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Osmolarity is a measure of the osmoles of solute per liter of solution, while the osmolality is a measure of the osmoles of solute per kilogram of solvent. Molarity and osmolarity are not commonly used in osmometry because they are temperature dependent; that is, water changes its volume with temperature. However, if the concentration is very low, osmolarity and osmolality are considered equivalent. In calculations for these two measurements, salts are presumed to dissociate into their component ions. For example, a mole of glucose in solution is one osmole, whereas a mole of sodium chloride in solution is two osmoles (one mole of sodium and one mole of chloride). Both sodium and chloride ions affect the osmotic pressure of the solution.
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 (e.g. for sucrose). For salts, electrostatic effects cause φ to be smaller than 1 even if 100% dissociation occurs (see Debye-Hückel equation);
- n is the number of particles (e.g. ions) 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.
Osmolarity is expressed in units of osmoles per liter of solution (osmol/L), while osmolality is defined as osmoles per kilogram solvent (osmol/kg).
Osmolality can be measured using an osmometer which measures colligative properties, such as Freezing-point depression, Vapor pressure, or Boiling-point elevation.
Osmolality/osmolarity vs. tonicity
While similar, osmolarity and tonicity are not the same. The key difference between the two is osmolarity is a measure of all solutes in solution, whereas tonicity is a measure of impermeable solutes. Osmolarity compares the amount of solutes in two solutions, whereas tonicity compares the osmotic pressure gradient. If a solution in compartment A is hypertonic to a solution in compartment B, water will flow from compartment B to compartment A in an effort to dilute the solutes in compartment A. This allows the two compartments to have equal solute concentration.
The derivatives of the term: isosmotic, hyperosmotic, and hypoosmotic, should not be confused with isotonic, hypertonic and hypotonic.
Example: Solution A contains 2 mM of sucrose (impermeable) and 5 mM of urea (permeable)
Solution B contains 2 mM of sucrose (impermeable)
Solution A is hyperosmotic to solution B because it contains more total solutes.
Solution A is isotonic to solution B because the amount of impermable solutes is equal. Therefore, there is no osmotic pressure or water flow.
See also
References
