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
and then mechanically induce the sample to freeze. The heat of fusion
sud-denly liberated causes the sample temperature to rise toward a pla-
teau, wherein the intrinsic sample thermodynamics maintain a tempo-
rary adiabatic liquid/solid equilibrium. The time over which equilibrium
develops and is maintained is a function of the thermal characteristics
of the sample, the temperature differential between the sample and its
environment, and the ability of the intervening materials to conduct
heat. The equilibrium temperature is, by definition, the freezing point of
the solution.
Optimum precision by this method results from a carefully engineered
instrument. In the 4C3, the sample environment is temperature con-
trolled to maximize the efficiency of the instrument without distorting
the freezing plateau. A sensitive probe monitors the sample temperature,
controls the degree of supercooling and freeze induction, and measures
the freezing point of the sample. The tip of the sample probe is precise-
ly positioned in each sample. Automatic stirring during sample cooling
and crystallization minimizes sample temperature gradients and gen-
erates a uniform insulating slush which helps stabilize the equilibrium
plateau.
xvi
The Advanced
®
Model 4C3 User’s Guide
Figure 1: Standard Freezing Curve
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