CHAPTER IV.

VOLUMETRIC ASSAYS.


These have been already described as those in which the results are got
by measuring, either--(1) the volume of a reagent required to complete
some reaction, or (2) the volume of the resulting product. For example,
if a permanganate of potash solution be added to a solution containing a
weighed amount of iron, dissolved in sulphuric acid, the strong colour
of the permanganate of potash will be removed until a certain quantity
of it has been added. Repeating the experiment, it will be found that
the same amount of iron decolorises the same volume of the permanganate
solution within certain narrow limits of variation, known as "error of
experiment." This error is due to variation in the method of working and
to slight differences in the weighings and measurings; it is present in
all experimental methods, although the limits of variation are wider in
some than in others. Apart from this error of experiment, however, it is
certain that a given volume of the permanganate of potash solution
corresponds to a definite weight of iron, so that if either is known the
other may be calculated. Similarly, if a known weight of zinc (or of
carbonate of lime) be dissolved in hydrochloric acid, a gas will be
given off which can be measured, and so long as the conditions of the
experiment do not vary, the same weight of zinc (or of carbonate of
lime) gives off the same volume of gas. The weight of the one can be
determined from the volume of the other.