Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

10 FILTKATION
It is quite wrong to collect a few grammes of substance on a funnel
six or more centimetres in diameter. In many cases, especially for
FIG. 6 FIG. 7
small amounts (5 g. or less), the Witt filter plate (Fig. 7) is to be
preferred. It presents the advantage that its cleanliness can be
checked much more readily than that of an opaque porcelain funnel,
and, especially, that much less solvent is required to wash the more
compact solid.
In order to prepare the filter paper a small piece of the paper is
folded over the upper edge of the filter plate and then a piece having
a radius 2-3 mm. greater is cut out with scissors. This piece is
moistened with the solvent and fitted closely to the funnel by
pressing, rubbing out small folds with a rounded glass rod or, in
the case of larger plates, with the finger-nail.
When minute amounts of substance (a few
hundred milligrammes or less) have to be filtered,
small glass plates 0-5-1 -0 cm. in diameter
are used as supports for the filter paper. These
plates are made from thin glass rods by heating
one end in the blow-pipe till soft and then
pressing out flat on an iron or earthenware
plate (Diepolder). The glass rods must be long
enough and thin enough to pass through the
delivery tube of a quite small funnel and to
project beyond its end. The pieces of filter
FIG. 8
paper which rest on the glass plates are cut
somewhat larger than the plates themselves and are made to
fit closely (Fig. 8).