Osmotic concentration

Osmotic concentration,formerly known asosmolarity,^[1]is the measure ofsolute concentration,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 themolarityof a solution is expressed as "M" (pronounced "molar" ). Whereas molarity measures the number ofmolesof solute per unitvolumeof solution, osmolarity measures the number ofosmoles of solute particlesper unit volume of solution.^[2]This value allows the measurement of theosmotic pressureof a solution and the determination of how the solvent will diffuse across asemipermeable membrane(osmosis) separating two solutions of different osmotic concentration.

Unit

The unit of osmotic concentration is theosmole.This is a non-SIunit of measurement that defines the number ofmolesof solute that contribute to the osmotic pressure of a solution. Amilliosmole(mOsm) is 1/1,000 of an osmole. Amicroosmole(μOsm) (also spelledmicro-osmole) is 1/1,000,000 of an osmole.

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 candissociatein 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]

Another example ismagnesium chloride(MgCl₂), which dissociates into Mg²⁺and 2Cl⁻ions. For every 1 mole of MgCl₂in 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 ofglucoseis 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]

Definition

The osmolarity of a solution, given in osmoles per liter (osmol/L) is calculated from the following expression: $\mathrm {osmolarity} =\sum _{i}\varphi _{i}\,n_{i}C_{i}$ where

$φ$ is theosmotic 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 (seeDebye–Hückel equation);
$n$ is the number of particles (e.g. ions) into which a molecule dissociates. For example:glucosehas $n$ 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 can be measured using anosmometerwhich measurescolligative properties,such asFreezing-point depression,Vapor pressure,orBoiling-point elevation.

Osmolarity vs. tonicity

Osmolarity andtonicityare 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 solutesandnon-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.

In medicine

Plasma osmolarity vs. osmolality

Plasma osmolarity, the osmolarity ofblood plasma,can be calculated fromplasma osmolalityby the following equation:^[4]

Osmolarity =osmolality

\times (ρ sol - c a)

where:

$ρ sol$ is thedensityof the solution in g/ml, which is 1.025 g/ml forblood plasma.^[5]
$c a$ is the (anhydrous) solute concentration in g/ml – not to be confused with the density of dried plasma

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 permassof solvent, whereas osmolarity is pervolumeof solution (thus the conversion by multiplying with the mass density of solvent in solution (kg solvent/litre solution). ${\text{osmolality}}=\sum _{i}\varphi _{i}\,n_{i}m_{i}$

where $m i$ is the molality of component $i$ .

Plasma osmolarity/osmolality is important for keeping proper electrolytic balance in the blood stream. Improper balance can lead todehydration,alkalosis,acidosisor 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.^[6]

Hyperosmolarity and hypoosmolarity

A concentration of an osmatically active substance is said to be hyperosmolar if a high concentration causes a change in osmatic pressure in a tissue, organ, or system. Similarly, it is said to be hypoossmolar if the osmolarity, or osmatic concentration, is too low. For example, if the osmolarity ofparenteral nutritionis too high, it can cause severe tissue damage.^[7]One example of a condition caused by hypoosmolarity iswater intoxication.^[8]

