J. G. Dinwiddle — Hydrofluoric and Fluosilicic Acids. . 423 



cc. of solution. 



f 10 35-78 



10_ 35-79 



No 2 10 - 35 " 80 



^°- *\ 10 35-80 



| 10 12-27 



^ 10 12-30 



H 9 SiF„ NaOH req. NaOH req. NaOH req. 



cc. of solution (in alcohol) (in water) (cc. per gram) 



cc. cc. 



f 10 38-23 



[ 10 38-30 



AT -J 10 : 38-15 



N °- 3 i 10 13-18 



| 10 13-20 



^ 30 39-85 



With each of the three samples, the alcoholic titration requires 

 from one to three per cent more than one-third of that required 

 in the water titration. 



This result might be explained in either of two ways : that 

 the fluosilicic acid, as sold commercially and as made in the 

 Fenfield-Offermann method for determining fluorine, may 

 always contain small amounts of free hydrofluoric acid, or the 

 solubility of potassium fluosilicate may be sufficient to allow it 

 to be appreciably acted upon by the alkali used. The latter of 

 these two explanations seems more plausible ; for when the 

 end-point has been reached, as shown by the pink of the 

 phenolphthalein, the color fades out upon short standing, and 

 continues to disappear after successive additions of alkali suf- 

 ficient to bring back the pink. Upon heating the mixture, the 

 hydrolysis goes much farther and practically all of the potas- 

 sium fluosilicate is converted to fluoride and silica. 



To test Katz's theory of the absorption of hydrofluoric acid 

 by potassium fluosilicate, 10 cc portions of fluosilicic acid were 

 mixed with varying amounts of hydrofluoric acid and these 

 mixtures were titrated, after addition of alcohol and potassium 

 chloride, with approximately 0-3 normal sodium hydroxide. 

 Since the amount of alkali required for each acid separately 

 was known, it was easy to calculate what the mixture should 

 require, were there no complications. These results are shown 

 in Table II. 



The fluosilicic acid used required for 10 oc in alcohol, lOtT 00 

 of standard alkali. Upon addition of increasing amounts of 

 hydrofluoric acid, that unaccounted for seems to reach a 

 maximum which is equivalent to approximately 1'3 CC of the 



