What Is a Cryoscope?

A cryoscope is a scientific instrument used in research and industrial laboratories to determine the freezing point of a liquid or solution. Cryoscopy, the measure of freezing point, is often used to determine the concentration of a solution, or the amount of solute that has been dissolved in a solvent. Since freezing point changes in proportion to the amount of dissolved solute in the solution, a cryoscope can help researchers determine concentration with great precision. This technique is especially used in the milk industry, where higher or lower freezing points often provide important information about the quality of the milk.

In order to determine freezing point, the cryoscope uses a principle known as supercooling. When a solution is supercooled, its temperature is lowered past its freezing point without it freezing. The solution remains in its liquid phase because there is no seed crystal or structure around which it can crystallize. When a structure is introduced or the solution is mechanically agitated, it will suddenly freeze solid.

After a sample solution is placed into the cryoscope, the device inserts a temperature probe and a stirring rod into the solution and begins to cool it down. Eventually the temperature of the solution passes its freezing point as it is supercooled to a specific level. Once the solution is supercooled, the stirring rod agitates the liquid, causing an abrupt freeze to occur. The heat released in the crystallization reaction of freezing, known as the heat of fusion, causes the temperature of the solution to shoot back up to its freezing point, where it plateaus for an interval of time before dropping again. The cryoscope makes its measurement during this plateau, when the temperature of the solution is precisely at its freezing point.

Cryoscopes are widely used in the dairy industry to determine the quality of milk. Although the freezing point of milk varies based on the type of animal, its diet, and its breed, industry regulatory organizations have determined an acceptable freezing point for uncontaminated and undiluted cow’s milk. Cryoscopes use this point as a standard for calibration. If the freezing point of the milk sample is determined to be warmer than the standard, it has likely been diluted — that is, water has been added to it. Milk samples that have a colder freezing point than the norm may be contaminated or sour.

The addition of solutes such as those found in milk make it more difficult for water to freeze — in other words, they lower the freezing point, and the solution freezes at a colder temperature. More water, on the other hand, means that the solution freezes at a higher temperature — its freezing point is raised, since it is easier to freeze. By comparing the freezing points of milk samples, cryoscopes can determine not only whether water has been added to the milk, but also exactly how much water has been added. Industrial labs use these measurements to monitor and maintain milk quality.