32 THE ATOMIC WEIGHTS OF 



volatile ammonium borofluoride (NH 4 BF 4 ) and sodium fluoride 

 are produced. Reischle 3 recommended this procedure for the in- 

 direct determination of boric acid in borax, the only difference 

 being that he prefers to finish with sulphuric acid and to weigh 

 the resulting sodium sulphate. 



The determination, according to Reischle, is carried out as 

 follows: Powdered borax (the decahydrate) is mixed with six 

 times its weight of resublimed ammonium fluoride and gently 

 heated. The major portion of the boron is thus expelled as 

 ammonium borofluoride. After cooling, the residue is evaporated 

 with sulphuric acid, which, as the author states, removes the last 

 traces of boron fluoride and ammonium borofluoride. According 

 to Reischle this method is superior to all others, including Gooch's 

 distillation method. By the latter method Reischle found from 

 90.8 to 101.0 per cent, of the boric acid present. Without dis- 

 cussing the pros and cons, it seems entirely proper to say that these 

 poor results are not entirely chargeable to the Gooch method. 



The ammonium fluoride method has also been adopted, and 

 found satisfactory, by Waldbott 4 in the analysis of borax. The 

 method may be of great value for ordinary analytical purposes; 

 for the derivation of a ratio it was soon discovered to be of little 

 promise. 



In the present instance the initial substance, the substance 

 which had to be weighed, was fused borax. In order to produce 

 a thorough mixture with the reagent the anhydrous borax was 

 dissolved in water and evaporated with an excess of ammonium 

 fluoride, which was added either in solid form to the concentrated 

 borax solution or in the form of a solution obtained by combining 

 pure hydrofluoric acid with redistilled ammonia. It was found 

 that the resulting mixture is evaporated to dryness with great 

 difficulty, for even when the apparatus was heated in an oven the 

 formation of crusts retarded the evaporation to an annoying degree. 

 Several such treatments would have been required to make sure of 

 the complete removal of the boron with the production of pure 

 sodium fluoride, the weight of which was to be determined before 

 it was converted into the sulphate. In the final ignition, further- 

 more, appreciable quantities of sodium fluoride were volatilized, a 

 loss which is probably aggravated by the repeated sublimation 



J Z. anorg. Chem. 4, 111-116 (1893). * J. Am. Chem. Soc., 16, 410-418 (1894). 



