Synthetic Inorganic Chemistry

(^10) NOTES ON LABORATORY MANIPULATION
directions will be given when it is necessary to use wash liquids
other than pure water.
First, the solid is allowed to drain as completely as possible,
then the wash liquid is applied, preferably from the jet of a wash
bottle, so as to wet the whole mass and to rinse down the sides of the
filter. If suction is used, suck the solid as dry as possible, then stop
the suction while applying the washing liquid; after the solid is
thoroughly wet, suck out the liquid and repeat the washing.
A little thought will make it clear that the washing is much
more effective if the liquid is removed as completely as possible
each time before applying fresh wash liquid, and that a number of
washings with a small amount of liquid each time is more effective
than fewer washings with much greater quantities of wash liquid.
It is, of course, evident that with each washing the liquid should
penetrate to all parts of the solid material.
(b) Washing by Decantation. A very insoluble precipitate can
be washed most thoroughly and quickly by decantation. The
solid is allowed to settle in a deep vessel and then the clear liquid is
poured (decanted) or siphoned off. Following this the precipitate
is stirred up with fresh water and allowed to settle, and the liquid
is again decanted off. By a sufficient number of repetitions of this
process, the precipitate may be washed en-
tirely free from any soluble impurity, after
which it may be transferred to a filter,
drained, and then dried.
Most precipitates, even after they have
settled as completely as possible in the liquid
from which they were thrown down, are very
bulky, and their apparent volume is very
large as compared with the actual volume
of the solid matter itself. For example, a
precipitate of basic zinc carbonate (Prep. 31),
after it has settled as completely as possible
in a deep jar (Fig. 4), may still occupy a
volume of 400 cc. When this bulky precipi-
tate is dried, however, it shrivels up into a
few small lumps whose total volume is not
more than 4 or 5 cc.
If a precipitate, which is at first uniformly suspended in a liquid,
is allowed to settle in a tall jar until it occupies but one-fifth of the