Osmolality

What it determines

A solution freezes below the freezing point of pure water, and the size of that depression tracks the number of dissolved particles per kilogram of solvent. The osmometer cools a small volume of sample below its freezing point, seeds crystallization, and reads the temperature plateau that follows.

That plateau is converted to milliosmoles per kilogram against calibration standards of known osmolality. Every dissolved species contributes in proportion to the number of particles it produces in solution, so a salt that dissociates counts more than once.

Practical notes

• The measurement belongs to the solution as prepared. A lyophilized powder is reconstituted first.
• Buffer salts dominate the reading in most formulations, so the composition of the diluent shapes the result.
• Evaporation concentrates a solution in transit, and tightly closed containers keep the value stable.