Page 126 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

or nickel) vessel with the bottom of the vessel pierced with numerous holes. A
silica triangle, the legs of which are appropriately bent, is inserted inside the
bath for supporting an evaporating dish, crucible, etc. The whole is heated by
a Bunsen flame, which is shielded from draughts. The insulating layer of air
prevents bumping by reducing the rate at which heat reaches the contents of
the inner dish or crucible. An air bath of similar construction but with special
heat-resistant glass sides may also be used; this possesses the obvious advantage
of visibility inside the air bath.
Infrared lamps and heaters. Infrared lamps with interna1 reflectors are available
commercially and are valuable for evaporating solutions. The lamp may be
mounted immediately above the liquid to be heated: the evaporation takes place
rapidly, without spattering and also without creeping. Units are obtainable
which permit the application of heat to both the top and bottom of a number
of crucibles, dishes, etc., at the same time; this assembly can char filter papers
in the crucibles quite rapidly, and the filter paper does not catch fire.
Immersion heaters. An immersion heater consisting of a radiant heater encased
in a silica sheath, is useful for the direct heating of most acids and other liquids
(except hydrofluoric acid and concentrated caustic alkalis). Infrared radiation
passes through the silica sheath with little absorption, so that a large proportion
of heat is transferred to the liquid by radiation. The heater is almost unaffected
by violent thermal shock due to the low coefficient of thermal expansion of the
silica.
Heating mantles. These consist of a flexible 'knitted' fibre glass sheath which
fits snugly around a flask and contains an electrical heating element which
operates at black heat. The mantle may be supported in an aluminium case
which stands on the bench, but for use with suspended vessels the mantle is
supplied without a case. Electric poweris supplied to the heating element through
a control unit which may be either a continuously variable transformer or a
thyristor controller, and so the operating temperature of the mantle can be
smoothly adjusted.
Heating mantles are particularly designed for the heating of flasks and find
wide application in distillation operations. For details of the distillation
procedure and description of the apparatus employed, a textbook of practical
organic chemistry should be con~ulted.~
Crucibles and beaker tongs. Apparatus such as crucibles, evaporating basins
and beakers which have been heated need to be handled with suitable tongs.
Crucible tongs should be made of solid nickel, nickel steel, or other rustless
ferro-alloy. For handling hot platinum crucibles or dishes, platinum-tipped
tongs must be used. Beaker tongs are available for handling beakers of
100-2000 mL capacity. The tongs have jaws: an adjustable screw with locknut
limits the span of these jaws and enables the user to adjust them to suit the
container size.

3.22 DESICCATORS AND DRY BOXES
It is usually neessary to ensure that substances which have been dried by
heating (e.g. in an oven, or by ignition) are not unduly exposed to the atmosphere,
otherwise they will absorb moisture more or less rapidly. In many cases, storage