16 ON PYRITE AND MARCASITE. [bull. 186. 
reduced by the excess of pyrite. Upon the extent to which the sec- 
ond reaction lakes place, as compared with the first, depends the per- 
centage of sulphur oxidized in that portion of the mineral which is 
decomposed. 
If a ferric solution containing a known amount of iron be boiled 
with an indefinite excess of FeS 2 under appropriate precautions, and 
if the increment of iron in the solution be then determined, as well as 
the ferrous salt formed, this increment gives the amount of sulphide 
decomposed; three times this increment gives the quantity of ferrous 
salt formed in equation (1); while any excess of ferrous salt is that 
formed by oxidizing sulphur to sulphuric acid as in equation (2). 
From these data the percentage of sulphur oxidized may be com- 
puted. 
DEDUCTION OF AN EXPRESSION FOR OXIDIZED SULPHUR. 
It is unnecessary to know the absolute quantities of iron and sul- 
phur involved in these reactions, or the strength of the permanganate 
solution. It suffices, in deducing an expression for the oxidized sul- 
phur, to employ symbols expressing the permanganate equivalent of 
the iron, and the symbols used represent simply the volumes of per- 
manganate consumed by a given volume of the solution. 
For a given volume let 
a = iron in the original solution, 
b — resulting ferrous iron, 
c = resulting total iron. 
c — a = increment of iron resulting from decomposition of FeS 2 , 
2 ( _ii ) (c — a)= total sulphur in decompo.-ed sulphide. (A) 
V 55.60 ' 
:; (c — a) — ferrous iron produced according to equation (1) 
Then 
and 
Also, 
and 
b — 3 (c — a) = ferrous iron produced by oxidation ot sulphur. 
According to equation (2), 1 atom of sulphur requires for oxidation 
(') atoms of ferric iron, producing atoms of ferrous iron; hence, 
1 . §M 3 ( b - 3 (c -a))= sulphur oxidized. (B). 
6 55.60 V } J - 
('ailing the percentage of sulphur oxidized p, we obtain from (A) 
and (B) 
V 
100 31.83 / ? \ 
~6~\55.60V 6{C a) ) 8.3336 
1.83 \, s c-a 
•(S£>-> 
25. 
It thus appears that three titrations suffice to determine the per- 
centage of sulphur oxidized, and that neither the amount of FeS 2 , the 
