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Advanced
®
Cryoscope Model 4250 User’s Guide
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 plateau,
wherein the intrinsic sample thermodynamics maintain a temporary adi-
abatic liquid/solid equilibrium. The time over which equilibrium devel-
ops and is maintained is a function of the thermal characteristics of the
sample, the temperature differential between the sample and its environ-
ment, and the ability of the intervening materials to conduct heat. The
equilibrium temperature is, by definition, the freezing point of the solu-
tion.
Optimum precision by this method results from a carefully engineered
instrument. In the 4250, 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 gener-
ates a uniform insulating slush which helps stabilize the equilibrium
plateau.
Careful attention to sample-handling and to simple maintenance proce-
dures (e.g., heat transfer fluid replenishment, and probe and stir/ freeze
wire cleanliness) will ensure continued high performance.
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.
Although the freezing point plateau in a properly-designed cryoscope
appears flat for a considerable period, extending ideally for many min-
utes, an amplified plot of the plateau temperature would show a very
gradual curve, peaking as the temporary adiabatic condition breaks
down. The “freezing point” of a sample has been defined as the warmest
temperature achieved during equilibrium. Unfortunately, the warmest
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