Page 131 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

FILTRATION APPARATUS 3.24

Porosity 3 is suitable for precipitates of moderate particle size, and porosity 4
for fine precipitates such as barium sulphate. These crucibles should not be
heated above about 200 OC.
Silica crucibles of similar pattern are also available, and, although expensive,
have certain advantages in thermal stability.
Filter crucibles with a porous filter base are available in porcelain (porosity 4),
in silica (porosities 1,2,3,4), and in alumina (coarse, medium and fine porosities):
these have the advantage as compared with sintered crucibles, of being capable
of being heated to much higher temperatures. Nevertheless, the heating must
be gradua1 otherwise the crucible may crack at the join between porous base
and glazed side.
For filtering large quantities of material, a Buchner funnel is usually
employed; alternatively, one of the modified funnels shown diagrammatically
in Fig. 3.7 may be used. Here (a) is the ordinary porcelain Buchner funnel, (b)
is the 'slit sieve' glass funnel. In both cases, one or (better) two good-quality
filter papers are placed on the plate; the glass type is preferable since it is
transparent and it is easy to see whether the funnel is perfectly clean. Type (c) is a
Pyrex funnel with a sintered glass plate; no filter paper is required so that
strongly acidic and weakly alkaline solutions can be readily filtered with this
funnel. In al1 cases the funnel of appropriate size is fitted into a filter flask (d),
and the filtration conducted under the diminished pressure provided by a filter
pump or vacuum line.

Fig. 3.7

One of the disadvantages of the porcelain Buchner funnel is that, being of
one-piece construction, the filter plate cannot be removed for thorough cleaning
and it is difficult to see whether the whole of the plate is clean on both sides.
In a modern polythene version, the funnel is made in two sections which can
be unscrewed, thus permitting inspection of both sides of the plate.
In some circumstances, separation of solid from a liquid is better achieved
by use of a centrifuge than by filtration, and a small, electrically driven centrifuge
is a useful piece of equipment for an analytical laboratory. It may be employed
for removing the mother liquor from recrystallised salts, for collecting difficultly
filterable precipitates, and for the washing of certain precipitates by decantation.
It is particularly useful when small quantities of solids are involved; centrifuging,
followed by decantation and re-centrifuging, avoids transference losses and yields
the solid phase in a compact form. Another valuable application is for the
separation of two immiscible phases.