Preparation of Samples
To obtained the best results, the samples under test / observation should be
perfectly dry. Drying is best accomplished by storing the samples overnight in a
vacuum desiccator over a suitable drying agent such as phosphourus
pentoxide. A drying method must be employed in the case of hygroscopic
samples.
Provided the sample is dry there is usually little difficulty in introducing the
small amount required into the capillary tube. Tapping the tube on the bench to
compact the material at the base of the tube. It is important not to use too large
a sample, a quantity occupying no more than 1-2 mm of the bottom of the tube
should be used. If this is exceeded, accurate and sharp melting point may not
be obtained. Care should be taken to see the sample beds down to the bottom
of the tube, as large are pockets can cause difficulty.
Determination of Melting point.
In recording melting points, the operator may wish to observe four distinct
phases:
a) First signs of change (darkening, shrivelling, sintering, etc). These changes
may be due to loss of solvent or crystallization. Occasionally one can see
the solvent condensing on the cooler portion of the tube.
b) First signs of liquid formation.
c) Formation of meniscus.
d) Formation of a completely clear liquid.
Not all compounds will behave in this manner however. With many, especially
where the material is in a high state of purity, no change may be seen before
any liquid appears. Some materials will be seen to sublime, rather than melt
above 300°C
Decomposition may often be accompanied by evolution of gas.
Pure compounds will be expected to pass through the 3 stages a) c) and d),
within a range of 1°C.
Interpretation of Results.
It is generally accepted the formation of a meniscus should be regarded as the
figure to be recorded for a single temperature melting point. Where a range is
quoted, it can generally be assumed reference is being made to stages b) to d)
inclusive. The range will generally cover no more than 1°C for pure
compounds, but may well extend to 3 to 4°C for others. The range will often be
greater when decomposition or sublimation is experienced. When using the
equipment to determine the melting point of more than one sample of the same
compound at a time, the average of the meniscus temperature (onset of melt)
should be taken.
Calibration Procedure.
For equipment calibration purposes and to minimise variation between repeat
measurements, it is recommended that onset of melt observations are used
against Certified Reference Material stated onset figures.
The measurements should be taken using 2mm diameter capillary tubes.
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