WASHING PRECIPITATES 9retained. With some very fine-grained precipitates, repeatedly-
pouring the filtrate through the same filter will finally give a clear
filtrate.
Special kinds of filter paper are made to retain very fine precipi-
tates, but they allow the liquid to pass much more slowly than
ordinary niters, and their use is not essential in any of the following
preparations.
Particles of colloidal size may be removed by boiling the liquid
with a little bone charcoal and subsequent nitration.
(/) To Keep Liquids Hot during Filtration. When liquids
must be kept hot during a slow nitration, as, for example, when
cooling would cause a separation of crystals that would clog the
filter, it sometimes becomes necessary to surround the funnel with
a jacket which is heated with steam or boiling water. In the
following preparations the use of such a device will not be neces-
sary, although there are several instances where it is necessary to
work quickly to avoid clogging the filter. It helps to keep the
funnel covered with a watch glass.
(g) Cloth Filters. In preparations made on a small scale, paper
niters placed in ordinary filter funnels are invariably used if the
liquid is not too corrosive. On a larger scale or in commercial
practice, cloth is much used for filters, and it can be made in the
shape of bags or it can be stretched over wooden frames. The
cloth or other filtering medium (asbestos, paper pulp, sand, etc.)
has to be chosen in each case with reference to the nature of the
precipitate and the corrosiveness of the liquid.
Many of the preparations in this book, if carried out on a larger
scale than given in the directions, would require the use of such
cloth niters. It is often advantageous to tack one piece of cloth
permanently across a wooden support and on top of this to lay a
second cloth. The precipitate can then be easily removed together
with the unfastened cloth.
For devices for rapid nitration and nitration in general on a large
scale, a work on industrial chemistry should be consulted.
- WASHING PRECIPITATES
(a) Washing on the Filter. Precipitates and crystals are washed
to remove the impurities contained in the mother liquor which
clings to them. Pure water is used for washing provided the solid
is not too soluble or is not decomposed (hydrolyzed) by it. Special