Precipitated as Thorium Fluoride. 371 



is ignited with the paper filter and the fluorine present is 

 calculated from the weight of the residual thorium 

 oxide, Th0 2 . 



In preliminary experiments upon this method it 

 became plain that the precautions relating to acidity and 

 excess of the precipitant must be more accurately defined 

 and later it was found that the tedious succession of par- 

 tial precipitations when unknown amounts of fluoride are 

 to be titrated may be obviated by a simple modification 

 of method. 



The experiments to be discussed were made with a 

 solution of thorium nitrate made by dissolving 10 grm. of 

 the hydrous salt, Th(N0 3 ) 4 .12H 2 0, in a liter of cold water 

 acidulated with 1 cm 3 of glacial acetic acid, and with a 

 nearly N/10 solution of sodium fluoride made by dissolv- 

 ing about 4-2 grm. of that salt in a liter of cold water con- 

 taining a small amount (0-5 grm. to 1 grm.) of sodium car- 

 bonate, to insure alkalinity of the solution. The exact 

 strength of the thorium nitrate solution was determined 

 by precipitating the thorium as oxalate and weighing the 

 ignited thorium oxide. The fluoride solution was stand- 

 ardized by precipitation as calcium fluoride and treat- 

 ment, according to the common procedure 2 and also by 

 the more recent procedure of Dinwiddie. 3 Portions of 

 these solutions were measured from burettes for the 

 experiments to be detailed, correction being made for 

 temperature variations. 



In the following table are given the results found in a 

 series of experiments made to determine approximately 

 the effect of thorium nitrate when added in excess to the 

 solution of the fluoride slightly acidulated with acetic 

 acid, the effect of varying amounts of acetic acid, and the 

 effects of free nitric acid and free sulphuric acid. 



The results of Table I show, first, that great care must 

 be taken in respect to the excess of thorium nitrate, used 

 as the precipitating reagent, and that the precipitate will 

 probably be sufficiently insoluble for the purpose of a 

 quantitative determination, if the excess of the thorium 

 salt does not exceed by more than 50% the amount theo- 

 retically needed in the metathesis. 



Furthermore, it is plain that free nitric acid, even in 

 very small amount, exerts a strong and inadmissible sol- 



2 Treadwell-Hall ; Analytical Chemistry, 4th ed., vol. 2, 471. 

 s This Journal, (4), 42, 421, 1916. 



