APPENDIX
E
Osmolality Standards
STANDARD INTERNATIONAL (Sl) UNITS OF OSMOLALITY
Osmolality, by definition, is an expression of the total number of
solute particles dissolved in one kilogram of solvent without regard
for particle size, density, configuration, or electrical charge.
Traditionally, osmolality has been expressed as milliosmols per kilo-
gram, with various abbreviations such as mOs/kg, mOsm/kg, and
mOsmol/kg. The letters "Os" signify that osmolality is defined as the
concentration, expressed on a molal basis, of the osmotically active
particles in true solution. Thus, one mole (1000 mmol) of sodium
chloride dissolved in a kilogram of water has an ideal osmolality of
2000 mOsm/kg, since a molecule of sodium chloride dissociates in
solution to produce two ions, that is, two osmotically active particles.
In fact, a molal solution of sodium chloride has an osmolality value
slightly less than the ideal because the residual mutual attraction of
the hydrated ions reduces their mutual independence due to the
osmotic coefficient. Since this coefficient varies with the solute con-
centration, the relation between osmolality and concentration of
solute is not linear. For this reason, measurements of osmolality
made on laboratory-diluted specimens, with subsequent multiplica-
tion by the dilution factor to calculate the original solution osmolality,
will not give valid results.
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