Page 13 of 24 MEL-TEMP M8160 Issue 1.5
7.2.2. Preparation of Samples.
To obtain the best results the samples under melt should be perfectly dry. Drying is best
accomplished by storing the samples in vacuum desiccators over a suitable drying
agent such as phosphorus pent oxide. Always dry hygroscopic samples.
Provided the sample is dry there is usually little difficulty in introducing a small amount
of sample into the capillary tube. Insert the tube into your sample to gather material.
Stand the tube upright and then tap the tube on the bench to compact the material at
the tube base. 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 is ample. If this is exceeded accurate and
sharp melt reading points may not be obtained. Care should be taken to see the sample
beds down to the bottom of the tube as large air pockets can cause false melt
observations.
Please not sample preparation should be undertaken independently of the IA
apparatus.
7.2.3. Determination of Melting point.
In recording melting points, the operator may wish to observe four distinct phases:
a) First signs of change (darkening, shriveling, 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.
7.2.4. 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 or
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.
