Advanced Instruments 3300, Руководство пользователя

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Freezing-Point Thermodynamics
The quickest and most precise way to measure the freezing point of a
solution is to supercool it several degrees below its freezing point. It is
unstable in this state, and a mechanical agitation induces crystallization.
The heat of fusion suddenly liberated causes the sample temperature to
rise toward a plateau temperature, where a liquid/solid equilibrium
occurs. The equilibrium temperature is, by definition, the freezing point
of the solution. Managing the plateau temperature for precise measure-
ment is the basis for several patents issued to Augustus Fiske.
The time over which liquid/solid equilibrium develops and is main-
tained, is a function of the speed with which the heat-of-fusion is liber-
ated vs. the speed it is transferred away, or absorbed, by the surrounding
environment. This ratio can be slowed and the equilibrium time
stretched, to give a distinct plateau height measurable to 0.001°C.
Sensitive thermistor probes monitor the sample temperature and control
the thermoelectric cooling element. Microprocessor control and auto-
mated operation minimize imprecision due to operator technique.
Figure 1 illustrates the temperature of a sample as it progresses through
the freezing cycle and shows the action of the instrument at each stage
of the cycle.
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Foreword
Figure 1: Standard Freezing Curve