Osmotic concentration
Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L (pronounced "osmolar"), in the same way that the molarity of a solution is expressed as "M" (pronounced "molar"). 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. This value allows the measurement of the osmotic pressure of  a solution and the determination of how the solvent will diffuse across a semipermeable membrane (osmosis) separating two solutions of different osmotic concentration. Unit. The unit of osmotic concentration is the osmole. This is a non-SI unit of measurement that defines the number of moles of solute that contribute to the osmotic pressure of a solution. A milliosmole (mOsm) is 1/1,000 of an osmole. A microosmole (μOsm) (also spelled micro-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 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. 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. Another example is magnesium chloride  (MgCl), which dissociates into Mg and 2Cl ions. For every 1 mole of MgCl 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 of glucose is 1 osmol/L. 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, given in osmoles per liter (osmol/L) is calculated from the following expression: where Osmolarity can be measured using an osmometer which measures colligative properties, such as Freezing-point depression, Vapor pressure, or Boiling-point elevation. Osmolarity vs. tonicity. Osmolarity and tonicity are related but distinct 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 non-freely penetrating solutes "only". 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; therefore, the movement of water across the cell membrane (i.e., osmosis) 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 (in other words, a penetrating solute), drawing water with it, thus causing no net change in solution volume. Plasma osmolarity vs. osmolality. Plasma osmolarity can be calculated from plasma osmolality by the following equation: Osmolarity = osmolality * (ρ − c) 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). In simpler terms, osmolality is an expression of solute osmotic concentration per "mass" of solvent, whereas osmolarity is per "volume" of solution (thus the conversion by multiplying with the mass density of solvent in solution (kg solvent/litre solution). where m is the molality of component i. Plasma osmolarity/osmolality is important for keeping proper electrolytic balance in the blood stream. Improper balance can lead to dehydration, 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.