J. G. Dinwiddle — Fluorine in Soluble Fluorides. 465 



1. Since the fluoride lias to be treated with acid, it cannot 

 be filtered on asbestos because the hydrofluoric acid set free 

 will attack the mineral of the Alter. 



2. If an ordinary filter is used, particles of the precipitate 

 adhere to it and, when burnt along with the paper, the cal- 

 cium sulphate is partly turned to sulphide and leads to incor- 

 rect results. 



3. If the mixed precipitate is heated to redness in order to 

 obtain a constant weight, the mass fuses together and it is 

 almost impossible to completely decompose the solid mass with 

 sulphuric acid so as to convert all of the fluoride to sulphate. 



4. When the excess of sulphuric acid is being driven off so 

 that the residue of calcium sulphate may be weighed, great care 

 has to be exercised to prevent spattering if the heat is supplied 

 by placing a bunsen burner beneath the crucible. 



The detailed directions for the determination of fluorine by 

 use of powdered calcium sulphate as mentioned above will now 

 be given, and it will be made clear how each of the above diffi- 

 culties was surmounted. 



The solution of the fluoride, which should occupy as small a 

 volume as practicable, say about thirty or forty cubic centi- 

 meters, and should be neutral, is heated to boiling and pow- 

 dered calcium sulphate is added. After standing from thirty 

 minutes to one hour, with frequent stirring, the precipitate of 

 fluoride and sulphate of calcium is washed by decantation sev- 

 eral times and then is put onto the filter for final washing. 

 The filter consists of a perforated platinum crucible in the 

 bottom of which is a small disc of ashless filter paper, cut so as 

 to fit exactly in the bottom without being bent np around 

 the sides. By keeping gentle suction upon the crucible, the 

 disc is held in place and the filtrate comes through without 

 the least turbidity. As soon as the precipitate has been 

 sufficiently washed, it is transferred with the aid of a fine 

 jet of water from a wash bottle to an ordinary platinum cruci- 

 ble ; the disc of paper is washed free of the precipitate and is 

 ignited on the lid of the crucible, while the precipitate in the 

 crucible is evaporated on the steam bath to dryness. If now 

 this residue is heated to redness to obtain a constant weight, it 

 melts and becomes very difficult to decompose with sulphuric 

 acid. By experimenting it was found that at a temperature 

 around 300° C. the calcium sulphate loses all of its crystal 

 water and a constant weight is obtained. The proper temper- 

 ature is obtained by heating the platinum crucible within an 

 ordinary iron crucible of diameter about three inches at the 

 top, used as a radiator. In order to equalize the heat a thin 

 piece of asbestos was placed in the bottom and upon this was 

 placed a small triangle for the platinum crucible to rest upon. 



