Osmotic concentration and osmometers
The clinical unit of osmotic concentration is the milliosmole (mOsm); 1 mOsm is defined as the quantity of a substance that
dissociates to produce 1 millimole (mmol) of particles in solution.
Let's illustrate the concept this way: Consider the effects of relatively large albumin particles (molecular weight = 68,000),
much smaller glucose molecules (molecular weight = 180) and tiny sodium chloride molecules (molecular weight = 58) on the
osmolality of urine. Does albumin, glucose or sodium chloride have the greater effect on osmolality? We know 1 mmol of albumin
provides 1 mOsm of solute because albumin does not dissociate in urine to form an increased quantity of solute. Likewise,
1 mmol of glucose provides 1 mOsm of solute because glucose does not dissociate in urine to form an increased quantity of
solute. But 1 g/dl of glucose has a greater effect on osmolality than 1 g/dl of albumin because the number of particles in
1 g/dl of glucose is many times greater than the number of particles of albumin in 1 g/dl.
What about sodium chloride? In urine, 1 mmol of sodium chloride will dissociate to form 2 mOsm (one sodium ion and one chloride
ion) in solution. Thus, a 1 g/dl solution of sodium chloride has many hundred times the osmotic activity of a 1 g/dl solution
of albumin because undissociated and dissociated sodium chloride contribute many small molecules in large numbers while the
same weight of protein contributes fewer large molecules.
However, because of their molecular weight, protein molecules could substantially affect specific gravity measurements. When
urine contains 1 g/dl of protein, 0.003 must be subtracted from the observed specific gravity. In contrast, the effect of
1 g/dl of protein on urine osmolality is negligible (less than 1 mOsm/kg).
In clinical medicine, the osmotic concentration of solutions is usually measured with osmometers that determine freezing points
(freezing point osmometers) or vapor pressure (vapor pressure osmometers). Now, consider this—as solute (e.g., sodium chloride)
is added to urine:
> Osmotic pressure increases.
> Vapor pressure (the pressure at which rate of evaporation is equal to the rate of condensation) decreases.
> Boiling point increases.
> Freezing point decreases.
For example, 1 Osm of an ideal solute in 1 kg of water will have a freezing point of -1.86 C compared with pure water. Commercially
manufactured osmometers determine osmolality by measuring relative changes in freezing point or vapor pressure of unknown
solutions, using standard solutions as reference points. Currently available equipment uses microprocessors to provide rapid
digital readout of data on samples as small as 0.2 ml. Unfortunately, compared with refractometers, osmometers are expensive.