NOTES ON LABORATORY MANIPULATION(6) Filtering with Suction. With a fine-grained crystal meal,
or a precipitate which is not of such a slimy character as to clog
the pores of the filter paper, a suction filter is most advantageously
used. A 5-inch filter funnel should be fitted tightly by means of
a rubber stopper into the neck of a 500-cc. filter bottle (Fig. 3).
Place a 1^-inch perforated filter plate in the funnel and on this a
disk of filter paper cut so that its edges will turn up about 3 mm.
on the side of the funnel all the way around. Hold the disk of
dry paper in the right posi-
tion, wet it with a jet from
the wash bottle, draw it
firmly down against the filter
plate by applying the suc-
tion, and press the edges
firmly against the side of the
funnel, so that no free chan-
nel shall remain. In pouring
the liquid, direct it with a
stirring rod (Note 2) on to
the middle of the filter; do
not allow it to run down the
side of the funnel, as this
might turn up the edge of
the paper and allow some of
FIG. 3 the precipitate to pass by.
After all the solid has been
brought upon the filter it may be freed from a large part of the
adhering liquid by means of the suction, and it may then be purified
by washing with a suitable liquid (see Note 5).
The suction filter is very generally useful for the purpose of
separating a solid product from a liquid. If the liquid runs slowly,
the rate of filtration can be increased by using a larger filter plate
or still better a Btichner funnel and thereby increasing the filtering
area. The student should, however, avoid using the suction
indiscriminately, for in many cases, as explained in paragraph
(c), it is a positive disadvantage.
Suction. The most convenient source of suction is the Rich-
ards water pump, which can be attached directly to the water tap.
If the water is supplied at a pressure of somewhat over one at-
mosphere (34 feet of water), a vacuum of very nearly an atmos-