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Osmolality

The osmolality is a measure of the total number of particles in a solution. The osmolality is dependent on;

a The molar concentration of dissolved solute

b the dissociation of the solute

c the deviation from ideality

a Molar concentration of dissolved solute.

One gram mole of any compound contains the 6.021 x 1023 particles (Avogadro’s number) The number of particles is therefore dependent on the weight (in millimoles) of the dissolved solute. For example 180 grams of dextrose (MWt 180 Daltons) will dissolve in solution to give 6.021 x 1023 particles. 18 g would give 6.021 x 1022 particles. The osmolality is therefore dependent on the molar concentration.

b The dissociation of the solute

Osmolality is dependent on the number of particles. Molecules such as sodium chloride dissociate in solution into two particles namely sodium and chloride. 58.5 g of sodium chloride (the gram molecular weight) dissolves into 12.042 x 1023 particles in water. The number of particles into which a salt dissociates has a major effect on the osmolality.

c Deviation from ideality

In practice, salts such as sodium chloride do not completely dissociate into sodium and chloride ions. A little remains as undissociated sodium chloride. The amount of undissociated salt increases as the concentration increases. The measured osmotic pressure of salt solutions is therefore always slightly less than the theoretical osmotic pressure. Deviation from ideality factors can be found in such texts as Geigy Scientific Tables .

Why is osmolality important?

Normal human plasma has an osmolality in the range of 285-295 miliosmol/kg. Pharmaceutical solutions which have an osmolality higher than 600 milliosm/kg cause crenation (Shrinking) of red blood cells and significant pain. Solutions which have an osmolality less than about 150milliosml/kg cause haemolysis (rupture) and pain at the site of injection. Normal plasma osmolality is tightly controlled by homeostatic mechanisms in the body, a change of 3 milliosmol represents a change from minimal to maximal ADH output. Large volume infusions should therefore have an osmolality as close to 287 milliosmol (the average plasma normal value) as possible.

Osmolality measurement

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Last modified: 07/05/06