264 AGRICULTURAL ANALYSIS 



sodium carbonate solution is added; it is heated to boiling and 

 an excess of solution of calcium chlorid is added. The precipi- 

 tate of calcium fluorid and carbonate must be boiled for a few 

 minutes, when it can be easily filtered and washed with hot 

 water. The precipitate is then washed from the filter into a 

 small platinum dish and evaporated to dryness, while the filter, 

 after being partially dried and used to wipe off any particles of 

 the precipitate adhering to the dish in which it was formed, is 

 burned, and the ash is added to the main precipitate. This, 

 when dry, is ignited, and allowed to cool ; dilute acetic acid is 

 added in excess, and the whole is evaporated to dryness, being 

 kept on the water bath until all odor of acetic acid has disap- 

 peared. The residue is then treated with hot water, digested, 

 filtered on a small filter, washed with hot water, partially dried 

 put into a crucible, carefully ignited, and weighed as calcium 

 fluorid. The calcium fluorid is then dissolved in sulfuric acid 

 by gentle heating and agitation, evaporated to dryness on a 

 radiator, ignited at full red heat, and weighed as calcium sulfate. 

 From this weight the equivalent weight of calcium fluorid should 

 be calculated, and this should be very close to that actually 

 found as above, but should never exceed it. The difference 

 which is generally about a milligram (sometimes more), is due 

 to silica precipitated with the fluorid. The percentage of fluorin 

 is, therefore, always calculated from the weight of the sulfate. 

 and not from that of the fluorid obtained. 



The main improvements in this method are the use of sodium 

 bicarbonate to separate the silica, and the keeping of the earlier 

 solutions as dilute as possible, which can not be done if ammo- 

 nium carbonate be used for the separation of the silica. These 

 changes make the fluorin estimation, although still tedious, far 

 more rapid than before, and the results are very satisfactory. 



227. Modification of Wyatt. By reason of the tediousness of 

 the method of Chatard given above, Wyatt has sought to shorten 

 the process by the following modification. 24 



The presence of fluorin having been established by a qualita- 

 tive test, its estimation is secured as follows : 

 24 Phosphates of America, 4th Edition, 1892 : 149. 



