Pratt — Determination of Ferrous Iron in Silicates. 149 



Akt. XX. — On the Determination of Ferrous Iron in 

 Silicates; by J. H. Pratt. 



The determination of ferrous iron in silicates made by dis- 

 solving tliem in a mixture of hydrofluoric and sulphuric acids 

 and titrating with potassium permanganate is one of the well 

 recognized methods of analytical chemistry. The apparatus 

 described by Prof. J. P. Cooke,* by means of which the solu- 

 tion can be made in an atmosphere of steam and carbon di- 

 oxide, is well adapted for preventing any oxidation by the air 

 but the necessary appliances are not always at hand. Prof. 

 S. L. Pen field has suggested to the author a simpler means for 

 accomplishing the same purpose, namely to displace the air by 

 carbon dioxide led directly into the crucible by means of a 

 platinum tube, and at his request a series of experiments has 

 been made to determine the accuracy of the method. 



The arrangement of the apparatus is similar to that devised 

 by H. Posef for igniting precipitates in hydrogen. The solu- 

 tion can be accomplished by boiling in a crucible directly over 

 a small flame and after cooling the contents are transferred to 

 a beaker or caserole and titrated. With a 50 cc crucible one- 

 third full, the boiling may be continued for ten minutes with 

 safety, but if too much of the water and hydrofluoric acid are 

 evaporated the resulting hot concentrated sulphuric acid will 

 act as an oxidizing agent. The solution may also be made on 

 a water bath, but experiments with black tourmaline which is 

 a very difficult mineral to dissolve, have shown that nearly as 

 much was dissolved by boiling for ten minutes over a flame as 

 by heating quietly for two hours on the water bath. 



For testing the method, weighed quantities of ferrous sul- 

 phate were used. In the first series of experiments dilute 

 sulphuric acid was taken as a solvent, and the results indicate 

 that the heating may be continued for a considerable time 

 without fear of oxidation. 



Time. 



Method of heating. 



FeO taken. 



FeO found. 



Error. 



30 minutes 



water bath 



•0786 



•0782 



•0004 — 



3-J- hours 



water bath 



•0711 



•0709 



•0002 — 



30 minutes 



water bath 



•0785 



•0785 



•0000 



1 hour 



water bath 



•0978 



•0979 



•0001 + 



15 minutes 



water bath 



•1012 



•1014 



•0002 + 



10 minutes 



flame 



•0810 



•0808 



•0002 — 



10 minutes 



flame 



•0835 



•0836 



•0001 + 



* This Journal, II, xliv, 347. 



T Handbuch der Anal. Chem., 1871, p. 77. Also Fresenius' Quantitative 

 Analysis, American edition by O. D. Allen, p. 251. 



