that N. N.
Beketoff (1887) prepared oxide of potassium, K2O, by heating the peroxide, KO, in the vapour of potassium (disengaged from its alloy with silver), and showed that in dissolving in an excess of water it evolves (for the above-given molecular weight) 67,400 calories (while 2KHO in dissolving in water evolves 24,920 cal.; so that K2O + H2O gives 42,480 cal.), whence (knowing that K2 + O + H2O in an excess of water evolves 164,500) it follows that K2 + O evolves 97,100 cal. This quantity is somewhat less than that (100,260 cal.) which corresponds to sodium, and the energy of the action of potassium upon water is explained by the fact that K2O evolves more heat than Na2O in combining with water (see Chapter II. Note 138). Just as hydrogen displaces half the Na from Na2O forming NaHO, so also N. N. Beketoff found from experiment and thermochemical reasonings that hydrogen displaces half the potassium from K2O forming KHO and evolving 7,190 calories. Oxide of lithium, Li2O, which is easily formed by igniting Li2CO3 with carbon (when Li2O + 2CO is formed), disengages 26,000 cals. with an excess of water, while the reaction Li2 + O gives 114,000 cals.
and the reaction Li2 + H2O gives only 13,000 cals., and metallic lithium cannot be liberated from oxide of lithium with hydrogen (nor with carbon). Thus in the series Li, Na, K, the formation of R2O gives most heat with Li and least with K, while the formation of RCl evolves most heat with K (105,000 cals.) and least of all with Li (93,500 cals.). Rubidium, in forming Rb2O, gives 94,000 cals. (Beketoff). Cæsium, in acting upon an excess of water, evolves 51,500 cals., and the reaction Cs2 + O evolves about 100,000 cals.—i.e. more than K and Rb, and almost as much as Na—and oxide of cæsium reacts with hydrogen (according to the equation Cs2O + H = CsHO + Cs) more easily than any of the oxides of the alkali metals, and this reaction takes place at the ordinary temperature (the hydrogen is absorbed), as Beketoff showed (1893). He also obtained a mixed oxide, AgCsO, which was easily formed in the presence of silver, and absorbed hydrogen with the formation of CsHO.