8 NOTES ON LABORATORY MANIPULATIONAfter the precipitate is collected in the filter and drained, it
should if necessary be washed (see Note 5).
Both nitration and washing take place much more rapidly if the
liquid is hot. Time can also usually be saved if the precipitate is
allowed to settle as completely as possible before commencing to
filter. The clear liquid can then be decanted off, or if necessary
poured rapidly through the filter before the latter becomes clogged
with the main part of the precipitate.
(d) Filtering Corrosive Liquids. Solutions of very strong
oxidizing agents, concentrated solutions of the strong acids and
bases, and concentrated solutions of a few salts of the heavy
metals — notably zinc chloride and stannous chloride — attack
filter paper. Ordinary paper is thus unserviceable for nitration,
but a felt made of asbestos fibers is frequently very useful.
Shredded asbestos, which has been purified by boiling with hydro-
chloric acid and subsequent washing, is suspended in water;
the suspension is poured onto a perforated plate placed in a filter
funnel; and suction is applied whereby the water is removed and
the fibers are drawn together to form a compact felt over the filter
plate. Enough asbestos should be used to make a felt 1 to 3 mm.
thick, and care must be taken to see that it is of uniform thickness
and that no free channels are left through which solid matter
may be drawn. Before it is ready for use a considerable amount
of water should be drawn through the filter, and the loose fibers
should be rinsed out of the filter bottle. Before pouring the liquid
onto the filter the suction should be started gently, and the liquid
should be directed by means of a stirring rod (Note 2) onto the
middle of the filter. If these precautions are not observed the
felt may become turned up in places, so that the precipitate will
pass through.
A wad of glass wool in the bottom of a glass funnel may some-
times be used to filter corrosive liquids. Another method which
can be used in separating crystals from a corrosive liquid consists
in putting a glass marble into a funnel. The crystals form a mat in
the small space between the marble and the sides of the funnel and
the liquid can be removed by suction.
(e) Cloudy Filtrates. When a nitrate at first comes through
cloudy, it is usually sufficient to pour the first portion through the
filter a second time. The pores of the filter soon become partially
closed with the precipitate, so that even the finest particles are