Osmotic concentration, formerly known as osmolarity, is the measure of solute
concentration, defined as the number of osmoles of solute per litre of solution.
The osmolarity of a solution is usually expressed as Osm/L, in the same way that
the molarity of a solution is expressed as "M". Whereas molarity measures the
number of moles of solute per unit volume of solution, osmolarity measures
the number of osmoles of solute particles per unit volume of solution.
Types of solutes Osmolarity is distinct from molarity
because it measures osmoles of solute particles rather than moles of solute.
The distinction arises because some compounds can dissociate in solution,
whereas others cannot. Ionic compounds, such as salts, can
dissociate in solution into their constituent ions, so there is not a
one-to-one relationship between the molarity and the osmolarity of a
solution. For example, sodium chloride dissociates into Na+ and Cl− ions. Thus,
for every 1 mole of NaCl in solution, there are 2 osmoles of solute particles.
Both sodium and chloride ions affect the osmotic pressure of the solution.
Nonionic compounds do not dissociate, and form only 1 osmole of solute per 1
mole of solute. For example, a 1 mol/L solution of glucose is 1 osmol/L.
Multiple compounds may contribute to the osmolarity of a solution. For example, a
3 Osm solution might consist of: 3 moles glucose, or 1.5 moles NaCl, or 1 mole
glucose + 1 mole NaCl, or 2 moles glucose + 0.5 mole NaCl, or any other
such combination. Definition
The osmolarity of a solution can be calculated from the following
expression: where
φ is the osmotic coefficient, which accounts for the degree of non-ideality
of the solution. In the simplest case it is the degree of dissociation of the
solute. Then, φ is between 0 and 1 where 1 indicates 100% dissociation. However,
φ can also be larger than 1. For salts, electrostatic effects cause φ to be
smaller than 1 even if 100% dissociation occurs;
n is the number of particles into which a molecule dissociates. For example:
glucose has n of 1, while NaCl has n of 2;
C is the molar concentration of the solute;
the index i represents the identity of a particular solute.
Osmolality can be measured using an osmometer which measures colligative
properties, such as Freezing-point depression, Vapor pressure, or
Boiling-point elevation. Osmolarity vs. tonicity
Osmolarity and tonicity are related but distinct concepts. Thus, the terms
ending in -osmotic are not synonymous with the terms ending in -tonic. The
terms are related in that they both compare the solute concentrations of two
solutions separated by a membrane. The terms are different because osmolarity
takes into account the total concentration of penetrating solutes and
non-penetrating solutes, whereas tonicity takes into account the total
concentration of only non-penetrating solutes.
Penetrating solutes can diffuse through the cell membrane, causing momentary
changes in cell volume as the solutes "pull" water molecules with them.
Non-penetrating solutes cannot cross the cell membrane, and therefore osmosis of
water must occur for the solutions to reach equilibrium.
A solution can be both hyperosmotic and isotonic. For example, the intracellular
fluid and extracellular can be hyperosmotic, but isotonic – if the
total concentration of solutes in one compartment is different from that of
the other, but one of the ions can cross the membrane, drawing water with it,
thus causing no net change in solution volume.
Plasma osmolarity vs. osmolality Plasma osmolarity can be calculated from
plasma osmolality by the following equation:
Osmolarity = osmolality * where:
ρsol is the density of the solution in g/ml, which is 1.025 g/ml for blood
plasma. ca is the solute concentration in g/ml –
not to be confused with the density of dried plasma
Since ca is slightly larger than 0.03 g/ml, plasma osmolarity is 1–2% less
than osmolality. According to IUPAC, osmolality is the
quotient of the negative natural logarithm of the rational activity of
water and the molar mass of water, whereas osmolarity is the product of the
osmolality and the mass density of water.
In simpler terms, osmolality is an expression of solute osmotic
concentration per mass of solvent, whereas osmolarity is per volume of
solution. where mi is the molality of component i.
Plasma osmolarity/osmolality is important for keeping proper
electrolytic balance in the blood stream. Improper balance can lead to
dehydration, alkalosis, acidosis or other life-threatening changes.
Antidiuretic hormone is partly responsible for this process by
controlling the amount of water the body retains from the kidney when filtering
the blood stream. See also
Molarity Molality
Plasma osmolality Tonicity
van't Hoff factor References
D. J. Taylor, N. P. O. Green, G. W. Stout Biological Science
External links Information on osmotic concentration
from IUPAC Gold Book Online Serum Osmolarity/Osmolality
calculator