Freezing Point Thermodynamics
The “freezing point” in osmometry is that unique temperature, at atmospheric pressure, when solid and liquid phases co-exist in equilibrium. The Standard Freeze Curve 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.
The freezing point technique operates by rapidly Super Cooling the sample to a predetermined temperature below the expected Sample Freezing Point. This is identified in the diagram as the Fast Cool and Slow Cool regions of the freeze curve. While the sample is in the super-cooled state, crystallization is initiated by some external means, such as the rapid vibration of a stirring rod or a solenoid knocking. This external catalyst is called the Freeze Pulse. As a result of the initial freezing caused by the freeze pulse, only part of the sample crystallizes, forming a layer of slush around the temperature probe.
The Heat of Fusion resulting from the crystallization process raises the sample temperature to a Plateau where ice/water equilibrium is maintained. This equilibrium temperature is the Sample Freezing Point of the solution. The sample remains at this temperature for prolonged periods of time. During this time, the osmometer uses a thermistor probe to sense the sample temperature, control the degree of super cooling and freeze induction, and measure the freezing point of the sample. This information is calculated and the Readout displayed in milliosmoles per kilogram of water (mOsm/Kg H20).
Eventually, all of the liquid in the sample freezes out and the sample assumes the temperature of its environment. This is seen in the gradually decreasing temperature of the freeze curve after the readout has occurred.
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