Page 148 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECIBIQUES

precipitate is said to be dried or ignited respectively. The meaning will be
adequately conveyed for Our purpose if we designate drying when the temperature
is below 250 OC (the maximum temperature which is readily reached in the usual
thermostatically controlled, electric drying-oven), and ignition above 250 OC up
to, Say 1200 OC. Precipitates that are to be dried should be collected on filter
paper, or in sintered-glass or porcelain filtering crucibles. Precipitates that are
to be ignited are collected on filter paper, porcelain filtering crucibles, or silica
filtering crucibles. Ignition is simply effected by placing in a special ignition dish
and heating with the appropriate burner; alternatively, these crucibles (and,
indeed, any type of crucible) rnay be placed in an electrically heated muffle
furnace, which is equipped with a pyrometer and a means for controlling the
temperature.
Attention is directed to the information provided by thermogravimetric
anal~sis~"~~ concerning the range of temperature to which a precipitate
should be heated for a particular composition. In general, thermal gravimetric
curves seem to suggest that in the past precipitates were heated for too long a
period and at too high a temperature. It must, however, be borne in mind that
in some cases the thermal gravimetric curve is influenced by the experimental
conditions of precipitation, and even if a horizontal curve is not obtained, it is
possible that a suitable weighing form rnay be available over a certain
temperature range. Nevertheless, thermograms do provide valuable data
concerning the range of temperature over which a precipitate has a constant
composition under the conditions that the thermogravimetric analysis was made;
these, at the very least, provide a guide for the temperature at which a precipitate
should be dried and heated for quantitative work, but due regard must be paid
to the general chemical properties of the weighing form.
Although precipitates which require ignition will usually be collected in
porcelain or silica filtering crucibles, there rnay be some occasions where filter
paper has been used, and it is therefore necessary to describe the method to be
adopted in such cases. The exact technique will depend upon whether the
precipitate rnay be safely ignited in contact with the filter paper or not. It must
be remembered that some precipitates, such as barium sulphate, rnay be reduced
or changed in contact with filter paper or its decomposition products.
A. Incineration of the filter paper in the presence of the precipitate. A silica
crucible is first ignited to constant weight (i.e. to within 0.0002 g) at the same
temperature as that to which the precipitate is ultimately heated. The well-
drained filter paper and precipitate are carefully detached from the funnel; the
filter paper is folded so as to enclose the precipitate completely, care being taken
not to tear the paper. The packet is placed point-down in the weighed crucible,
which is supported on a pipe-clay, or better, a silica triangle resting on a ring
stand. The crucible is slightly inclined, and partially covered with the lid, which
should rest partly on the triangle. A oery small jlame is then placed under the
crucible lid; drying thus proceeds quickly and without undue risk. When the
moisture has been expelled, the flame is increased slightly so as to carbonise
the paper slowly. The paper should not be allowed to inflame, as this rnay cause
a mechanical expulsion of fine particles of the precipitate owing to the rapid
escape of the products of combustion: if, by chance, it does catch fire, the flame
should be extinguished by momentarily placing the cover on the mouth of the
crucible with the aid of a pair of crucible tongs. When the paper has completely