In freezing point osmometers, the sample is typically cooled to approximately what temperature?

• A. -7C
• B. 0C
• C. -20C
• D. 25C

Answer: (A)

Freezing point osmometers determine osmolality from freezing point depression: adding solutes lowers the temperature at which the solvent freezes, and the amount of this depression relates to the osmolarity of the sample. To obtain a reliable and reproducible reading, the instrument cools the sample beyond its depressed freezing point until a definite freezing event occurs and can be detected clearly. Cooling to about -7°C provides a stable, unambiguous ice–water phase change for typical clinical samples, while staying well below the depressed freezing point so supercooling and detection variability are minimized. This temperature is a practical operating point chosen for consistent readings across samples.

The other options don’t fit because 0°C would be the freezing point of pure water and isn’t low enough to reflect the presence of dissolved particles; -20°C is unnecessarily cold and can introduce instability or nonlinearity in the measurement; 25°C is far above any freezing point and meaningless for this method.