Page 129 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

STlRRlNC APPARATUS 3.23

be covered with a watch or clock glass, so that no mechanical loss ensues as a
result of the removal or admission of air. Air must be admitted slowly into an
exhausted desiccator: if the substance is very hygroscopic, a drying train should
be attached to the stopcock. In order to maintain a satisfactory vacuum within
the desiccator, the flanges on both the lid and the base must be well lubricated
with Vaseline or other suitable grease. In some desiccators an elastomer ring
is incorporated in a groove in the flange of the lower component of the desiccator:
when the pressure is reduced, the ring is compressed by the lid of the desiccator,
and an air-tight seal is produced without the need for any grease. The same
desiccants are used as with an ordinary desiccator.
Dry boxes (glove boxes) which are especially intended for the manipulation
of materials which are very sensitive to atmospheric moisture (or to oxygen),
consist of a plastic or metal box provided with a window (of glass or clear
plastic) on the upper side, and sometimes also on the side walls. A pair of rubber
or plastic gloves are fitted through air-tight seals through the front side of the
box, and by placing the hands and forearms into the gloves, manipulations may
be carried out inside the box. One end of the box is fitted with an air-lock so
that apparatus and materials can be introduced into the box without disturbing
the atmosphere inside. A tray of desiccant placed inside the box will maintain
a dry atmosphere, but to counter the unavoidable leakages in such a system,
it is advisable to supply a slow current of dry air to the box; inlet and outlet
taps are provided to control this operation. If the box is flushed out before use
with an inert gas (e.g. nitrogen), and a slow stream of the gas is maintained
while the box is in use, materials which are sensitive to oxygen can be safely
handled. For a detailed discussion of the construction and uses of glove boxes,
see Ref. 10.

3.23 STlRRlNC APPARATUS
Many operations involving solutions of reagents require the thorough mixing
of two or more reactants, and apparatus suitable for this purpose ranges from
a simple glass stirring rod to electrically operated stirrers.
Stirring rods. These are made from glass rod 3-5 mm in diameter, cut into
suitable lengths. Both ends should be rounded by heating in the Bunsen or
blowpipe flame. The length of the stirring rod should be suitable for the size
and the shape of the vessel for which it is employed, e.g. for use with a beaker
provided with a spout, it should project 3-5 cm beyond the lip when in a resting
position.
A short piece of Teflon or of rubber tubing (or a rubber cap) fitted tightly
over one end of a stirring rod of convenient size gives the so-called 'policeman';
it is used for detaching particles of a precipitate adhering to the side of a vessel
which cannot be removed by a stream of water from a wash bottle: it should
not, as a rule, be employed for stirring, nor should it be allowed to remain in
a solution.
Boiling rods. Boiling liquids and liquids in which a gas, such as hydrogen
sulphide, sulphur dioxide etc., has to be removed by boiling can be prevented
from superheating and 'bumping' by the use of a boiling rod (Fig. 3.6). This
consists of a piece of glass tubing closed at one end and sealed approximately
1 cm from the other end; the latter end is immersed in the liquid. When the rod