Page 125 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

HEATING APPARATUS 3.21

Iron ware. Iron crucibles may be substituted for those of nickel in sodium
peroxide fusions. They are not so durable, but are much cheaper.
Stainless-steel ware. Beakers, crucibles, dishes, funnels, etc., of stainless steel
are available commercially and have obvious uses in the laboratory. They will
not rust, are tough, strong, and highly resistant to denting and scratching.

3.21 HEATING APPARATUS
Various methods of heating are required in the analytical laboratory ranging
from gas burners, electric hot plates and ovens to muffle furnaces.
Burners. The ordinary Bunsen burner is widely employed for the attainment
of moderately high temperatures. The maximum temperature is attained by
adjusting the regulator so as to admit rather more air than is required to produce
a non-luminous flame; too much air gives a noisy flame, which is unsuitable.
Owing to the differing combustion characteristics and calorific values of the
gaseous fuels which are commonly available [natural gas, liquefied petroleum
(bottled) gas], slight variations in dimensions, including jet size and aeration
controls, are necessary: for maximum efficiency it is essential that, unless the
burner is of the 'Al1 Gases' type which can be adjusted, the burner should be
the one intended for the available gas supply.
Hot plates. The electrically heated hot plate, preferably provided with three
controls - 'Low', 'Medium' and 'High' - is of great value in the analytical
laboratory. The heating elements and the interna1 wiring should be totally
enclosed; this protects them from fumes or spilled liquids. Electric hot plates
with 'stepless' controls are also marketed; these permit a much greater selection
of surface temperatures to be made. A combined electric hot plate and magnetic
stirrer is also available. For some purposes a steam bath may be used.
Electric ovens. The most convenient type is an electrically heated, thermostatically
controlled drying oven having a temperature range from room temperature to
about 250-300 OC; the temperature can be controlled to within + 1-2 OC. They
are used principally for drying precipitates or solids at comparatively low
controlled temperatures, and have virtually superseded the steam oven.
Microwave ovens. These also find application for drying and heating operations.
They are particularly useful for determining the moisture content of materials
since the elimination of water takes place very rapidly on exposure to microwave
radiation.
Muffle furnaces. An electrically heated furnace of muffle form should be
available in every well-equipped laboratory. The maximum temperature should
be about 1200 OC. If possible, a thermocouple and indicating pyrometer should
be provided; otherwise the ammeter in the circuit should be calibrated, and a
chart constructed showing ammeter and corresponding temperature readings.
Gas-heated muffle furnaces are marketed; these may give temperatures up to
about 1200 OC.
Air baths. For drying solids and precipitates at temperatures up to 250 OC in
which acid or other corrosive vapours are evolved, an electric oven should not
be used. An air bath may be constructed from a cylindrical metal (copper, iron,