§ 44. POTASSIUM. 45 



presence of free hydrochloric acid. No precipitate will 

 be given by the reagent in a A'ery dilute solution of the 

 potassic salt, but if such a solution is evaporated nearly 

 to dryness with a little platinic chloride, and alcohol is 

 added to the residue, the yellow double salt remains un- 

 dissolved. 



If a drop of a solution of a potassic salt is evaporated 

 to dryness in the platinum-wire loop, and the loop with 

 the residue on it is held at the end of the inner blowpipe 

 flame, or in the corresponding part of the flame of the 

 Bunsen gas-burner, a violet color is communicated to the 

 flame beyond the wire. Viewed through thick blue glass, 

 this color has a more reddish appearance, but the light is 

 not entirely absorbed ; the presence of sodium, barium, 

 calcium, and copper, may interfere with this reaction. 



In a silicate, this reaction for jDotassium may be ob- 

 tained by fusing it, in a fine powder, with pure gypsum, 

 treating the fused mass with w^ater, filtering, and testing 

 the filtrate. 



Quantitatiye estimation. — Potassium may be deter- 

 mined as potassic chloride, KCl, potassic sulphate, K^SO^, 

 or potassic platinic chloride, K^PtCl^. 



The first two salts are soluble in water, and therefore 

 cannot be obtained by precipitation ; other metals and 

 acids being removed from the solution by methods here- 

 inafter described, the pure salt is then left as a residue on 

 evaporation to dryness. 



a. Determination as potaSSic cllloridc. — The solution 

 being freed from other metals and acids, evaporate it to 

 dryness over the water-bath, and ignite the residue in a 

 w^ell covered platinum crucible, very gently for a consid- 

 erable time at first, to avoid the decrepitation and conse- 

 quent loss that might result from too rapid heating; 

 finally, heat the crucible to a dull red for a short time. 

 The residue contains 52.41° 1^ of potassium. 



