34 THE ATOMIC WEIGHTS OF 



Possibly the only drawback involved in the use of formic 

 acid, at this stage of the reaction, is its volatility a property, 

 however, which it shares with a number of other available acids 

 and which is of the greatest importance in the final transposition 

 of the formate to the fluoride. The volatility of formic acid, it 

 is true, favors a slight reversion of the original reaction between 

 this acid and borax; that is to say, sodium formate and boric acid, 

 in very concentrated solution or in the dry state, tend to react 

 again (when heated) with the re-formation of some sodium borate. 

 The extent of this reversion, however, is not alarming under the 

 conditions and is easily counteracted by the addition of a drop 

 or two of strong formic acid to the alcohol in the subsequent 

 evaporations, particularly to the last portions of alcohol used. 

 This mode of procedure insures the complete decomposition of 

 the last traces of borax and was adhered to throughout the work. 



The complete removal of the boric acid was carried out exactly 

 as indicated in the description of experiment iv and need not be 

 repeated. It may be of interest to indicate the quantities of 

 methyl alcohol used in some of these experiments. In experiment 

 ix 82 grams of methyl alcohol were used. This represents the 

 total and includes the final portions which were practically super- 

 fluous. The total quantities of methyl alcohol used in experiments 

 x and xi were 61 and 85 grams, respectively. Considering the 

 quantities of boric acid that had to be expelled these amounts 

 of methyl alcohol actually used seem rather moderate. 



The sodium formate was now ready to be converted into the 

 fluoride. It was therefore dissolved in a small quantity of water, 

 an excess of pure hydrofluoric acid (see p. 11) added, and the 

 mixture slowly evaporated to dryness in a current of air. The 

 residue thus obtained was finally heated to about 200, allowed 

 to cool, and again moistened with a little strong hydrofluoric 

 acid. After the expulsion of the latter it was quite safe to assume 

 that the residue consisted of pure sodium fluoride. In the pre- 

 liminary experiments it was found that, for most purposes, a 

 single evaporation of the dry, porous formate with an excess of 

 strong hydrofluoric acid is quite sufficient, for subsequent treat- 

 ments with the haloid acid produced no appreciable change in 

 weight. 



To be doubly sure of the complete conversion of the formate, 



