286, 287] Portion Insoluble in Hydrochloric Acid 169 



to be continued until nearly all the acid has been volatilised. 

 When the capsule is nearly dry, allow it to cool, dilute with 

 water acidulated with hydrochloric acid, filter, wash thoroughly, 

 dry, ignite, and weigh. The residue consists of sand and 

 amorphous silica. The silica which results from the decom- 

 position of the clay may be dissolved out with strong sodium 

 carbonate solution, and the residue weighed again as sand. 

 The filtrate is analysed as described in paragraph 272. 



286. Hydrofluoric Acid Method.— This method de- 

 pends upon the fact that when hydrofluoric acid acts upon 

 silica a gaseous product is formed according to the equation 



Si0 2 + 4HF = 2 H 2 + SiF 4 . 



Thus all the silica will pass off in the course of evaporation. 



Weigh out about 2 grams of the soil in a platinum dish, 

 ignite, and weigh accurately as described in paragraph 285. 

 Now add concentrated HF until the substance is just covered. 

 Digest on the water bath for an hour, then add another portion 

 of the acid, and digest again for half-an-hour. Now add 2 c.c. 

 of sulphuric acid (equal parts acid and water), and heat up 

 gradually over an Argand. The heating must be continued 

 until the acid fumes cease to come off. Allow the residue to 

 cool, and treat with 20 c c. of strong HC1. Allow to stand on 

 the water bath for half-an-hour, then dilute. A clear solution 

 should be obtained. Should any insoluble matter be present, 

 decant the liquor into a beaker, and treat with HF again. 



The liquid eventually obtained may be analysed according 

 to the table shown in paragraph 272, the silica being estimated 

 by difference. 



287. The Fusion Method.— For this method two 

 portions must be used, one of which is fused with a mixture of 

 potassic and sodic carbonates, which form alkaline silicates and 

 so render the substance soluble; whilst the other is heated 



