ESTIMATION OF FLUORIN 263 



Two grams of the phosphate are intimately mixed in a large 

 platinum crucible with three grams of precipitated silica and 12 

 grams of pure sodium carbonate, and the mixture is gradually 

 brought to clear fusion over the blast-lamp. When the fusion 

 is complete the melt is spread over the walls of the crucible, 

 which is then rapidly cooled (preferably by a blast of air). If 

 this has been properly done, the mass separates easily from the 

 crucible, and the subsequent leaching is hastened. The mass, 

 detached from the crucible, is put into a platinum dish into which 

 whatever remains adhering to the crucible or its lid is also washed 

 with hot water. A reasonable amount of hot water is now put 

 into the dish, which is covered and digested on the water bath 

 until the mass is thoroughly disintegrated. To hasten this, the 

 supernatant liquid may, after a while, be poured off, the residue 

 being washed into a small porcelain mortar, ground up, returned 

 to the dish and boiled with fresh water until no hard grains are 

 left. The total liquid is filtered, and the residue is washed with 

 hot water. The filtrate (which should amount to about half a 

 liter) is nearly neutralized with nitric acid (methyl orange being 

 used as indicator), some pure sodium bicarbonate is at once 

 added, and the solution (in a platinum dish, if one large enough 

 is at disposal, otherwise in a beaker) is placed on the water 

 bath, when it speedily becomes turbid through separation of 

 silica. As soon as the solution is warm it is removed from the 

 bath, stirred, allowed to stand for two or three hours, and then 

 filtered by means of the filter-pump and washed with cold water. 



The filtrate is concentrated to about a quarter of a liter and 

 nearly neutralized, as before, some sodium carbonate is added, 

 and the phosphoric acid is precipitated with silver nitrate in 

 excess. The precipitate is separated by filtration and washed 

 with hot water, and the excess of silver in the filtrate is removed 

 with sodium chlorid. 



The filtrate from the silver chlorid (after addition of some 

 sodium bicarbonate) is evaporated to its crystallizing point, then 

 cooled and diluted with cold water ; still more sodium bicarbon- 

 ate is added, and the whole is allowed to stand, when additional 

 silica will separate, and this is to be removed by filtration. 



This final solution is nearly neutralized, as before; a little 



