Determination of Pyrite and Marcasite. 373 



of iron and 5 g. free sulphuric acid per liter, is always used in 

 the same quantity, 250 cc in each experiment. The sulphide, 

 finely ground and purified, is used in large excess (about 1 g.). 

 The water of the solution is kept constant by a reflux con- 

 denser, while the influence of atmospheric oxygen is prevented 

 by a stream of moist carbon dioxide which traverses the 

 apparatus during the process. At the end of the test the ferric 

 iron of the solution should be entirely reduced, partly by iron 

 and partly by sulphur. A quantity of iron in the ferrous state 

 which is constant for each mineral has passed into solution 

 and a constant percentage of the sulphur has been oxidized. 

 From the iron dissolved in our own experiments, we calculate 

 that about 155 milligrams of marcasite are required in the 

 reduction of 250 cc of the standard solution, while only about 

 65 milligrams of pyrite are required for the same work. This 

 of course is due to the fact that far less of the sulphur in mar- 

 casite is effective in reduction, nearly 90 per cent of it being 

 precipitated in the free state. The complete data are given in 

 the footnote.* 



A. Formula for the percentage of sulphur oxidized.-^- 

 Stokes determined directly the iron dissolved by the ferric 

 salt, and calculated stochiometrically the percentage of sulphur 

 which was oxidized. Similarly, the percentage of sulphur 

 oxidized in unknown mixtures of the two sulphides was esti- 

 mated by him experimentally and a curve plotted which was 

 used to determine their composition. The formula for the 

 precentage of sulphur oxidized or the " oxidation coefficient " 

 of either sulphide or of any mixture of the two was derived 



O.QQ7, 



directly from equations a and &, viz. (1) p = 25, where 



c a 



* I. Pyrite. The increase of iron in solution after reduction with pyrite 

 was equivalent to 446 g. KMn0 4 solution per 100 CC , or for 250 cc . 

 2-5 x 4-46 x -001544 (value of KMn0 4 solution) = KMn0 4 .-_. 0-0172 g 



Equivalent to Fe "0304 g 



Equivalent to FeS 2 - "0652 g 



Total sulphurin '0652 g. FeS 2 .-. _ '0348 g 



Fe reduced by Fe (see equation a) -. -0608 g 



Fe reduced by S, 0-2500 -'0608 -1892 g 



S reducing Fe, or S oxidized (see equation b) "0181 g 



•0181 

 Percentage of S oxidized ;wTo 52.$ 



II. Marcasite. The increase of iron in solution after reduction with 

 marcasite was equivalent to 10*57 g. KMn0 4 solution per 100 cc . 



Hence 2'5 x 10'57 x 001544 = KMn0 4 00408 g. 



Equivalent to Fe .. '0721 g. 



Equivalent to FeS 2 -- "1546 g. 



Total sulphur in 0-1546 g. FeS 2 -0825 g. 



Fe reduced by Fe (see equation a) "1441 g. 



Fe reduced by S, 0'2500--1441 "1059 g. 



S reducing Fe, or S oxidized (see equation b) "0101 g. 



Percentage of S oxidized 12'3# 



'0825 



