stokes.] ACTION OF FERRIC SOLUTIONS. 17 
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 composi- 
tion and a standard temperature, and to continue the action to com- 
plete reduction. 
STANDARD SOLUTION AND TEMPERATURE. 
The standard solution contains 1 gram ferric iron per liter, and is 
prepared by dissolving in water 8.61 grams clear crystals of pure fer- 
ric ammonium alum, adding 16 cm. 3 25 per cent sulphuric acid and 
diluting to 1 liter. One hundred cm. 3 of this solution must show a 
decided change of color with one drop of the permanganate solution, 
containing about 1.5 grams per liter, and must be free from nitrate. 
The sulphuric acid is added to prevent precipitation of basic salt. 
The experiment is conducted at the boiling temperature. The 
strength of the ferric solution ma}^ vary 1 per cent without apprecia- 
ble influence, and at least 99.5 per cent must be reduced. 
OXIDATION COEFFICIENT. 
The percentage of sulphur oxidized under these conditions and with 
the precautions explained in detail below (p. 20) may be called briefly 
the oxidation coefficient of the pyrite or marcasite. The oxidation 
coefficient of pyrite varies between 00 and <il, and that of marcasite 
between 16.5 and 18. The oxidation coefficients of mixtures of the 
two indicate the relative amount of each present, by reference to an 
empirical curve obtained with artificial mixtures of known composi- 
tion. A theoretical deduction of the relative proportions from the 
oxidation coefficient is impossible in the absence of the necessary 
physical data. 
THEORY OF THE REACTION. 
The mass law states that the active mass of a solid phase is constant; 
hence, the final composition of the resulting solution must be the 
same, irrespective of the relative amount of reagent and sulphide 
used, provided only that the latter, the solid phase, be always in 
excess. With more, or more finely powdered, material 1 — that is, 
with a relatively greater exposure of surface, equilibrium is sooner 
reached, but the composition of the resulting solution, and therefore 
the percentage of sulphur oxidized, is the same. It is unnecessary 
^stwald (Zeitschr. physik. Chemie, Vol. XXXIV, 1900, p. 495) has shown for certain cases 
that an extremely finely powdered substance is more soluble than the same in coarser particles, 
and that the composition of the resulting solution is therefore subject to slight variations. 
Such effects were looked for in the present instance by using coarse and very fine material 
obtained by sedimentation, but if they exist they are smaller than the errors of the method. 
Bull. 186—01 2 
