Osmotic concentration


Osmotic concentration, formerly known as osmolarity.


Osmotic concentration is the measurement  of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). 


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.


It allows the measurement of the osmotic pressure of a solution and the determination of how the solvent will diffuse across a semipermeable membrane separating two solutions of different osmotic concentration.


The unit of osmotic concentration is the osmole. 

The osmolarity of a solution is usually expressed as Osm/L.

A milliosmole (mOsm) is 1/1,000 of an osmole.


A microosmole (μOsm) is 1/1,000,000 of an osmole.

This is unit of measurement that defines the number of moles of solute that contribute to the osmotic pressure of a solution.

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. 


Sodium chloride (NaCl) dissociates into Na+ and Cl− ions. 


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.


Magnesium chloride (MgCl2), which dissociates into Mg2+ and 2Cl− ions. For every 1 mole of MgCl2 in the solution, there are 3 osmoles of solute particles.


Nonionic compounds do not dissociate, and form only 1 osmole of solute per 1 mole of solute: a 1 mol/L solution of glucose is 1 osmol/L.


Multiple compounds may contribute to the osmolarity of a solution: e.g.

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.


Osmolarity can be measured using an osmometer: measures colligative properties, such as Freezing-point depression, Vapor pressure, or Boiling-point elevation.


Osmolarity and tonicity are distinct concepts. 


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. 


Osmolarity takes into account the total concentration of penetrating solutes and non-penetrating solutes.


Tonicity takes into account the total concentration of non-freely penetrating solutes only.


Penetrating solutes can diffuse through the cell membrane, causing changes in cell volume as the solutes  pull water molecules with them. 


Non-penetrating solutes cannot cross the cell membrane; therefore, the movement of water across the cell membrane must occur for the solutions to reach equilibrium.


A solution can be both hyperosmotic and isotonic.


Osmolality expresses solute osmotic concentration per mass of solvent, whereas osmolarity is per volume of solution.


Plasma osmolarity/osmolality balance is required for keeping proper electrolytic levels.


Improper osmolarity/osmolality 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.







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