Appendix D: Theory Of Operation - Elitech FreezePoint 6000 Series User Manual

Appendix D: Theory of Operation

The Freezing Point Osmometer is a non-invasive in-vitro diagnostic device used to determine the
osmolality of aqueous solutions. The use of the freezing point to determine osmolality is particularly suited
for the fields of medicine, industry and research.
The instrument measures the total osmolality of an aqueous solution. The osmolality of a solution is
defined as the number 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 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 mole 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:
Cosm = ΔT / K
Cosm = osmolality [Osmol/kg]
ΔT = freezing point depression [°C]
K = 1.858°C kg/Osmol (cryoscopic constant)
The 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. A sample is pipetted
into a measurement vessel. The measurement vessel is placed over the thermistor probe and lowered into
the lower cooling system. The sample is cooled to a defined temperature and the crystallization process
is initiated by ice crystals produced in the upper cooling system. The osmolality of the unknown sample
solutions is determined using the calibration values.
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