OSMOMETER

Solutes or ions in water cause a lowering of the freezing point. The magnitude of the freezing point depression depends on the number of particles per kilogram of water and this number is expressed in Osmols where one Osmol is Avogadro's number of particles per kilogram. Blood or urine osmolality measurements may be required for patients in intensive care. An osmometer measures osmolality by detecting the 'freezing point depression'. At one Osmol, the freezing point is 1.858[d]C below normal.

Freezing point is found by detecting the temperature at which a small quantity of the fluid (e.g. 3 ml) freezes in a small cuvette. A small stirrer or vibrator and a thermistor probe are immersed in the fluid and the cuvette is lowered into a freezing bath or Peltier cooling device. The stirrer is arrested by the freezing of the solution and this triggers the measurement of temperature. True freezing point is measured just after the first freezing and this is slightly higher than the lowest temperature reached. The instrument displays osmolality directly. It may also be called an osmolality, or osmolarity meter.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details