Hileman — Alkalimetrie Estimation of Silicon Fluoride. 329 



Art. XXXI. — The Elimination and Alkalimetric Estimation 

 of Silicon Fluoride in the Analysis of Fluorides y by 

 Albert Hileman. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. cxlviii.] 



The errors of the processes for the determination of fluorine, 

 in which that element is eliminated as silicon fluoride to be 

 subsequently estimated volumetrically, naturally fall into two 

 categories. First, there are those errors which are due to 

 imperfect elimination and collection of the silicon fluoride 

 from the decomposition flask, and, secondly, there are the 

 errors of the titration processes. For the present purpose it 

 will be most convenient to consider the latter class of errors 

 first. 



The Process of Titration. 



As has been indicated, methods have been used for the 

 determination, by volumetric processes, of silicon fluoride. 

 First, the method of Penlield, which depends upon the collec- 

 tion of the silicon fluoride in an alcoholic solution of potassium 

 chloride and the titration, without removal of the precipitated 

 potassium fluosilicate, of the hydrochloric acid set free. It is 

 assumed that in the alcoholic solution the fluosilicic acid and 

 potassium fluosilicate are not appreciably hydrolized and that 

 ammonia may exactly neutralize the hydrochloric acid without 

 attacking the precipitated potassium fluosilicate. Cochineal is 

 used as the indicator. 



3SiF 4 + 4 H 2 4- 4KC1 = 2K,SiF 6 + Si0 4 H 4 + 4HC1 

 NH 4 OH + HCl = NH 4 C1 + H 2 



The second method of titrating the silicon fluoride, resem- 

 bling that of Penfielcl, except that standard sodium hydroxide 

 or potassium hydroxide is employed instead of ammonium 

 hydroxide in titrating the free acid, which is the method of 

 Bullnheimer* and used by Tread well and Koch,f involves very 

 similar reactions. 



3SiF 4 + 4H„0 + 4KCl = K 2 SiF fi + Si0 4 H 4 + 4HCl 

 XaOH + HCl = NaCl + H a O 



The third method, that of Offer man,;): differs from the pre- 

 ceding methods in this respect, that the silicon fluoride is 

 allowed to act upon water and the fluosilicic acid and hydro- 

 fluoric acid thus formed are acted upon by standard potassium 



* Zeitschr. Angew. Chem. 101. 1901. 

 f Zeitschr. Anal. Chem. xliii, 444, 1904. 

 % Zeitschr. Angew. Chem. 615, 1890. 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 130.— October, 1906. 

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