370 Gooch and Kobayashi — Determination of Fluorine 



Art. XXIV. — The Gravimetric and Volumetric Deter- 

 mination of Fluorine Precipitated as Thorium Flu- 

 oride; by F. A. Gooch and Matsusuke Kobayashi. 



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



The work of which an account is here given is the 

 result of an attempt to base a volumetric method for the 

 determination of fluorine upon the recent method of 

 Pisani 1 for the gravimetric determination of that element 

 by precipitation as hydrous thorium fluoride and the con- 

 version of the last to thorium oxide by ignition. Inci- 

 dentally, it became necessary to modify somewhat the 

 procedure of Pisani when applied either gravimetrically 

 or as preliminary step in the volumetric determination 

 of fluorine. 



According to the procedure of Pisani, thorium nitrate 

 is added gradually in limited amounts to the solution of 

 an alkali fluoride slightly acidulated with acetic acid, or 

 even with nitric acid, and the heavy gelatinous precipi- 

 tate allowed to settle after every small partial precipita- 

 tion before the addition of more of the precipitant. 

 When the precipitate is considerable enough water must 

 be added to permit the decanting of a large portion of 

 the clear liquid into a conical vessel for further treat- 

 ment with the precipitant, to make it sure that no fur- 

 ther precipitation will take place. If necessary, the 

 deposition is allowed to continue for some hours and too 

 great an excess of the precipitant must not be added at 

 once, because, in a concentrated liquid, thorium nitrate 

 may dissolve a little of the precipitate. When, however, 

 the content in fluorine is known approximately, the 

 necessary quantity of thorium nitrate may be added at 

 the beginning in sufficiently large amount to complete 

 the action at once. The delicacy of the reaction is very 

 great, permitting the recognition of as little as one part 

 of fluorine in ten thousand parts of the liquid. In the 

 washing many decantations are employed and the clear 

 liquid is decanted into a large vessel and. allowed to stand 

 for some time before filtering, to prevent the small 

 amount of suspended material from passing the filter. 

 The precipitate of hydrous thorium fluoride, ThF 4 .4H 2 0, 



1 Compt. rend., 162, 791, 1916. 





