By adding 1 c.c. to the quantity of gas obtained, and taking 0.4604 as
the standard, the calculated results will agree with those found with a
variation of 0.1 c.c. When a large number of assays of the same kind are
being made, this method of calculation is convenient; when, however,
only one or two determinations are in question, it is easier to make a
couple of standardisings, taking quantities as nearly as possible the
same as those present in the assays.

Sometimes it is necessary to draw up a table which will show, without
calculation, the weight of substance equivalent to a given volume of gas
or of solution. The substance used for standardising should be, whenever
possible, a pure sample of the substance to be determined--that is, for
copper assays pure copper should be used, for iron assays pure iron, and
so on; but when this cannot be got an impure substance may be used,
provided it contains a known percentage of the metal, and that the
impurities present are not such as will interfere with the accuracy of
the assay. Including compounds with these, the standard may be
calculated by multiplying the standard got in the usual way, by the
percentage of metal in the compound or impure substance, and dividing by
100. If, for example, the standard 1.008 gram was obtained by using a
sample of iron containing 99.7 per cent. of metal, the corrected
standard would be 1.008◊99.7/100 = 1.005.

In volumetric analysis the change brought about must be one in which the
end of the reaction is rendered prominent either by a change of colour
or by the presence or absence of a precipitate. If the end of the
reaction or finishing-point is not of itself visible, then it must be
rendered visible by the use of a third reagent called an indicator.