For example, the action of sulphuric acid upon soda results in nothing
which makes the action conspicuous; if, however, litmus or
phenolphthalein be added the change from blue to red in the first case,
or from red to colourless in the second, renders the finishing-point
evident. Some indicators cannot be added to the assay solution without
spoiling the result; in which case portions of the assay solution must
be withdrawn from time to time and tested. This withdrawal of portions
of the assay solution, if rashly done, must result in loss; if, however,
the solution is not concentrated, and if the portions are only withdrawn
towards the end of the titration, the loss is very trifling, and will
not show-up on the result. The usual plan adopted is to have a solution
of the indicator placed in drops at fairly equal intervals distributed
over a clean and dry white porcelain-plate: a drop or two of the
solution to be tested is then brought in contact with one of these and
the effect noted. Another plan is to have thin blotting-paper, moistened
with a solution of the indicator and dried; a drop of the solution to be
tested placed on this shows the characteristic change. When the assay
solution contains a suspended solid which interferes with the test, a
prepared paper covered with an ordinary filter-paper answers very well;
a drop of the solution to be tested is placed on the filter-paper, and,
sinking through, shows its effect on the paper below.

Except when otherwise stated, all titrations should be made at the
ordinary temperature; cooling, if necessary, by holding the flask under
the tap. When a titration is directed to be made in a boiling solution,
it must be remembered that the standard solution is cold, and that every
addition lowers the temperature of the assay.

On running the solution from the burette into the assay, do not let it
run down the side of the flask. If a portion of the assay has to be