416 H. JV. /Stokes — Pyrite and Marcasite. 



Also, 



3 (c—a) = ferrous iron produced according to equation (1) 

 and 



b-S (c—a) = ferrous iron produced by oxidation of sulphur. 



According to equation (2), 1 atom of sulphur requires for 

 oxidation 6 atoms of ferric iron, producing 6 atoms of ferrous 

 iron: hence, 



1 31*83 / \ 



— . ' ( b— 3(c — a) ) = sulphur oxidized. (B) 



b 55'bO \ J 



Calling the percentage of sulphur oxidized p, we obtain 

 from (A) and (B) 



100 31-83/, n , A 



. lb — 3 (c— a)\ 



p = 6 55-60 V l ■ >) = 8-333^ _ ^ 



c—a 



/3l'83\ 



(two) {c ~ { 



It thus appears that three titrations suffice to determine the 

 percentage of sulphur oxidized, and that neither the amount 

 of FeS 2 , the volume of the ferric solution used, nor the absolute 

 titer of the latter or of the permanganate need be known. As, 

 however, the proportion of sulphur oxidized varies with the 

 strength of the ferric solution, the extent of reduction, and the 

 temperature, it is necessary, in order to obtain comparable 

 results, to use a solution of standard composition and a standard 

 temperature, and to continue the action to complete reduction. 



The value p, or the percentage of sulphur oxidized, may be 

 called the oxidation coefficient. The oxidation coefficient of 

 pyrite, as established by the study of various specimens, varies 

 between 60 and 61 with a mean of 604, while that of marca- 

 site varies between 16*5 and 18. In duplicate determinations, 

 properly made, it may differ about one-half unit. It is clear 

 then that in these figures we have perfectly characteristic con- 

 stants of the two minerals. It may be noted that the figures 

 are independent of any contaminations which do not affect the 

 titer of the solution, or which can be removed by previous 

 treatment. 



The explanation of the oxidation coefficient and its difference 

 in the two cases is probably the following: the molecules of 

 FeS 2 are acted on by a reagent which is capable of oxidizing 

 the iron more rapidly than the sulphur. The rate of solution 

 of the pyrite is greater than the oxidation rate of the sulphur, 

 hence a portion of the latter, under the given conditions about 

 40 per cent, escapes and when once in the free state, as I have 

 found, is scarcely attacked ; at the same time the limit of the 



