Page 10 - A HANDBOOK OF ORGANIC ANALYSIS QUALITATIVE AND QUANTITATIVE

PRELIMINARY INVESTIGATION 5

An apparatus bas been developed
by Johnston and Lynn for the estima-
tion on the foregoing principle of the
setting-point of any organic substance
that melts without decomposition.
The sample is melted in a narrow test-
tube, and a thermometer is immersed
in the liquid. When cold, the solidified
sample, with test-tube and thermo-
meter attached, is placed within an
electrical heating coil which in turn is
surrounded by a vacuum jacket (Fig.
4). The coil must be placed in shunt
with a variable resistance, which should
be capable of giving 25o ohms for a
d.c. supply at no volts. To determine I
the melting range, an electric current is
supplied to the coil, such that the
equilibrium temperature (previously
determined by calibration of the in-
strument) lies 30--40° above the melt-
ing-point of the substance. Tempera-
ture readings are taken every minute
and plotted against time. At the melt-
ing temperature the readings become
nearly constant over the time interval
during which the sample (if it is nearly
pure) melts. With impure samples the
normal heating curve of the apparatus,
for the selected setting of the resist-
ance, undergoes a marked extension,
but this is not horizontal.
An even sharper break in con- F16. 4.
tinuity is obtained on plotting the
cooling curve. To determine this, the sample is heated
Piate communication.