An osmometer measures the osmolality of solutions using freezing point depression. Osmolality is the measure of constituents in solution – solutes. Units for osmolality are Osm/kg or osmoles/kilogram. An osmole can be the same as 1 mole as in glucose but 1 mole of NaCl is 2 osmoles because Sodium and Chlorine disassociate in solution to Na+ and Cl- ions. Osmolality is preferred to osmolarity which is osmoles per unit of volume, Osm/L and is temperature dependent; osmolality is independent of temperature.
Osmometers use freezing point depression to measure the osmolality of samples. The principal of freezing point depression is based on the process in which solutes lower the freezing point of a solvent compared to the freezing point of the pure solvent. The lower freezing point can be correlated to the amount of solutes in the solvent.
Osmolality is measured in clinical applications such as serum, urine or other bodily fluids. A microbiologist will use an osmometer to determine the solute concentration in media preparations. osmolality is also used to determine added water in milk. Osmometers have pharmaceutical and industrial applications as well.