I I'll •/. <;. Dinxoiddu 1 'V 'uorine in Soluble Fluorides. 



Art. XLA". — Determination of Fluorine in Soluble Fluo- 

 rides ; l)v .1. (t. Dinwiddie. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — cclxxxiv. ] 



In determining fluorine gravimetrically there are several 

 methods in use. The method of Hose* consists in precipitat- 

 ing; calcium carbonate together with calcium fluoride so that 

 the precipitate of calcium fluoride may, with some degree of 

 satisfaction, he filtered and washed. After being ignited, the 

 calcium carbonate is dissolved out by means of 1*5 N acetic 

 acid and the residue of calcium fluoride is washed, ignited and 

 weighed. This procedure is open to two objections : that the 

 calcium fluoride is very appreciably soluble in the dilute acetic 

 acid, and that two filtrations are required. Starck andThorinf 

 precipitate calcium fluoride along with a known weight of cal- 

 cium oxalate and determine the fluorine by difference. These 

 authors claim that the precipitate so formed is granular and 

 easy to wash but Adolph^; found it very hard to handle. 

 Starck§ makes use of the mixed chloride and fluoride of lead. 

 This method is said to give good results provided that care is 

 taken to use very little wash water. 



In the attempt to precipitate fluorine so that it could be 

 separated from fluosilicic acid by filtration, an excess of pow- 

 dered calcium sulphate (CaS0 4 .2H„0) was used. This gave a 

 precipitate of calcium fluoride and sulphate which was easier 

 to filter and wash and had almost no tendency to run through 

 the pores of the filter. It was thought that it might be possible 

 to adapt this to the determination of fluorine in soluble fluo- 

 rides. Calcium fluoride when treated with sulphuric acid is 

 converted to sulphate, the fluorine being expelled as hydro- 

 fluoric acid. Now if a mixture of fluoride and sulphate of cal- 

 cium be similarly treated with sulphuric acid, the only change 

 will be the conversion of the fluoride to sulphate. A gram 

 molecule of calcium fluoride, 78, when changed to sulphate 

 will weigh 136, so that an increase of 58 parts by weight will 

 mean that the precipitate contained 78 parts of calcium 

 fluoride. 



In perfecting this method there were several difficulties 

 which arose in connection with the filtering, ignition, etc., of 

 the precipitates. These will be enumerated, and then will be 

 described the expedients which were used to avoid these diffi- 

 culties. 



*Eose, Liebig's Annalen, lxxii, 343, 1849. 

 f Starck and Thorin, Zeit. Anal. Chem.. li, 1912. 

 JAdolph, J. Am. Ch. Soc, xxxvii. 2500, 1915. 

 § Starck, Zeitschr. anorg. Chem., lxx, 173, 1911. 