References

D. J. Taylor, N. P. O. Green, G. W. StoutBiological Science

^^a ^bMcNaught, A. D.; Wilkinson, A.; Chalk, S. J. (1997).IUPAC. Compendium of Chemical Terminology (the "Gold Book" )(2nd ed.). Oxford: Blackwell Scientific Publications.ISBN 0-9678550-9-8.Retrieved23 January2022.
^^a ^b ^c ^d ^e ^f ^gWidmaier, Eric P.; Hershel Raff; Kevin T. Strang (2008).Vander's Human Physiology, 11th Ed.McGraw-Hill. pp.108–12.ISBN 978-0-07-304962-5.
^Costanzo, Linda S. (2017-03-15).Physiology.Preceded by: Costanzo, Linda S., 1947- (Sixth ed.). Philadelphia, PA.ISBN 9780323511896.OCLC 965761862.{{cite book}}:CS1 maint: location missing publisher (link)
^Martin, Alfred N.; Patrick J Sinko (2006).Martin's physical pharmacy and pharmaceutical sciences: physical chemical and biopharmaceutical principles in the pharmaceutical sciences.Philadelphia, Pennsylvania: Lippincott Williams and Wilkins. p. 158.ISBN 0-7817-5027-X.
^Shmukler, Michael (2004). Elert, Glenn (ed.)."Density of blood".The Physics Factbook.Retrieved2022-01-23.
^Earley, L. E.; Sanders, C. A. (1959)."The Effect of Changing Serum Osmolality on the Release of Antidiuretic Hormone in Certain PAtients with Decompensated Cirrhosis of the Liver and Low Serum Osmolality".Journal of Clinical Investigation.38(3): 545–550.doi:10.1172/jci103832.PMC293190.PMID 13641405.
^Panganiban, Jennifer; Mascarenhas, Maria R. (2021),"Parenteral Nutrition",Pediatric Gastrointestinal and Liver Disease,Elsevier, pp. 980–994.e5,doi:10.1016/b978-0-323-67293-1.00088-8,ISBN 978-0-323-67293-1,retrieved2024-05-10
^Donaldson, D. (1994),"Psychiatric Disorders of Biochemical Origin",Scientific Foundations of Biochemistry in Clinical Practice,Elsevier, pp. 144–160,doi:10.1016/b978-0-7506-0167-2.50013-3,ISBN 978-0-7506-0167-2,retrieved2024-05-10

External links

Online Serum Osmolarity/Osmolality calculator

[gold_book-1] McNaught, A. D.; Wilkinson, A.; Chalk, S. J. (1997).IUPAC. Compendium of Chemical Terminology (the "Gold Book" )(2nd ed.). Oxford: Blackwell Scientific Publications.ISBN 0-9678550-9-8.Retrieved23 January2022.

[Widmaier-2] ^^a ^b ^c ^d ^e ^f ^gWidmaier, Eric P.; Hershel Raff; Kevin T. Strang (2008).Vander's Human Physiology, 11th Ed.McGraw-Hill. pp.108–12.ISBN 978-0-07-304962-5.

[3] Costanzo, Linda S. (2017-03-15).Physiology.Preceded by: Costanzo, Linda S., 1947- (Sixth ed.). Philadelphia, PA.ISBN 9780323511896.OCLC 965761862.{{cite book}}:CS1 maint: location missing publisher (link)

[martin-4] Martin, Alfred N.; Patrick J Sinko (2006).Martin's physical pharmacy and pharmaceutical sciences: physical chemical and biopharmaceutical principles in the pharmaceutical sciences.Philadelphia, Pennsylvania: Lippincott Williams and Wilkins. p. 158.ISBN 0-7817-5027-X.

[5] Shmukler, Michael (2004). Elert, Glenn (ed.)."Density of blood".The Physics Factbook.Retrieved2022-01-23.

[6] Earley, L. E.; Sanders, C. A. (1959)."The Effect of Changing Serum Osmolality on the Release of Antidiuretic Hormone in Certain PAtients with Decompensated Cirrhosis of the Liver and Low Serum Osmolality".Journal of Clinical Investigation.38(3): 545–550.doi:10.1172/jci103832.PMC293190.PMID 13641405.

[7] Panganiban, Jennifer; Mascarenhas, Maria R. (2021),"Parenteral Nutrition",Pediatric Gastrointestinal and Liver Disease,Elsevier, pp. 980–994.e5,doi:10.1016/b978-0-323-67293-1.00088-8,ISBN 978-0-323-67293-1,retrieved2024-05-10

[8] Donaldson, D. (1994),"Psychiatric Disorders of Biochemical Origin",Scientific Foundations of Biochemistry in Clinical Practice,Elsevier, pp. 144–160,doi:10.1016/b978-0-7506-0167-2.50013-3,ISBN 978-0-7506-0167-2,retrieved2024-05-10

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