12.
Appendix
12.1.
Principle of the freezing point determination
The freezing point is the temperature at which the solid phase and the liquid phase of a
substance or mixture of substances are present simultaneously and are in equilibrium. The
best suitable method for determining the freezing point has proved to be cooling down the
sample and then triggering the freezing. In doing so, use is made of the fact that a liquid
can be cooled down to below its actual freezing point while being carefully agitated. This
state is not stable. The freezing process can be triggered by vibrations, e.g. by beats on
the wall of the sample vial. The freezing is also triggered by introducing a small ice crystal
into the supercooled liquid. During freezing, the fusion heat is released and the
temperature rises until the freezing point is reached. There, the fusion heat released by the
freezing solid stays in equilibrium with the heat loss due to the surrounding cooling bath
until the sample is completely frozen solid and the temperature then drops.
The graph on the left shows the
principal temperature curve. However, if
the curve in the area of the plateau is
considered with higher resolution
(second graph), it can clearly be seen
that this plateau is not completely flat.
Therefore a binding regulation
(standard) is needed which specifies
how the freezing point must be
determined from the curve.
For this reason, it has been precisely
specified in the national and
international standards how the
chronological progress of the
measurement curve must be evaluated
in order to determine the standardised
freezing point. The currently applicable
standard defines the plateau as follows:
The temperature value must not
increase by more than 0.5m°C within a
period of at least 20 seconds. The
freezing point determination equipment
from Funke-Gerber can be freely adjusted in all the important parameters for the
measurement. For this reason, the equipment complies with all national and international
standards and possible corresponding changes to the standards. The equipment from
Funke-Gerber provide the possibility of being able to select from various methods:
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