•/. (1 . Dinwiddle — Fluorine in Soluble Fl uorides. 4G7 



tate was so hard to handle, in that it ran tli rough into the fil- 

 trate and clogged up the pores of the paper as well, that the 

 attempt was abandoned. Since the solution of which the 

 standard was desired contained no other compound besides 

 sodium fluoride, its concentration was finally determined by 

 evaporating measured portions to dryness in a platinum cruci- 

 ble, igniting to about 300° C. and weighing. The following 

 results were obtained on several portions. 10°° portions of the 

 sodium fluoride solution gave 0-2724, 0-2721, 0-2723, 0-2725, 

 0-2721 grams of sodium fluoride. One determination with 20 cc 

 gave 0-5450 grams. The average of all gave for the standard 

 of the solution that 10 cc contained 0-2724 grams of sodium 

 fluoride. 



The table following shows the results obtained by carrying 

 out the determinations as outlined above : 









Table VI. 









Sol. NaF 





Increase 



NaF 









used 



NaF 



F 2 -» S0 4 



equiv. 



Diff. 



% error 



a 



10 cc 



0-2724 



0-1894 



•2740 



+ ■0016 



+ 0-62 



b 



]0 cc 



0-2724 



0-1888 



•2731 



+ •0007 



+ 0-25 



c 



10 cc 



0-2724 



0-1885 



•2727 



+ •0003 



+ 0-11 



d 



10 cc 



0-2724 



0-1882 



•27223 



— •00017 



— 0-062 



e 



10 cc 



0-2724 



0-1883 



•27237 



— -00003 



— 0-015 



f 



]0 co 



0-2724 



0-1876 



•2714 



— •0010 



— 0-37 



9 



20 ec 



0-5448 



0-3752 



•5427 



— •0021 



— 0-385 



h 



]0 cc 



0-2724 



0-1881 



•2721 



— ■0003 



— 0-082 



i 



10 cc 



C-2724 



0-1886 



•2728 



+ •0004 



+ 0-11 



On account of the solubility of calcium fluoride, there will 

 be a tendency for the results to run low and, unless the filtrate 

 and washings are kept to a low volume, large negative errors 

 are liable to occur. In several determinations the filtrate was 

 about 200 cc and the results here were about 1*4 per cent low. 

 On account of this danger, a solution saturated with pure cal- 

 cium fluoride and calcium sulphate was used for wash water 

 in order to eliminate the solubility error and results very close 

 to the calculated value were obtained even when the filtrate 

 was allowed to get quite large. These results are shown in h 

 and i of Table VI. 



The weighed precipitate of calcium fluoride and sulphate 

 might be converted, according to Loczke,* into chloride and 

 sulphate, by evaporating with hydrochloric acid, and the cal- 

 cium sulphate be weighed, the fluoride being determined by 

 difference. However this would require another filtration and 

 would be less accurate. 



* Loczke, Zeitschr. anal. Cliera., xlix, 329. 1910. 



