Which osmometry method is used to measure osmolality based on freezing-point depression?

Which osmometry method is used to measure osmolality based on freezing-point depression?

• A. Vapor Pressure Osmometry
• B. Membrane Osmometry
• C. Freezing-point Depression Osmometry
• D. Cryoscopic Osmometry

Answer: (C)

Freezing-point depression is used to determine osmolality. When solutes are present, they lower the temperature at which a solution freezes. The greater the number of dissolved particles, the more the freezing point drops. An osmometer measuring osmolality based on this principle detects the sample’s freezing point and uses the observed depression to calculate osmolality, often via the relation ΔTf = i Kf m, where ΔTf is the freezing-point change, i is the van’t Hoff factor, and Kf is the solvent’s freezing-point depression constant. In practice, this yields the osmolality in mOsm/kg after calibration. This approach is distinct from vapor-pressure osmometry, which uses vapor pressure changes, and from membrane osmometry, which measures osmotic pressure across a semipermeable membrane. Cryoscopic osmometry is another term for the same freezing-point method.

Freezing-point depression is used to determine osmolality. When solutes are present, they lower the temperature at which a solution freezes. The greater the number of dissolved particles, the more the freezing point drops. An osmometer measuring osmolality based on this principle detects the sample’s freezing point and uses the observed depression to calculate osmolality, often via the relation ΔTf = i Kf m, where ΔTf is the freezing-point change, i is the van’t Hoff factor, and Kf is the solvent’s freezing-point depression constant. In practice, this yields the osmolality in mOsm/kg after calibration. This approach is distinct from vapor-pressure osmometry, which uses vapor pressure changes, and from membrane osmometry, which measures osmotic pressure across a semipermeable membrane. Cryoscopic osmometry is another term for the same freezing-point method.