Determination of Ferrous Oxide. 113 



temperature required to drive off the water some, if not all, of 

 the ferrous oxide will take up oxygen, the amount taken up de- 

 pending on the time and the temperature to which it is heattd. 



3. Determination of Ferrous Oxide. 



The determination of ferrous oxide in insoluble silicates by 

 the use of hydrofluoric acid in the presence of carbon dioxide 

 presents so many difficulties that in many cases it has not been 

 attempted, although essential to a proper understanding of the 

 constitution of the mineral under consideration. In 1867, Prof. 

 Cooke described* a method which gives excellent results, and 

 which requires the use of only simple apparatus, the principal 

 objection to it being the difficulty of obtaining pure concen- 

 trated hydrofluoric acid. In 1877, Prof. Leeds, at a meeting of 

 the New York Academy of Science, read a paperf on a new 

 method, where the hydrofluoric acid used is manufactured dur- 

 ing the progress of the analysis, within the apparatus employed. 

 Yery good results are also obtained by this method, but the ap- 

 paratus is somewhat complicated. A third method is given by 

 Ddlter;}: which seems to be quite satisfactory, but here again 

 hydrofluoric acid is employed. 



We have used ammonium fluoride, a substance which can 

 readily be obtained pure, and can be kept unchanged. The 

 method is in short as follows: About 0*5 gm. of the pulverized 

 mineral is put into a large platinum crucible over a water bath, 

 and a slow stream of carbon dioxide is carried into it. When 

 the air is expelled, a few drops of concentrated sulphuric acid 

 are added, then some pure ammonium fluoride and the whole 

 stirred with a platinum spatula. Similar additions are made 

 from time to time, until the mineral is completely decomposed, 

 when the contents of the crucible are emptied in a beaker con- 

 taining cold water, the solution diluted, and the iron determined 

 as usual by means of potassium permanganate. 



There are several points with reference to apparatus and 

 manipulation that it is desirable to describe more in detail. 



Fine pulverization of the mineral is quite necessary, and we 

 have found it desirable to rub the powder a second time in the 

 agate mortar, after the sample had received the usual prepara- 

 tion for decomposition by fusion. 



The platinum crucible should be of at least 50 c.c. capacity 

 so that it can be immersed in the hot water of the water bath to 

 the height that the frothing reaches inside at the beginning of 

 the operation. Otherwise some of the material carried up by 

 the froth may stick to the sides of the crucible, and not be thor- 

 oughly decomposed. 



*This Journal, IT, xliv, p. 347, 1867. f Am. Chemist, vii, p. 396, 1877: 



JZeitschr. f. Anal. Chem., xviii, 50, 1879. 



Am. Jour. Sci — Third Series, Vol. XXXIV, No. 200.— August, 1887. 



