Page 143 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

FILTER PAPERS 3.34

The simplest method for decomposing an organic sample is to heat it in an
open crucible until al1 carbonaceous matter has been oxidised leaving a residue
of inorganic components, usually as oxide. The residue can then be dissolved
in dilute acid giving a solution which can be analysed by appropriate procedures.
This technique is referred to as dry ashing; it is obviously inapplicable when
the inorganic component is volatile. Under these conditions the wet ashing
procedure described under perchloric acid must be used. A full discussion of
the destruction of organic matrices is given in Ref. 20.

The conditions for precipitation of inorganic substances are given in Section 11.6.
Precipitations are usually carried out in resistance-glass beakers, and the
solution of the precipitant is added slowly (for example, by means of a pipette,
burette, or tap funnel) and with efficient stirring of the suitably diluted solution.
The addition must always be made without splashing; this is best achieved by
allowing the solution of the reagent to flow down the side of the beaker or
precipitating vessel. Only a moderate excess of the reagent is generally required;
a very large excess may lead to increasing solubility (compare Section 2.6) or
contamination of the precipitate. After the precipitate has settled, a few drops
of the precipitant should always be added to determine whether further
precipitation occurs. As a general rule, precipitates are not filtered off immediately
after they have been formed; most precipitates, with the exception of those which
are definitely colloidal, such as iron(II1) hydroxide, require more or less digestion
(Section 11.5) to complete the precipitation and make al1 particles of filterable
size. In some cases digestion is carried out by setting the beaker aside and
leaving the precipitate in contact with the mother liquor at room temperature
for 12-24 hours; in others, where a higher temperature is permissible, digestion
is usually effected near the boiling point of the solution. Hot plates, water baths,
or even a low flame if no bumping occurs, are employed for the latter purpose;
in al1 cases the beaker should be covered with a clockglass with the convex side
turned down. If the solubility of the precipitate is appreciable, it may be necessary
to allow the solution to attain room temperature before filtration.

3.33 FILTRATION
This operation is the separation of the precipitate from the mother liquor, the
object being to get the precipitate and the filtering medium quantitatively free
from the solution. The media employed for filtration are: (1) filter paper;
(2) porous fritted plates of resistance glass, e.g. Pyrex (sintered-glass filtering
crucibles), of silica (Vitreosil filtering crucibles), or of porcelain (porcelain
filtering crucibles): see Section 3.24.
The choice of the filtering medium will be controlled by the nature of the
precipitate (filter paper is especially suitable for gelatinous precipitates) and
also by the question of cost. The limitations of the various filtering media are
given in the account which follows.

3.34 FILTER PAPERS
Quantitative filter papers must have a very small ash content; this is achieved
during manufacture by washing with hydrochloric and hydrofluoric acids. The