128 J. G. Dinwiddii — Hydrofluoric and Fhiosilicic Acids. 



showing that all of the water had not hecn driven off. Upon 

 increasing the heat until a low red was reached, there was a 

 continued decrease until the loss in weight was innch greater 

 than that required to account for the loss of water. Therefore, 

 it was found that this method is of no use for distinguishing 

 hetween hydrofluoric and fluosilicic acids, and its value in 

 determining the water of crystallization as given by Stolba is, 

 at least, very doubtful. 



For the determination of fluorine in soluble fluorides, Greef* 

 takes advantage of the fact that ferric chloride forms with 

 sodium fluoride an insoluble complex fluoride, FeF 3 .3NaF, 

 which does not give a red color with potassium sulphocyanide. 

 By addition of a large amount of sodium chloride, the complex 

 fluoride is made sufficiently insoluble, and the end-point of the 

 reaction is obtained by adding 5 cm 8 of a 5 per cent solution of 

 potassium sulphocyanide and 20 cc of a mixture of equal parts 

 of alcohol and ether. When ferric chloride has been added in 

 quantity sufficient to form the sodinm-iron-fluoride, the next 

 drop after vigorous shaking gives a pink color to the alcohol 

 ether layer. Knowing the concentration of the ferric chloride, 

 the amount of fluoride taking part in the reaction is readily 

 calculated according to the equation 



6NaF + FeCl 3 = FeF 3 .3NaF + 3NaCl. 



The results of this method are quite accurate provided that 

 care is used in detecting the first permanent pink, that con- 

 ditions of concentration of the different titrations are uniform, 

 and that the solution of the fluoride as well as the ferric 

 chloride standard be strictly neutral. 



Since the presence of free acid causes a marked shifting of 

 the end-point of this reaction, it was thought worth while to 

 make a series of titrations, varying the amount of free hydro- 

 chloric acid in order to ascertain the effect on the quantity of 

 ferric chloride necessary to give a permanent pink. Table TV 

 gives the results of these titrations. 



From the results as shown in this table, it can be seen that 

 very small amounts of free acid cause a relatively large decrease 

 in the quantity of ferric chloride solution required. Thus in 

 a volume of approximately fifteen cubic centimeters only 0'1 CC 

 of normal hydrochloric acid causes a decrease of nearly four 

 per cent in the result, while as much as 2 CC causes a difference 

 of almost 50 per cent. In numbers 12 and 13 a small 

 amount of acid was added and then the solution was neutral- 

 ized before titration just to ascertain how accurate the deter- 

 mination could be made when the solution was acid to begin 



* Greef, Berichte, xlvi, 251, 1913; see also Guyot, Comptes ReDdus, 

 lxxi, 2T4, 1870. 



