vogel_practical_ochem_3.pdf

80 PRACTICAL ORGANIC CHEMISTRY [II.
first melted, e.g., by immersing the tube in warm
water, the apparatus set up as in the figure, and
the appropriate cooling mixture placed in the beaker
(cold water, ice or a freezing mixture, according
to the melting point of the substance) so that the
rate of cooling is about 0*5° per minute. Both the
liquid substance and the cooling bath are kept well
stirred, and temperature readings are taken at
half-minute or one minute intervals ; this is continued
until the compound has fully solidified.
Upon plotting temperatures (ordinates) against
time (abscissae), a horizontal portion will be obtained
in the cooling curve, if the substance is
pure (compare Section 1,13); this corresponds to
the freezing (or setting) point. It is advisable to
determine the melting point also. The cooling bath
is removed and replaced, if the substance melts
above room temperature, by a bath of warm water.
Stirring is continued as before and temperatures
rf
m are observed at regular intervals until the substance
is completely fluid. Upon plotting the temperaturetime
graph (heating curve), the break in the curve
(horizontal portion for a pure substance) takes
place at the melting point. For pure compounds
Fig. //, 10, 4.
the melting point and freezing point are identical.
It must be emphasised that the above method applies only to substances
which melt without decomposition.
11,11. MISCELLANEOUS FORMS OF MELTING POINT
APPARATUS
Several forms of apparatus employing electrical heating will be described.
A simple form may be readily constructed from a domestic " electric
iron " of 400-500 watts rating. The handle is removed, and two holes of
8 mm. diameter are drilled through the base (ca. 11 mm. thick) so that they
meet in the centre of the block. One hole is for a 360° thermometer (small
bulb) ; the other hole is spare and can be used for comparison with a
standard thermometer. The heater is mounted on a sheet of thick asbestos
board which is fixed to an appropriate wooden base. The wires from
the heating unit are connected to two insulated terminals fitted on the
board (Fig. //, 11, 1). The rate of heating is controlled by either of the
following methods :
(i) A rheostat (e.g., a Zenith double-tubular sliding adjustable resistance
type T> with lead-screw motion, 230 ohms resistance to carry 2*8
amperes, is suitable).
(ii) A variable voltage transformer (e.g., a Variac transformer, enclosed
model, type 200-CMH, is satisfactory; a compact assembly is thus
obtained).
With a little experience, the correct setting for any desired rate of
heating (from rapid heating to about 1° per minute) may be obtained.