CHAPTER XIII POTASSIUM, RUBIDIUM, CÆSIUM, AND LITHIUM. SPECTRUM ANALYSIS
Tutton's researches (1894) upon the analogy of the crystalline forms of K2SO4, Rb2SO4 and Cs2SO4 may be taken as a typical example of the comparison of analogous compounds. We cite the following data from these excellent researches: the sp. gr. at 20°/4° of K2SO4 is 2·6633 of Rb2SO4, 3·6113, and of Cs2SO4, 4·2434. The coefficient of cubical expansion (the mean between 20° and 60°) for the K salt is 0·0053, for the Rb salt 0·0052, for the Cs salt 0·0051. The linear expansion (the maximum for the vertical axis) along the axis of crystallisation is the same for all three salts, within the limits of experimental error. The replacement of potassium by rubidium causes the distance between the centres of the molecules in the direction of the three axes of crystallisation to increase equally, and less than with the replacement of rubidium by cæsium. The index of refraction for all rays and for every crystalline path (direction) is greater for the rubidium salt than for the potassium salt, and less than for the cæsium salt, and the differences are nearly in the ratio 2 : 5.
The lengths of the rhombic crystalline axes for K2SO4 are in the ratio 0·5727 : 1 : 0·7418, for Rb2SO4, 0·5723 : 1 : 0·7485, and for Cs2SO4, 0·5712 : 1 : 0·7521. The development of the basic and brachy-pinacoids gradually increases in passing from K to Rb and Cs. The optical properties also follow the same order both at the ordinary and at a higher temperature. Tutton draws the general conclusion that the crystallographic properties of the isomorphic rhombic sulphates R2SO4 are a function of the atomic weight of the metals contained in them (see Chapter XV.) Such researches as these should do much towards hastening the establishment of a true molecular mechanics of physico-chemical phenomena.
Carnallite belongs to the number of double salts which are directly decomposed by water, and it only crystallises from solutions which contain an excess of magnesium chloride. It may be prepared artificially by mixing strong solutions of potassium and magnesium chlorides, when colourless crystals of sp. gr. 1·60 separate, whilst the Stassfurt salt is usually of a reddish tint, owing to traces of iron. At the ordinary temperature sixty-five parts of carnallite are soluble in one hundred parts of water in the presence of an excess of the salt. It deliquesces in the air, forming a solution of magnesium chloride and leaving potassium chloride. The quantity of carnallite produced at Stassfurt is now as much as 100,000 tons a year.
The method of separating sodium chloride from potassium chloride has been described in Chapter I. On evaporation of a mixture of the saturated solutions, sodium chloride separates; and then, on cooling, potassium chloride separates, owing to the difference of rate of variation of their solubilities with the temperature. The following are the most trustworthy figures for the solubility of potassium chloride in one hundred parts of water (for sodium chloride, see Chapter X., Note 593):— 10° 20° 40° 60° 100° 32 35 40 46 57 When mixed with solutions of other salts the solubility of potassium chloride naturally varies, but not to any great extent.