Page 146 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

capsule and dried to constant weight at the same temperature as that which
will be subsequently used in drying the precipitate. For temperatures up to
about 250 OC a thermostatically controlled electric oven should be used. For
higher temperatures, the crucible may be heated in an electrically heated muffle
furnace. In al1 cases the crucible is allowed to cool in a desiccator before weighing.
When transferring a precipitate into the crucible, the same procedure is
employed as described in Section 3.34 referring to the use of filter papers: care
must be taken that the liquid level in the crucible is never less than 1 cm from
the top of the crucible.
Care must be taken with both sintered glass and porous base crucibles to
avoid attempting the filtration of materials that may clog the filter plate. A new
crucible should be washed with concentrated hydrochloric acid and then with
distilled water. The crucibles are chemically inert and are resistant to al1 solutions
which do not attack silica; they are attacked by hydrofluoric acid, fluorides,
and strongly alkaline solutions.
Crucibles fitted with permanent porous plates are cleaned by shaking out as
much of the solid as possible, and then dissolving out the remainder of the solid
with a suitable solvent. A hot 0.1 M solution of the tetrasodium Salt of the
ethylenediaminetetra-acetic acid is an excellent solvent for many of the
precipitates [except metallic sulphides and hexacyanoferrates(III)] encountered
in analysis. These include barium sulphate, calcium oxalate, calcium phosphate,
calcium oxide, lead carbonate, lead iodate, lead oxalate, and ammonium
magnesium phosphate. The crucible may either be completely immersed in the
hot reagent or the latter may be drawn by suction through the crucible.

Most precipitates are produced in the presence of one or more soluble
compounds. Since the latter are frequently not volatile at the temperature at
which the precipitate is ultimately dried, it is necessary to wash the precipitate
to remove such material as completely as possible. The minimum volume of
the washing liquid required to remove the objectionable matter should be used,
since no precipitate is absolutely insoluble. Qualitative tests for the removal of
the impurities should be made on small volumes of the filtered washing solution.
Furthermore, it is better to wash with a number of small portions of the washing
liquid, which are well drained between each washing, than with one or two
large portions, or by adding fresh portions of the washing liquid while solution
still remains on the filter (see Section 11.8).
The ideal washing liquid should comply as far as possible with the following
conditions.
1. It should have no solvent action upon the precipitate, but dissolve foreign
substances easily.
2. It should have no dispersive action on the precipitate.
3. It should form no volatile or insoluble product with the precipitate.
4. It should be easily volatile at the temperature of drying of the precipitate.
5. It should contain no substance which is likely to interfere with subsequent
determinations in the filtrate.
In general, pure water should not be used unless it is certain that it will not
dissolve appreciable amounts of the precipitate or peptise it. If the precipitate