§ 53. SILICIC ACID. 7^ 



1. Decomposition with hydrofluoric acid.— Provide a 



leaden cup about 16 cm. in diameter and 16 cm. deep 

 with a close-fitting cover, with projections on the sides 

 about 8 cm. from the bottom, supporting a perforated 

 shelf, and with a shallow tray in the bottom about 12 cm. 

 in diameter and 3 cm. deep, all made of lead ; spread a 

 layer of finely powdered fluor spar about 12 mm. deep, 

 over the bottom of the tray in the cup, and mix it with 

 enough concentrated sulphuric acid to make a thin paste ; 

 23ut the shelf in its place and on the shelf a shallow plat- 

 inum dish, such as a crucible cover, containing 1-2 grms. 

 of the very finely pulverized and carefully weighed sub- 

 stnnce, spread over the surface of the dish in as thin a 

 layer as possible and moistened with sulphuric acid ; 

 put the cover on tlie cup, and set it in a warm j^lace 

 wliere the temperature is about 60° or 70° C, and lift 

 the cover a few times in the course of the digestion ; 

 the evolution of the hydrofluoric acid should be main- 

 tained all the time. After 48 hours take the substance 

 out, expel most of the sulphuric acid by heat, boil the 

 residue with dilute hydrochloric acid, and, if anything 

 remains undissolved, treat this residue with hydrofluoric 

 acid in the same manner as above described. The 

 alkaline metals can be determined in this solution by 

 hydrochloric acid. 



2. Decomposition hy ammonic fluoride. — This method 

 is considered by many to be easier of execution and more 

 certain in its results than the other. Mix the very finely 

 pulverized silicate with 4-5 times its weight of ammonic 

 fluoride in a platinum dish, moisten the mixture thorough- 

 ly with concentrated sulphuric acid, and heat the whole 

 on the water-bath in a place where the fumes of hydro- 

 fluoric acid will be carried off speedily; after a time, 

 when the evolution of acid fumes has ceased, moisten the 

 residue again with sulphuric acid, and heat it, directly 

 over the lamp at last, until it is completely dry and all 



