260 METALS AND THEIR COMBINATIONS. 



Iodide and iodate of potassium are formed, and may be separated 

 by crystallization. A better method, however, is to boil to dryness 

 the liquid containing both salts, and to heat the mass after having 

 mixed it with some charcoal, in a crucible, when the iodate is con- 

 verted into iodide : 



KIO 3 + 30 = KI + SCO. 



Experiment 21. Add to a solution of about 3 grammes of potassium hydroxide 

 in about 25 c.c. of water (or to the solution obtained by making Experiment 

 20) iodine until the brown color no longer disappears. (How much iodine will 

 be needed for 3 grammes of KOH?) Evaporate the resulting solution (What 

 does this solution contain now ?) to dryness, mix the powdered mass with about 

 10 per cent, of powdered charcoal and heat the mixture in a crucible until 

 slight deflagration has taken place. Dissolve the cold mass in hot water, filter 

 and set aside for crystallization ; if too much water has been used for dissolving, 

 the liquid must be concentrated by evaporation. 



Potassium iodide forms colorless, cubical crystals, which are soluble 

 in 0.5 part of boiling and 0.8 part of cold water, also soluble in 12 

 parts of alcohol, and 2.5 parts of glycerin. When heated it fuses, 

 and at a bright-red heat is volatilized without decomposition. 



Potassium bromide, Potassii bromidum, KBr = 118.22, may be 

 obtained in a manner analogous to that given for potassium iodide, 

 by the action of bromine upon potassium hydroxide, etc. 



Or it may be made by the decomposition of a solution of ferrous 

 bromide by potassium carbonate : 



Ferrous carbonate is precipitated, while potassium bromide remains 

 in solution, from which it is obtained by crystallization. 



Potassium salts of interest, which have not yet been mentioned, will be con- 

 sidered under the head of their respective acids. Some of these salts are 

 potassium chromate and permanganate, and the salts formed from organic 

 acids, such as potassium tartrate, acetate, etc. 



Tests for potassium. 



(Potassium chloride, KC1, or nitrate, KNO 3 , may be used.) 

 1. To a solution of any potassium salt add some solution of chloro- 

 platinic acid. A yellow crystalline precipitate of potassium chloro- 

 platinate is obtained : 



2KN0 3 + H 2 PtCl 6 = K 2 PtCl 6 + 2HN0 3 ; 



or 2K- + 2NQ,' + 2H + PtCl 6 " = K 2 PtCl 6 + 2H' + 2NQ/ . 

 This test is not very delicate, as 1 part of the precipitate is soluble in 

 about 100 parts of water. It is much less soluble in alcohol, which 

 is usually added to facilitate precipitation. 



