468 J. (r. Dinwiddle — Fluorine in Soluble Fluorides. 



Since, during the precipitation of the fluoride of calcium by 

 the sulphate, an equivalent amount of sulphate ion is set free, 

 it is obvious that this method is adapted without any modifica- 

 tions to the separation and determination of fluorides in the 

 presence of sulphates. It may also be used for the separation 

 of fluorine from other radicals which do not form insoluble 

 compounds with calcium. 



The method of Bunsen for determining fluoride in the 

 presence of phosphoric acid would be applicable here provided 

 that it were accurate. This consists in weighing a mixed pre- 

 cipitate of orthophosphate and fluoride of calcium and then 

 converting it to phosphate and sulphate by ignition with sul- 

 phuric acid. He claims that the final residue after ignition 

 consists entirely of calcium orthophosphate and calcium sul- 

 phate, but Treadwell and Koch,* in experimenting to decide 

 upon the best method for this separation in wines and beers, 

 heated a weighed amount of calcium orthophosphate with sul- 

 phuric acid, as directed by Bunsen, until a constant weight was 

 obtained. Instead of obtaining the original weight they 

 obtained a much greater weight. Upon testing the residue, 

 they found that the precipitate contained a large amount of 

 calcium sulphate and that metaphosphoric acid had been formed 

 and condensed partly on the lid of the crucible. 



* Treadwell and Koch, Zeitschr. anal. Chem. , xliii, 469. 



