(2) 1 gram Fe = 35.6 c.c. KMnO_{4} + 0.5 gram MnO_{2}
.'. 100 c.c. KMnO_{4} = 35.6 c.c. KMnO_{4} + 0.5 gram MnO_{2}
(100-35.6) c.c. KMnO_{4} = 0.5 gram MnO_{2}
64.4 c.c. KMnO_{4} = 0.5 gram MnO_{2}

The iron does not enter into the calculation if the same quantity is
present in the two experiments.

An indirect titration thus requires three determinations, but if more
than one assay is to be carried on, two of these need not be repeated.
The standard is calculated in the usual way.

~Colorimetric Assays.~--These are assays in which the colour imparted to
a solution by some compound of the metal to be determined is taken
advantage of; the depth of colour depending on the quantity of metal
present. They are generally used for the determination of such small
quantities as are too minute to be weighed. The method of working is as
follows:--A measured portion of the assay solution (generally 2/3, 1/2,
1/3, or 1/4 of the whole), coloured by the substance to be estimated, is
placed in a white glass cylinder standing on a sheet of white paper or
glazed porcelain. Into an exactly similar cylinder is placed the same
amount of re-agents, &c., as the portion of the assay solution contains,
and then water is added until the solutions are of nearly equal bulk.
Next, a standard solution of the metal being estimated is run in from a
burette, the mixture being stirred after each addition until the colour
approaches that of the assay. The bulk of the two solutions is equalised
by adding water. Then more standard solution is added until the tints
are very nearly alike. Next, the amount added is read off from the
burette, still more is poured in until the colour is slightly darker
than that of the assay, and the burette read off again. The mean of the