Osmotic concentration, formerly known asosmolarity,[1] is the measure ofsoluteconcentration, defined as the number ofosmoles (Osm) of solute perlitre (L) ofsolution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed asOsm/L (pronounced "osmolar"), in the same way that themolarity of a solution is expressed as "M" (pronounced "molar").
Whereas molarity measures the number ofmoles of solute per unitvolume of solution, osmolarity measures the number of particles on dissociation of osmotically active material (osmoles of solute particles) per unit volume of solution.[2] This value allows the measurement of theosmotic pressure of a solution and the determination of how the solvent will diffuse across asemipermeable membrane (osmosis) separating two solutions of different osmotic concentration.
The unit of osmotic concentration is theosmole. This is a non-SI unit of measurement that defines the number ofmoles of solute that contribute to the osmotic pressure of a solution. Amilliosmole (mOsm) is one thousandth of an osmole. Amicroosmole (μOsm) (also spelledmicro-osmole) is one millionth of an osmole.
Osmolarity is distinct from molarity because it measures osmoles of solute particles rather than moles of solute. The distinction arises because some compounds candissociate in solution, whereas others cannot.[2]
Ionic compounds, such assalts, can dissociate in solution into their constituentions, so there is not a one-to-one relationship between the molarity and the osmolarity of a solution. For example,sodium chloride (NaCl) dissociates into Na+ and Cl− ions. Thus, for every 1 mole of NaCl in solution, there are 2 osmoles of solute particles (i.e., a 1 mol/L NaCl solution is a 2 osmol/L NaCl solution). Both sodium and chloride ions affect the osmotic pressure of the solution.[2][Note: NaCl does not dissociate completely in water at standard temperature and pressure, so the solution will be composed of Na+ ions, Cl- ions, and some NaCl molecules, with actual osmolality = Na+ concentration x 1.75]
Another example ismagnesium 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. For example, a 1 mol/L solution ofglucose is 1 osmol/L.[2]
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.[2]
The osmolarity of a solution, given in osmoles per liter (osmol/L) is calculated from the following expression:where
Osmolarity can be measured using anosmometer which measurescolligative properties, such asFreezing-point depression,Vapor pressure, orBoiling-point elevation.
Osmolarity andtonicity are related but distinct concepts. Thus, the terms ending in-osmotic (isosmotic, hyperosmotic, hypoosmotic) are not synonymous with the terms ending in-tonic (isotonic, hypertonic, hypotonic). 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 solutesand non-penetrating solutes, whereas tonicity takes into account the total concentration of non-freely penetrating solutesonly.[3][2]
Penetrating solutes can diffuse through thecell membrane, causing momentary 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 (i.e.,osmosis) must occur for the solutions to reachequilibrium.
A solution can be both hyperosmotic and isotonic.[2] 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 (in other words, a penetrating solute), drawing water with it, thus causing no net change in solution volume.
Plasma osmolarity, the osmolarity ofblood plasma, can be calculated fromplasma osmolality by the following equation:[4]
where:
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 (also known as osmotic concentration).[1]
In simpler terms, osmolality is an expression of solute osmotic concentration permass of solvent, whereas osmolarity is pervolume of solution (thus the conversion by multiplying with the mass density of solvent in solution (kg solvent/litre solution).
wheremi is the molality of componenti.
Plasma osmolarity/osmolality is important for keeping proper electrolytic balance in the blood stream. Normally, sodium is the major contributor to plasma (or serum) osmolality; glucose and urea also contribute.[6] Improper balance can lead todehydration,alkalosis,acidosis or other life-threatening changes.Antidiuretic hormone (vasopressin) is partly responsible for this process by controlling the amount of water the body retains from the kidney when filtering the blood stream.[7]
A concentration of an osmotically active substance is said to be hyperosmolar if a high concentration causes a change in osmotic pressure in a tissue, organ, or system. Hypernatremia, diabetic ketoacidosis, and uremia are pathologic causes of hyperosmolarity.[6] Similarly, it is said to be hypoossmolar if the osmolarity, or osmatic concentration, is too low. For example, if the osmolarity ofparenteral nutrition is too high, it can cause severe tissue damage.[8] One example of a condition associated with hypoosmolarity iswater intoxication.[9]
{{cite book}}: CS1 maint: location missing publisher (link)