Page 144 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

sizes generally used are circles of 7.0, 9.0, 11.0, and 12.5 cm diameter, those of
9.0 and 11.0 cm being most widely employed. The ash of a 11 cm circle should
not exceed 0.0001 g; if the ash exceeds this value, it should be deducted from
the weight of the ignited residue. Manufacturers give values for the average ash
per paper: the value may also be determined, if desired, by igniting several filter
papers in a crucible. Quantitative filter paper is made of various degrees of
porosity. The filter paper used must be of such texture as to retain the smallest
particles of precipitate and yet permit of rapid filtration. Three textures are
generally made, one for very fine precipitates, a second for the average precipitate
which contains medium-sized particles, and a third for gelatinous precipitates
and coarse particles. The speed of filtration is slow for the first, fast for the third,
and medium for the second. 'Hardened' filter papers are made by further
treatment of quantitative filter papers with acid; these have an extremely small
ash, a much greater mechanical strength when wet, and are more resistant to
acids and alkalis: they should be used in ail quantitative work. The characteristics
of the Whatman series of hardened ashless filter papers are shown in Table 3.7.

Table 3.7 'Whatman' quantitative filter papers
Filter paper Hardened ashless
Number 540 54 1 542
Speed Medium Fast Slow
Particle size retention Medium Coarse Fine
Ash (%) 0.008 0.008 0.008

The size of the filter paper selected for a particular operation is determined
by the bulk of the precipitate, and not by the volume of the liquid to be filtered.
The entire precipitate should occupy about a third of the capacity of the filter
at the end of the filtration. The funnel should match the filter paper in size; the
folded paper should extend to within 1-2 cm of the top of the funnel, but never
closer than 1 cm.
A funnel with an angle as nearly 60" as possible should be employed; the
stem should have a length of about 15 cm in order to promote rapid filtration.
The filter paper must be carefully fitted into the funnel so that the upper portion
beds tightly against the glass. To prepare the filter paper for use, the dry paper
is usually folded exactly in half and exactly again in quarters. The folded paper
is then opened so that a 60" cone is formed with three thicknesses of paper on
the one side and a single thickness on the other; the paper is then adjusted to
fit the funnel. The paper is placed in the funnel, moistened thoroughly with
water, pressed down tightly to the sides of the funnel, and then filled with water.
If the paper fits properly, the stem of the funnel will remain filled with liquid
durins. the filtration.
TO-carry out a filtration, the funnel containing the properly fitted paper is
placed in a funnel stand (or is supported vertically in some other way) and a
clean beaker placed so that the stem of the funnel just touches the side; this
will prevent splashing. The liquid to be filtered is then poured down a glass rod
into the filter, directing the liquid against the side of the filter and not into the
apex; the lower'end of the stirring rod should be very close to, but should not
quite touch, the filter paper on the side having three thicknesses of paper. The