BORON AND FLUORINE. 33 



of rather large quantities of ammonium salts. The method 

 therefore was not suited to an atomic weight determination. 



The principle of the method finally adopted for the conversion 

 of sodium borate into sodium fluoride has been used in experiment 

 iv, where it was applied in the corresponding conversion, involving 

 sodium chloride as an end-product. The method depends on the 

 facts (1) that formic acid reacts " normally" with a solution of 

 borax, that is to say, it liberates boric acid with the production 

 of sodium formate without any disturbing secondary reactions; 

 (2) that sodium formate is easily transposed to a halide by 

 evaporation with a haloid acid. 



It may not be amiss to add a few more words as to the appli- 

 cability of formic acid in this reaction. In the mere expulsion of 

 boric acid by the volatilization method, irrespective of the com- 

 position of the residual sodium salt, acids other than formic may, 

 of course, be used to advantage. In the present case, however, 

 the remaining sodium salt was to be the fluoride, and here formic 

 acid served admirably, if only as a "go-between." In view of the 

 fact that the direct use of hydrofluoric acid presented many 

 difficulties (see p. 2) it would perhaps be difficult to find another 

 acid more suitable than formic as an auxiliary reagent in the con- 

 version of borax into sodium fluoride. 



The chief advantages of formic acid for this purpose seem to 

 be the following: The sodium salt of this acid is most easily 

 dehydrated, and therefore, in the present instance, the only water 

 present in the dry residue (consisting of sodium formate and boric 

 acid) is in combination with the latter, probably forming meta- 

 boric acid. A minimum of free or loosely combined water, of 

 course, is favorable to the formation of the boric ester when the 

 mass is treated with methyl alcohol. It has already been men- 

 tioned that acetic acid, for example, would not answer as well 

 because its sodium salt is dehydrated with difficulty and at a 

 much higher temperature. In the second place, sodium formate 

 is exceedingly soluble in water and also soluble in methyl alcohol. 

 Consequently, only small quantities of water need be used when 

 the residue is dissolved and evaporated in order to expose fresh 

 surfaces before another treatment with the alcohol. The solu- 

 bility of the formate in the alcohol also facilitates the interaction 

 of the substances in question. 



