METHOD OF LASNE 267 



229. Burk's Modification of Carnot's Method. Carnot's method 

 is based on the digestion of a substance containing silica and a 

 fluorid with sulfuric acid and conducting the silicon tetrafluorid 

 evolved into a solution of potassium fluorid. 20 The reactions 

 which take place are expressed by the following formulas : 



1. CaF 2 + H,SO 4 = 2HF + CaSO 4 . 



2. 4HF+ Si6 2 = SiF 4 + 2 H 2 O. 



3. SiF 4 + 2KF = K 2 SiF c . 



The potassium fluosilicate separates in part and is completely 

 precipitated by 90 per cent, alcohol. After standing, the precipi- 

 tate is collected on a filter, washed free of potassium fluorid by 90 

 per cent, alcohol, and dried to constant weight. 



Two-thirds of the fluorin in the dried product is derived from 

 the mineral under investigation. The number representing the 

 weight of the precipitate multiplied by 0.34511 gives the fluorin, 

 and this multiplied by 2.0527 gives the weight of the calcium 

 fluorid corresponding thereto. Burk describes the apparatus suited 

 to the conduct of the work, and states the conditions under which 

 it is accurate. 27 The chief sources of error are: 



1. Moisture in the air or tubes through which the silicon tetra- 

 fluorid passes. 



2. Fumes of sulfuric acid carried over in the air current or 

 otherwise. 



3. Insoluble addition products of potassium fluorid with the 

 silica of the glass. 



The methods of avoiding these sources of error are set forth 

 in detail in the paper cited above. 



230. Method of Lasne. The difficulties which have been men- 

 tioned led Lasne to modify the method of separating the hydro- 

 fiuosilicic acid in such a way as to render it practicable and 

 exact. 28 



The modification of Lasne consists essentially in removing the 

 hydrofluosilicic acid evolved by the action of concentrated sul- 

 furic acid on natural phosphate by a current of dry air conducted 



26 Comptes rendus, 1892, 114 : 75, 1003. 



27 Journal of the American Chemical Society, 1901, 23 : 825. 



m Bulletin de la Socie'te' chimique de Paris, 1888, [2], 50 : 167. 

 Annales de Chimie analytique, 1897, 2 : 161, 182. 



