Osmole (unit)
Encyclopedia
Osmolarity is the measure of solute
concentration
, defined as the number of osmole
s (Osm) of solute per litre
(L) of solution
(osmol/L or Osm/L). 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.
Osmolality is a measure of the osmoles of solute per kilogram
of solvent
(osmol/kg or Osm/kg).
Molarity and osmolarity are not commonly used in osmometry because they are temperature
dependent. This is because water
changes its volume with temperature (See: Vapour pressure of water
). However, if the concentration of solutes is very low, osmolarity and osmolality are considered equivalent.
in solution, whereas others cannot.
Ionic compounds, such as salt
s, can dissociate in solution into their constituent ion
s, 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 M NaCl solution is a 2 Osm NaCl solution). 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 M solution of glucose
is 1 Osm.
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.
where
Osmolality can be measured using an osmometer
which measures colligative properties
, such as Freezing-point depression
, Vapor pressure
, or Boiling-point elevation
.
are related, but different concepts. Thus, the terms ending in -osmotic (isosmotic, hyperosmotic, hyposmotic) 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 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 and thus causing no net change in solution volume.
by the following equation:
Osmolarity = osmolality * (ρsol - ca)
Where:
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 (also known as osmotic concentration).
In more simple terms, osmolality is an expression of solute osmotic concentration per mass, whereas osmolarity is per volume of solvent (thus the conversion by multiplying with the mass density).
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
, defined as the number of osmole
Osmole (unit)
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"...
s (Osm) of solute per litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
(L) of solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
(osmol/L or Osm/L). 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
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
of solute per unit volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of solution, osmolarity measures the number of osmoles of solute particles per unit volume of solution.
Osmolality is a measure of the osmoles of solute per kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
of solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
(osmol/kg or Osm/kg).
Molarity and osmolarity are not commonly used in osmometry because they are temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
dependent. This is because water
H2O
H2O is the chemical formula for water and is also used as an abbreviation for the word "water". H2O or H2O It may also refer to:* H2O , a punk band**H2O , their self-titled debut album...
changes its volume with temperature (See: Vapour pressure of water
Vapour pressure of water
The vapour pressure of water is the pressure at which steam is saturated. At higher pressures water would condense. Vapour pressure is a function of temperature...
). However, if the concentration of solutes is very low, osmolarity and osmolality are considered equivalent.
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 dissociateDissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
in solution, whereas others cannot.
Ionic compounds, such as salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s, can dissociate in solution into their constituent ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s, so there is not a one-to-one relationship between the molarity and the osmolarity of a solution. For example, sodium chloride
Sodium chloride
Sodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
(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 M NaCl solution is a 2 Osm NaCl solution). 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 M solution of glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
is 1 Osm.
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 coefficientOsmotic coefficientAn osmotic coefficient φ is a quantity which characterises the deviation of a solvent from ideal behaviour, referenced to Raoult's law. The osmotic coefficient on a molality basis is defined by:and on an amount fraction basis by:...
, 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 equationDebye-Hückel equationThe Debye–Hückel equation and Debye–Hückel limiting law, were derived by Peter Debye and Erich Hückel, who developed a theory with which to calculate activity coefficients of electrolyte solutions. Activities, rather than concentrations, are needed in many chemical calculations because solutions...
); - n is the number of particles (e.g. ions) into which a molecule dissociates. For example: glucoseGlucoseGlucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
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
Osmometer
An osmometer is a device for measuring the osmotic strength of a solution, colloid, or compound.There are several different techniques employed in osmometry:...
which measures colligative properties
Colligative properties
Colligative properties are properties of solutions that depend on the number of molecules in a given volume of solvent and not on the properties/identity of the molecules. Colligative properties include: relative lowering of vapor pressure; elevation of boiling point; depression of freezing point...
, such as Freezing-point depression
Freezing-point depression
Freezing-point depression describes the phenomenon in which the freezing point of a liquid is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a non-volatile solute is added to a pure solvent, such as water...
, Vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
, or Boiling-point elevation
Boiling-point elevation
Boiling-point elevation describes the phenomenon that the boiling point of a liquid will be higher when another compound is added, meaning that a solution has a higher boiling point than a pure solvent. This happens whenever a non-volatile solute, such as a salt, is added to a pure solvent, such...
.
Osmolarity vs. tonicity
Osmolarity and tonicityTonicity
Tonicity is a measure of the osmotic pressure gradient of two solutions separated by a semipermeable membrane. It is commonly used when describing the response of cells immersed in an external solution...
are related, but different concepts. Thus, the terms ending in -osmotic (isosmotic, hyperosmotic, hyposmotic) 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 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
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
, 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
Osmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
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 and thus causing no net change in solution volume.
Plasma osmolarity vs. osmolality
Plasma osmolarity can be calculated from plasma osmolalityPlasma osmolality
-Measured osmolality :Osmolality can be measured on an analytical instrument called an osmometer. It works on the method of depression of freezing point.Plasma osmolality is affected by changes in water content...
by the following equation:
Osmolarity = osmolality * (ρsol - ca)
Where:
- ρsol is the densityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
of the solution in g/ml, which is 1.025 g/ml for blood plasmaBlood plasmaBlood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...
. - ca is the (anhydrousAnhydrousAs a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
) 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 (also known as osmotic concentration).
In more simple terms, osmolality is an expression of solute osmotic concentration per mass, whereas osmolarity is per volume of solvent (thus the conversion by multiplying with the mass density).