Note 3 bis — Chapter XIII · POTASSIUM, RUBIDIUM, CÆSIUM, AND LITHIUM. SPECTRUM ANALYSIS

The specific gravity of the solid salt is 1·99—that is, less than that of sodium chloride. All the salts of sodium are specifically heavier than the corresponding salts of potassium, as are also their solutions for equal percentage compositions. If the specific gravity of water at 4° = 10,000, then at 15° the specific gravity of a solution of p p.c. potassium chloride = 9,992 + 63·29p + 0·226p2, and therefore for 10 p.c. = 1·0647, 20 p.c. = 1·1348, &c.

Potassium chloride combines with iodine trichloride to form a compound KCl + ICl₃ = KICl₄, which has a yellow colour, is fusible, loses iodine trichloride at a red heat, and gives potassium iodate and hydrochloric acid with water. It is not only formed by direct combination, but also by many other methods; for instance, by passing chlorine into a solution of potassium iodide so long as the gas is absorbed, KI + 2Cl₂ = KCl,ICl₃. Potassium iodide, when treated with potassium chlorate and strong hydrochloric acid, also gives this compound; another method for its formation is given by the equation KClO₃ + I + 6HCl = KCl,ICl₃ + 3Cl + 3H₂O. This is a kind of salt corresponding with KIO₂ (unknown) in which the oxygen is replaced by chlorine. If valency be taken as the starting-point in the study of chemical compounds, and the elements considered as having a constant atomicity (number of bonds)—that is, if K, Cl, and I be taken as univalent elements—then it is impossible to explain the formation of such a compound because, according to this view, univalent elements are only able to form dual compounds with each other; such as, KCl, ClI, KI, &c., whilst here they are grouped together in the molecule KICl₄. Wells, Wheeler, and Penfield (1892) obtained a large number of such poly-haloid salts. They may all be divided into two large classes: the tri-haloid and the penta-haloid salts. They have been obtained not only for K but also for Rb and Cs, and partially also for Na and Li. The general method of their formation consists in dissolving the ordinary halogen salt of the metal in water, and treating it with the requisite amount of free halogen. The poly-haloid salt separates out after evaporating the solution at a more or less low temperature. In this manner, among the tri-haloid salts, may be obtained: KI₃, KBr₂I, KCl₂I, and the corresponding salts of rubidium and cæsium, for instance, CsI₃, CsBrI₂, CsBr₂I, CsClBrI,CsCl₂I, CsBr₃, CsClBr₂, CsCl₂Br, and in general MX₃ where X is a halogen. The colour of the crystals varies according to the halogen, thus CsI₃ is black, CrBr₃ yellowish red, CrBrI₂ reddish brown, CsBr₂I red, CsCl₂Br yellow. The cæsium salts are the most stable, and those of potassium least so, as also those which contain Br and I separately or together; for cæsium no compounds containing Cl and I were obtained. The penta-haloid salts form a smaller class; among these salts potassium forms KCl₄I, rubidium RbCl₄I, cæsium CsI₅, CsBr, CsCl₄I, lithium LiCl₄I (with 4H₂O) and sodium NaCl₄I (with 2H₂O). The most stable are those salts containing the metal with the greatest atomic weight—cæsium (see Chapter XI., Note 63).