Page 145 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CRUCIBLES WITH PERMANENT POROUS PLATES 3.35

paper is never filled completely with the solution; the level of the liquid should
not rise closer than to within 5-10mm of the top of the paper. A precipitate
which tends to remain in the bottom of the beaker should be removed by holding
the glass rod across the beaker, tilting the beaker, and directing a jet of water
from a wash bottle so that the precipitate is rinsed into the filter funnel. This
procedure may also be adopted to transfer the last traces of the precipitate in
the beaker to the filter. Any precipitate which adheres firmly to the side of the
beaker or to the stirring rod may be removed with a rubber tipped rod or
'policeman' (Section 3.23).
Filtration by suction is rarely necessary: with gelatinous and some finely
divided precipitates, the suction will draw the particles into the pores of the
paper, and the speed of filtration will actually be reduced rather than increased.

3.35 CRUCIBLES WITH PERMANENT POROUS PLATES
Reference has already been made in Section 3.24 to these crucibles and to
crucibles with a porous base. In use, the crucible is supported in a special holder,
known as a crucible adapter, by means of a wide rubber tube (Fig. 3.12); the
bottom of the crucible should be quite free from the side of the funnel and from
the rubber gasket, the latter in order to be sure that the filtrate does not come
into contact with the rubber. The adapter passes through a one-holed rubber
bung into a large filter flask of about 750mL capacity. The tip of the funnel
must project below the side arm of the filter flask so that there is no risk that
the liquid may be sucked out of the filter flask. The filter flask should be coupled
with another flask of similar capacity, and the latter connected to a water filter
pump; if the water in the pump should 'suck back', it will first enter the empty
flask and the filtrate will not be contaminated. It is advisable also to have some
sort of pressure regulator to limit the maximum pressure under which filtration
is conducted. A simple method is to insert a glass tap in the second filter flask,
as in the figure; alternatively, a glass T-piece may be introduced between the
receiver and the pump, and one arm closed either by a glass tap or by a piece
of heavy rubber tubing ('pressure' tubing) carrying a screw clip.

Fig. 3.12

When the apparatus is assembled, the crucible is half-filled with distilled
water, then gentle suction is applied to draw the water through the crucible.
When the water has passed through, suction is maintained for 1-2 minutes to
remove as much water as possible from the filter plate. The crucible is then
placed on a small ignition dish or saucer or upon a shallow-form Vitreosil