Page 130 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 CONMON APPARATUS AND BASIC TECHNIQUES

Fig. 3.6

is removed, the liquid in the open end must be shaken out and the rod rinsed
with a jet of water from a wash bottle. This device should not be used in solutions
which contain a precipitate.
Stirring may be conveniently effected with the so-called magnetic stirrer. A
rotating field of magnetic force is employed to induce variable-speed stirring
action within either closed or open vessels. The stirring is accomplished with
the aid of a small cylinder of iron sealed in Pyrex glass, polythene, or Teflon,
which is caused to rotate by a rotating magnet.
The usual type of glass paddle stirrer is also widely used in conjunction with
an electric motor fitted with either a transformer-type, or a solid-state, speed
controller. The stirrer may be either connected directly to the motor shaft or
to a spindle actuated by a gear box which forms an integral part of the motor
housing; by these means, wide variation in stirrer speed can be achieved.
Under some circumstances, e.g. the dissolution of a sparingly soluble solid,
it may be more advantageous to make use of a mechanical shaker. Various
models are available, ranging from 'wrist action shakers' which will accommodate
small-to-moderate size flasks, to those equipped with a comparatively powerful
electric motor and capable of shaking the contents of large bottles vigorously.

3.24 FILTRATION APPARATUS
The simplest apparatus used for filtration is the filter funnel fitted with a filter
paper. The funnel should have an angle as close to 60" as possible, and a long
stem (15 cm) to promote rapid filtration. Filter papers are made in varying
grades of porosity, and one appropriate to the type of material to be filtered
must be chosen (see Section 3.34).
In the majority of quantitative determinations involving the collection and
weighing of a precipitate, it is convenient to be able to collect the precipitate
in a crucible in which it can be weighed directly, and various forms of filter
crucible have been devised for this purpose. Sintered glass crucibles are made
of resistance glass and have a porous disc of sintered ground glass fused into
the body of the crucible. The filter disc is made in varying porosities as indicated
by numbers from O (the coarsest) to 5 (the finest); the range of pore diameter
for the various grades is as follows:

Porosity O 1 2 3 4 5
Pore diameter pm 200-250 100-120 40-50 20-30 5-10 1-2