POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM 541 



is no difficulty in obtaining any salt of potassium for example, the 

 sulphate, 9 bromide, and iodide 10 by the action of the corresponding 



iPofautium fnil/iJnite, K.^SO.!, crystallises from its solutions in an anhydrous condi- 

 tion, in which respect it differs from the corresponding sodium salt, just as potassium car- 

 bonate differs from sodium carbonate. In general, it must be observed that the majority 

 of sodium salts combine more easily with water of crystallisation than the potassium 

 salts. The solubility of potassium sulphate does not show the same peculiarities as 

 sodium sulphate, because it does not combine with water of crystallisation ; at the 

 ordinary temperature 100 parts of water dissolve about 10 parts of the salt, at 8'8' 

 parts, and at 100 about 2(5 parts. The acid sulphate, KHSO 4 , obtained easily by 

 heatiiiLT crystals of the normal salt with sulphuric acid, is frequently employed in 

 chemical practice. On heating the mixture of acid and salt, fumes of sulphuric acid are 

 at first given off ; when they cease to be evolved, the acid salt is contained in the residue.. 

 At a higher temperature (of above 600) the acid salt parts with all the acid contained in 

 it, the normal salt being re-formed. The definite composition of this acid salt, and the 

 ease with which it decomposes, render it exceedingly valuable for certain chemical trans- 

 formations accomplished by means of sulphuric acid at a high temperature, because it is 

 possible to take, in the form of this salt, a strictly definite quantity of sulphuric acid, 

 and to act on a given substance at a high temperature, which it is often necessary to do, 

 more especially in chemical analysis. In this case, the acid salt acts in exactly the 

 same manner as sulphuric acid itself, but the action of the latter is inconvenient at 

 temperatures above 400, because it would all evaporate, while at that temperature the 

 acid salt still remains in a fused state, and acts with the elements of sulphuric acid on 

 the substance taken. Hence by its means the boiling-point of sulphuric acid is raised. 

 Thus the acid potassium sulphate is employed, for example, where for conversion of 

 certain oxides, such as the oxides of iron, aluminium, and chromium, into salts, a high 

 temperature is required. 



Weber, by heating potassium sulphate with an excess of sulphuric acid at 100,. 

 observed the formation of a lower stratum, which was found to contain a definite com- 

 pound containing eight equivalents of SO 3 per equivalent of KoO. The salts of 

 rubidium, caesium, and thallium give a similar result, but those of sodium and lithium 

 do not. 



10 The bromide and iodide of potassium are used, like the corresponding sodium 

 compounds, in medicine and photography. Potassium iodide is easily obtained in a pure- 

 state by saturating a solution of hydriodic acid with caustic potash. In practice, how- 

 ever, this method is rarely had recourse to, other more simple processes being em- 

 ployed although they do not give so pure a product. They aim at the direct formation 

 of hydriodic acid* in the liquid in the presence of potassium hydroxide or carbonate. 

 Thus iodine is thrown into a solution of pure potash, and hydrogen sulphide passed 

 through the mixture, the iodine being thus converted into hydriodic acid. Or a solution 

 is prepared from phosphorus, iodine, and water, containing hydriodic and phosphoric acid ; 

 lime is then added to this solution, and calcium iodide is obtained in solution, and" 

 calcium phosphate as a precipitate. The solution of calcium iodide gives, with potassium 

 carbonate, insoluble calcium carbonate and a solution of potassium iodide. If iodine is 

 added to a slightly-heated solution of caustic potash (free from carbonate that is, freshly 

 prepared), so long as the solution is not coloured from the presence of an excess of iodine, 

 then there is formed (as in the action of chlorine on a solution of caustic potash) a 

 mixture of potassium iodide and iodate. On evaporating the solution thus obtained and 

 igniting the residue, the iodate is destroyed and converted into iodide, the oxygen being 

 disengaged, and potassium iodide only is left behind. On dissolving the residue in water 

 and then evaporating, cubical crystals of the anhydrous salt are obtained, which are 

 soluble in water and alcohol, fuse and give an alkaline reaction, owing to the fact that 

 when ignited a portion of the salt decomposes, forming potassium oxide. The neutral 

 salt may be obtained by adding hydriodic acid to this alkaline salt until it gives an 

 acid reaction. It is best to add some finely-divided charcoal to the mixture of iodate- 



