ould be poured out, while hot, on
to a !dry! tile or iron surface.

IGNITION OF PRECIPITATES

Most precipitates may, if proper precautions are taken, be ignited
without previous drying. If, however, such precipitates can be dried
without loss of time to the analyst (as, for example, over night), it
is well to submit them to this process. It should, nevertheless, be
remembered that a partially dried precipitate often requires more care
during ignition than a thoroughly moist one.

The details of the ignition of precipitates vary so much with the
character of the precipitate, its moisture content, and temperature to
which it is to be heated, that these details will be given under the
various procedures which follow.

DETERMINATION OF CHLORINE IN SODIUM CHLORIDE

!Method A. With the Use of a Gooch Filter!

PROCEDURE.--Carefully clean a weighing-tube containing the sodium
chloride, handling it as little as possible with the moist fingers,
and weigh it accurately to 0.0001 gram, recording the weight at once
in the notebook (see Appendix). Hold the tube over the top of a beaker
(200-300 cc.), and cautiously remove the stopper, noting carefully
that no particles fall from it, or from the tube, elsewhere than into
the beaker. Pour out a small portion of the chloride, replace the
stopper, and determine by approximate weighing how much has been
removed. Continue this procedure until 0.25-0.30 gram has been taken
from the tube, then weigh accurately and record the weight beneath the
first in the notebook. The difference of the two weights represents
the weight of the chloride taken for analysis. Again weigh a second
portion of 0.25-0.30 gram into a second beaker of the same size as the
first. The beakers should be plainly marked to correspond with the
entries in the notebook. Dissolve each portion of the chloride in 150
cc. of distilled water and add about ten drops of dilute nitric acid
(sp. gr. 1.20) (Note 2). Calculate the volume of silver nitrate
solution required to effect complete precipitation in each case,
and add slowly about 5 cc. in excess of that amount, with constant
stirring. Heat the solutions cautiously to boiling, stirring
occasionally, and continue the heating and stirring until the
precipitates settle promptly, leaving a nearly clear supernatant
liquid (Note 3). This heating should not take place in direct sunlight
(Note 4). The beaker should be covered with a watch-glass, and both
boiling and