Page 149 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

DRYING AND ICNlTlNG PRECIPITATES 3.37

carbonised and vapours are no longer evolved, the flame is moved to the back
(bottom) of the crucible and the carbon slowly burned off while the flame is
gradually increased.* After al1 the carbon has been burned away, the crucible
is covered completely (if desired, the crucible may be placed in a vertical position
for this purpose) and heated to the required temperature by means of a Bunsen
burner. Usually it takes about 20 minutes to char the paper, and 30-60 minutes
to complete the ignition
When the ignition is ended, the flame is removed and, after 1-2 minutes, the
crucible and lid are placed in a desiccator containing a suitable desiccant
(Section 3.22), and allowed to cool for 25-30 minutes. The crucible and lid are
then weighed. The crucible and contents are then ignited at the same temperature
for 10-20 minutes, allowed to cool in a desiccator as before, and weighed again.
The ignition is repeated until constant weight is attained. Crucibles should
always be handled with clean crucible tongs and preferably with platinum-tipped
tongs.
It is important to note that 'heating to constant weight' has no real
significance unless the periods of heating, cooling of the cooered crucible, and
weighing are duplicated.

B. Incineration of the filter paper apart from the precipitate. This method is
employed in al1 those cases where the ignited substance is reduced by the burning
paper; for example, barium sulphate, lead sulphate, bismuth oxide, copper oxide,
etc. The funnel containing the precipitate is covered by a piece of qualitative
filter paper upon which is written the formula of the precipitate and the name
of the owner; the paper is made secure by crumpling its edges over the rim of
the funnel so that they will engage the outer conical portion of the funnel. The
funnel is placed in a drying oven maintained at 100-105 OC, for 1-2 hours or
until completely dry. A sheet of glazed paper about 25 cm square (white or
black, to contrast with the colour of the precipitate) is placed on the bench
away from al1 draughts. The dried filter is removed from the funnel, and as
much as possible of the precipitate is removed from the paper and allowed to
drop on a clockglass resting upon the glazed paper. This is readily done by
very gently rubbing the sides of the filter paper together, when the bulk of the
precipitate becomes detached and drops upon the clockglass. Any small particles
of the precipitate which may have fallen upon the glazed paper are brushed
into the clockglass with a small camel-hair brush. The clockglass containing
the precipitate is then covered with a larger clockglass or with a beaker. The
filter paper is now carefully folded and placed inside a weighed porcelain or
silica crucible. The crucible is placed on a triangle and the filter paper incinerated
as detailed above. The crucible is allowed to cool, and the filter ash subjected
to a suitable chemical treatment in order to convert any reduced or changed
material into the form finally desired. The cold crucible is then placed upon the
glazed paper and the main part of the precipitate carefully transferred from the
clockglass to the crucible. A small camel-hair brush will assist in the transfer.
Finally, the precipitate is brought to constant weight by heating to the necessary
temperature as detailed under A.

*If the carbon on the lid is oxidised only slowly, the cover may be heated separately in a flame. It
is, of course, held in clean crucible tongs.