Mineral Sulphides of Iron. 175 



depends on the difference in behavior between the two min- 

 erals toward a solution of ferric sulphate. In both cases the 

 sulphide which is used in excess reduces the ferric salt com- 

 pletely to ferrous sulphate, while the mineral itself is oxidized 

 to ferrous sulphate, sulphuric acid and free sulphur. It is in 

 the relative quantities of the products that the difference 

 between pyrite and marcasite shows itself. The reaction may 

 be represented in two stages as follows : 



1. FeS a + 'Fe,(SOj, = 3FeS0 4 + 2S 



2. 2S + 6Fe 2 (S0 4 ) 3 + 8H 2 = 12FeS0 4 + 8 H 2 S0 4 



In that portion of the sulphide which takes part in the reaction, 

 the percentage of the sulphur which is oxidized to sulphuric 

 acid depends on conditions, but, ceteris paribus, the quantity 

 is much greater for pyrite than for marcasite. In order to 

 distinguish between the two it is therefore necessary to deter- 

 mine what percentage of the sulphur is oxidized to sulphuric 

 acid. Stokes found it most advantageous to use a standard 

 solution (ferric ammonium alum) containing 1 g. of iron and 

 4*0 g.* free sulphuric acid per liter, and to operate at the boil- 

 ing temperature. Under these conditions, he found that* 60 

 per cent of the sulphur in pyrite was oxidized, and only 18 per 

 cent of the sulphur in marcasite. Instead of determining 

 directly the quantity of free sulphur or sulphuric acid formed 

 in the reaction, Stokes measured the increase in the concentra- 

 tion of ferrous iron in solution. By stochiometrical calcula- 

 tion, for the details of which the original paper must be con- 



suited, he derived the formula,^ = 25, where p is the 



c - ■* a 



percentage of sulphur in the reacting sulphide which is oxidized 

 to sulphuric acid, b the ferrous iron, c the total iron in the 

 solution at the end of the operation, and a the total iron in the 

 original solution. There is one point on which Stokes does 

 not express a perfectly clear view, viz : whether the solution 

 at the end of the oxidation should contain any ferric iron or 

 not. As a matter of fact, the reaction, when successful, pro- 

 ceeds to completion, so that all the iron at the end of the 

 operation is ferrous ; b and c are consequently identical, and 



the formula may therefore be simplified to p = t 25. 



Stokes determined the ferrous iron by direct titration with 

 permanganate. Afterwards he reduced the solution and titra- 

 ted again so as to determine the total iron, ferrous and ferric. 

 This reduction and subsequent titration are evidently super- 

 fluous. The process has been improved in several other 



*This is our interpretation of Stokes's statement: "16 cm3 of 25 per cent 

 sulphuric acid." 



