Page 11 - A HANDBOOK OF ORGANIC ANALYSIS QUALITATIVE AND QUANTITATIVE

6 ORGANIC ANALYSIS
electrically to a point well above its melting temperature ;
the resistance is set so that the equilibrium point lies about
40° below the solidification temperature, and readings are
made on the thermometer every minute. The temperature
falls along the normal curve until the sample is slightly cooler
than its setting point; it then rises.sharply (as crystallisation
sets in) lo the solidification temperature and remains almost
constant during the change of state. When less than r per
cent. of impurity is present, this portion of the cooling curve
slopes over less than one degree.
The melting-point as determined by the above apparatus
coincides with the setting point; it is as a rule slightly lower
(though- more accurate) than the melting point indicated by
the capillary tube method. The use of the apparatus is,
as above stated, applicable to the determination of melting
t point and purity of only those compounds that melt without
the least decomposition.
If the melting point as determined in the capillary tube
is not sharp (i.e. the range extends over appreciably more
than one degree), or if the extension in the cooling curve
deviates considerably from the horizontal, the original sample
must be recrystallised from some suitable solvent. Solvents
may be tried in the following order: alcohol, water, ligroin,
acetone, benzene, acetic acid, chloroform, ether. The
recrystallisation is effected by maintaining a certain quantity
of the solvent at its boiling-point in a conical flask--under
reflux if the solvent is highly volatile gradually adding
d
an
small quantities of the original substance until no more is
taken up into solution. A small volume of the solvent is
then added, the hot solution rapidly filtered through a
fluted flter-paper on a funnel without a stem, and the filtrate
allowed to cool, the vessel being occasionally scratched on
the inside with a glass rod, to induce crystallisation. When
cold, the solid which has separated is filtered off by suction,
pressed upon a porous plate, dried in a vacuum desiccator,
and the melting-point determined. This process must be