178 QUANTITATIVE ANALYSIS. 



water it must dissolve on the addition of hydrochloric acid 

 without evolving any gas. Calcium carbonate contains 56 

 per cent, of calcium oxide. 



5. DETERMINATION OF ALUMINIUM IN POTASH ALUM, 

 AlK(S0 4 ) 2 +i2H 2 0, AS ALUMINIUM OXIDE. 



Dissolve about 1 g. of potash alum in 150 cc. of water in 

 a porcelain dish, add 20 cc. of ammonium chloride solution, 

 heat till boiling just begins, and add ammonia to faintly 

 .alkaline reaction (instead of ammonium chloride and ammonia, 

 hydrochloric acid and ammonia may also be used). Heat 

 for some time, until the ammonia is for the most part driven 

 off, decant the supernatant liquid through an ash-free 

 filter, wash a few times by decantation, bring the entire pre- 

 cipitate on the filter, wash with hot water until the wash- 

 water no longer gives the reaction for chlorides, pour the 

 filter full of alcohol, then full of ether. After these have 

 evaporated, dry completely by heating in the air-bath and 

 incinerate, at first very cautiously with the cover on the 

 crucible, then heat for at least five minutes more with the 

 blast-lamp, etc., etc. Potash alum contains 10.75 per cent. 

 of A1 2 3 - 



6. DETERMINATION OF CHLORINE IN SODIUM CHLORIDE, 



NaCl. 



Weigh accurately between 0.2 and 0.3 g. of pure sodium 

 chloride (previously gently heated), place in a beaker, 

 dissolve in about 100 cc. of water, acidify with a few drops 

 of nitric acid, heat on the wire gauze, but not to boiling, 

 then add silver nitrate solution as long as a precipitate forms. 

 Continue the heating on the water-bath until the precipitate 

 has well settled, decant off the fluid through an ash-free thin 

 filter * of 9 cm. diameter, wash with hot water to which a few 



1 Use a Gooch crucible here and the method given on page 185. O. 



