Measuring Method Basics - Elitech FreezePoint 6000 Series User Manual

Section 2: Design and Function

2.2 Measuring Method Basics

Osmolality
The device measures the total osmolality of any aqueous solution.
The osmolality of a solution is defined as the number (or amount of substance) of the osmotically active
particles (e.g. salt ions, sugar, urea, proteins) present per kilogram of solvent (water).
The osmolality is specified in Osmol/kg or mOsmol/kg.
The device determines the total osmolality of a sample solution based on the freezing point depression
(see below).
The implemented measuring method is a relative measuring method where the device is first calibrated
based on the freezing points of distilled water and one or two calibration solutions with known osmolality.
Next, the osmolality of unknown sample solutions is determined with reference to this 2 or 3 point
calibration.
Freezing point depression
The freezing point of a solvent is depressed by adding soluble or mixable substances. The magnitude of
this effect depends less on the type and quantity of the dissolved substance, but rather on the number of
particles present in the solution afterwards.
Whereas water has a freezing point of 0°C, an aqueous solution with an osmotically active particle
concentration of 1 Osmol/kg has a freezing point of -1.858°C.
That means that one mol of an ideal non-dissociated substance (6.023 × 1023 parts diluted in one kilogram
of water) lowers the freezing point of a solution by 1.858°C.
The osmolality of a solution is directly proportional to the measured freezing point depression and can
therefore be calculated from this result. The relationship is as follows:
C = ΔT / K
osm
C = osmolality [Osmol/kg]
osm
ΔT = freezing point depression [°C]
K = 1.858°C kg/Osmol (cryoscopic constant)
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