Fenner's Complete Formulary - Southwest School of Botanical ...

WASHING PRECIPITATES.
The object of washing fresh precipitates is to free them from soluble
salts, or other substances with which they are associated, which are
soluble in water.
The usual manner of washing fresh precipitates, in a small way, is to
pour them upon a wet muslin strainer and filter water through them
until the soluble matter has all been washed out.
This method is open to several objections: ist, exposure to the
atmosphere, which rapidly oxidizes many salts, especially the iron salts,
rendering them insoluble; 2d, waste, as considerable of the precipitate is
washed away by this method; 3d, inconvenience, as it requires the
continued attention of the operator.
Another method is to wash the precipitate in a large jar or earthenware
crock, by pouring upon it a quantity of water and stirring thoroughly,
then allowing the precipitate to settle, drawing off the supernatant fluid
with a syphon, pouring on more fresh water, and thus continuing until
the soluble matter is washed out; and then draining the precipitate
upon a muslin strainer.
The best method, however, is to make the precipitate in a tall jar or
crock, rilled full of water; then, having fastened a piece of rubber tubing
to each end of a stick, insert it in the jar in such a manner that a stream
of water passing through one rubber tube will reach to the bottom of the
vessel, while the water at the top of the jar will be carried off by means
of the other tube, which acts as a syphon. The water to wash the
precipitate can be supplied from a water-pipe, or from a bucket set
above the washing apparatus, into which the tube is inserted as a
syphon. The same result will be accomplished by running the tube,
Fenner’s Complete Formulary - Part I-II - MISCELLANEOUS FORMULA - Page 51
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