Page 147 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

DRYINC AND ICNlTlNC PRECIPITATES 3.37

is appreciably soluble in water, a common ion is usually added, since any
electrolyte is less soluble in a dilute solution containing one of its ions than it
is in pure water (Section 2.7); as an example the washing of calcium oxalate
with dilute ammonium oxalate solution may be cited. If the precipitate tends
to become colloidal and pass through the filter paper (this is frequently observed
with gelatinous or flocculent precipitates), a wash solution containing an
electrolyte must be employed (compare Section 11.3). The nature of the
electrolyte is immaterial, provided it has no action upon the precipitate during
washing and is volatilised during the final heating. Ammonium salts are usually
selected for this purpose: thus ammonium nitrate solution is employed for
washing iron(II1) hydroxide. In some cases it is possible to select a solution
which will both reduce the solubility of the precipitate and prevent peptisation;
for example, the use of dilute nitric acid with silver chloride. Some precipitates
tend to oxidise during washing; in such instances the precipitate cannot be
allowed to run dry, and a special washing solution which re-converts the oxidised
compounds into the original condition must be employed, e.g. acidified hydrogen
sulphide water for copper sulphide. Gelatinous precipitates, like aluminium
hydroxide, require more washing than crystalline ones, such as calcium oxalate.
In most cases, particularly if the precipitate settles rapidly or is gelatinous,
washing by decantation may be carried out. As much as possible of the liquid
above the precipitate is transferred to the prepared filter (either filter paper or
filter crucible), observing the usual precautions, and taking care to avoid, as far
as possible, disturbing the precipitate. Twenty to fifty millilitres of a suitable
wash liquid is added to the residue in the beaker, the solid stirred up and allowed
to settle. If the solubility of the precipitate allows, the solution should be heated,
since the rate of filtration will thus be increased. When the supernatant liquid
is clear, as much as possible of the liquid is decanted through the filtering
medium. This process is repeated three to five times (or as many times as is
necessary) before the precipitate is transferred to the filter. The main bulk of
the precipitate is first transferred by mixing with the wash solution and pouring
off the suspension, the process being repeated until most of the solid has been
removed from the beaker. Precipitate adhering to the sides and bottom of the
beaker is then transferred to the filter with the aid of a wash bottle as described
in Section 3.34, using a 'policeman' if necessary to transfer the last traces of
precipitate. Finally, a wash bottle is used to wash the precipitate down to the
bottom of the filter paper or to the plate of the filter crucible.
In al1 cases, tests for the completeness of washing must be made by collecting
a small sample of the washing solution after it is estimated that most of the
impurities have been removed, and applying an appropriate qualitative test.
Where filtration is carried out under suction, a small test-tube is placed under
the crucible adapter.

3.37 DRYING AND IGNlTlNG PRECIPITATES
After a precipitate has been filtered and washed, it must be brought to a constant
composition before it can be weighed. The further treatment will depend both
upon the nature of the precipitate and upon that of the filtering medium; this
treatment consists in drying or igniting the precipitate. Which of the latter two
terms is employed depends upon the temperature at which the precipitate is
heated. There is, however, no definite temperature below or above which the