376 Gooch and Kobayashi — Determination of Fluorine. 



The approximately measured solution of alkali fluoride 

 is made barely aeid with acetic acid and a small amount 

 of this solution is taken in a convenient graduate. From 

 a burette filled with the standard thorium nitrate solu- 

 tion a small portion (a few cubic centimeters) of the 

 solution is drawn into a small beaker. To this small por- 

 tion of the thorium nitrate solution the fluoride solution 

 is added from the graduate, drop by drop, with stirring, 

 until a faint but distinct turbidity is formed immediately. 

 The amount of fluoride solution required to bring about 

 the immediate turbidity in the measured amount of tho- 

 rium nitrate solution is noted and from the relation of 

 these amounts of the solutions of fluoride and thorium 

 nitrate thus used is calculated the amount of thorium 

 nitrate solution which should stand in similar relation to 

 the entire amount of the fluoride solution. One half of 

 the amount of thorium nitrate thus calculated, which will 

 be about 25% more than is theoretically required and well 

 within the limit of safety in respect to an excess, will be 

 the proper amount to bring about the complete precipi- 

 tation of all the fluoride. 



The unused portion of the solution is acidified with 

 acetic acid until the acidity falls within the limits of 

 about 0-02 N and 0-2 N — amounting to a content of about 

 0-12 grm. to 1-2 grm. of free acetic acid in every 100 cm 3 

 of solution — and the small portion of the fluoride solu- 

 tion used in the test is returned to the unused solution. 

 The calculated proper amount of thorium nitrate solu- 

 tion (taking into account the amount of it already added 

 in the preliminary test) is added to the entire fluoride 

 solution and the mixture is allowed to stand a few hours. 

 From this point the procedure for the determination of 

 fluorine gravimetrically or volumetrically is that pre- 

 viously described. 



