650 PKINOIPLE& OF OHJSMISTBY 



-bromide, and iodide lo by the action of the corresponding acid on KCI 

 and especially on the carbonate, whilst the hydroxide, caustic potash^ 

 KHO, wliich is in many respects analogous to caustic soda, is easily ob- 



end to cause it 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 acta ' 

 in exactly the same manner as sulphuric acid itself, hut the- latter is inefficient aft 

 temperatures above 400, because it all evaporates, while at that temperature the acid 

 Bait 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, where for conversion of certain oxides, 

 euch as those 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 r 

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

 pound containing eight equivalents of 80s per equivalent of K 8 0. The salts off 

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

 do not. (See Note 1.) 



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 hydriodio acid. Or a solution, 

 is prepared from phosphorus, iodine, and water, containing hydriodio and phosphoric acid j 

 lime is then added to this solution, when 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 o potassium iodide. If iodine ia 

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

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

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

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

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

 soluble in Water and alcohol, and on fusion give an alkaline reaction, owing to the fact thab 

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

 salt may be obtained by adding hydriodio 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 

 and iodide before igniting it, as this facilitates the evolution of the oxygen from the iodate* 

 The iodate may also be converted into iodide by the action of certain reducing agents, 

 such as zinc amalgam, which when .boiled with a solution containing an iodate converts 

 it into iodide. Potassium iodide may also be prepared by mixing a solution of ferrous 

 iodide (it is best if the solution contain an excess of iodine) and potassium carbonate, ia 

 which case ferrous carbonate FeCO 5 , is precipitated (with an excess of iodine the pre- 

 cipitate is granular, and contains a compound of the suboxide and oxide of iron), while 

 potassium iodide remains in solution. Ferrous iodide, FeLj, is obtained by the direct 

 action of iodine on iron iri water. Potassium iodide considerably lowers the temperature 

 (by 24), when it dissolved in water, 100 parts of the salt dissolve in 78'5 parts of water aft 

 12-5, in 70 parts at 18, whilst the saturated solution which boils at 120 contains 100 

 parts of salt per 45 parts of water. Solutions of potassium iodide dissolve a considerable 

 amount of iodine ; strong solutions even .dissolving as much or more iodine than they 

 contain as potassium iodide (see Note 8 bis'and Chapter XI., Note 64). 



