166 J. G. Dimoiddie — Fluorine in Soluble Fluorides. 



By heating the bottom of the iron crucible with a bunsen 

 burner to a low red, the contents of the platinum crucible 

 reach a constant weight within one hour or less. 



When a constant weight has thus been obtained, the residue 

 is mixed with a little water and several cubic centimeters of 

 pure sulphuric acid. Tin's mixture is now evaporated on the 

 steam bath as far as it will go at this temperature and then, by 

 heating further, the sulphuric acid is driven off, the last traces 

 requiring the application of a red heat for a few moments. 

 As was stated above, there is great danger of spattering when 

 the excess of acid is being driven off. In order to avert this 

 danger and to permit the acid to be driven off very quickly 

 the following method was adopted : The lid is placed on the 

 crucible, which is resting on a triangle. A Meker burner is 

 fastened above and slightly to one side of the crucible by 

 means of an adjustable clamp. The flame of the burner is 

 allowed to impinge from above at an angle of about forty-five 

 degrees, on the farther side of the lid of the crucible. In this 

 way, the heat can easily be regulated so that the sulphuric acid 

 volatilizes rapidly and, since the heat radiates from above, 

 there is almost no tendency to spatter. The residue obtained 

 by igniting at 300° consists of a mixture of calcium fluoride 

 and sulphate while the residue remaining after volatilization 

 of the sulphuric acid consists entirely of calcium sulphate. 

 The increase in weight of the contents of the crucible is due to 

 the replacement of two atoms of fluorine by the sulphuric 

 acid radical. The changes which have taken place may be 

 represented by 



2NaF -> CaF, -> CaS0 4 . 



Thus 84 of sodium fluoride gives 780 of calcium fluoride and 

 136 - 07 of calcium sulphate. Therefore 58'07 of increase cor- 

 responds to 84 of sodium fluoride and 78 of calcium fluoride, 

 and to estimate the calcium fluoride present in the precipitate, 

 multiply the increase in weight by 78/58"07 = 1*3431 and for 

 the sodium fluoride multiply by 84/58-07 -— 1*4465. 



In order to test out the method which has been described in 

 detail, a solution of pure sodium fluoride was used. Com- 

 mercial sodium fluoride, even that marked C. P., contains silica 

 and so, for a standard solution, pure hydrofluoric acid was 

 neutralized to phenol phthalein with pure sodium hydroxide 

 obtained by allowing moist air to act on metallic sodium in 

 absence of carbon dioxide. This solution was diluted so that 

 it contained about three grams of sodium fluoride per 100 cubic 

 centimeters. To ascertain the exact concentration of this 

 solution, first the method of Rose, of precipitating the fluoride 

 along with the carbonate of calcium, was tried. This precipi- 



