Note 41 — Chapter XIII · POTASSIUM, RUBIDIUM, CÆSIUM, AND LITHIUM. SPECTRUM ANALYSIS

The salts of the majority of metals are precipitated as carbonates on the addition of ammonium carbonate—for instance, the salts of calcium, iron, &c. The alkalis whose carbonates are soluble are not, however, precipitated in this case. On evaporating the resultant solution and igniting the residue (to remove the ammonium salts), we obtain salts of the alkali metals. They may he separated by adding hydrochloric acid together with a solution of platinic chloride. The chlorides of lithium and sodium give easily soluble double salts with platinic chloride, whilst the chlorides of potassium, rubidium, and cæsium form double salts which are sparingly soluble. A hundred parts of water at 0° dissolve 0·74 part of the potassium platinochloride; the corresponding rubidium platinochloride is only dissolved to the amount of 0·134 part, and the cæsium salt, 0·024 part; at 100° 5·13 parts of potassium platinochloride, K2PtCl6, are dissolved, 0·634 part of rubidium platinochloride, and 0·177 part of cæsium platinochloride. From this it is clear how the salts of rubidium and cæsium may be isolated. The separation of cæsium from rubidium by this method is very tedious. It can be better effected by taking advantage of the difference of the solubility of their carbonates in alcohol; cæsium carbonate, Cs2CO3, is soluble in alcohol, whilst the corresponding salts of rubidium and potassium are almost insoluble. Setterberg separated these metals as alums, but the best method, that given by Scharples, is founded on the fact that from a mixture of the chlorides of potassium, sodium, cæsium, and rubidium in the presence of hydrochloric acid, stannic chloride precipitates a double salt of cæsium, which is very slightly soluble. The salts of Rb and Cs are closely analogous to those of potassium.