Osmolarity

Osmolarity is a measure of the total concentration of osmotically active particles in a solution, expressed as osmoles of solute per liter of solution (Osm/L). It reflects the number of particles that draw water by osmosis. Osmolarity is distinct from osmolality, which is the same concept measured per kilogram of solvent (Osm/kg). In dilute solutions, osmolarity and osmolality are usually similar, but they differ in concentrated solutions due to changes in density.

Osmotically active particles include ions, glucose, urea, and other small solutes that cannot freely cross a membrane. The osmolarity of a solution is approximately the sum of the molar concentrations of all solutes, each multiplied by its van't Hoff factor, which is the number of particles the solute dissociates into. For example, NaCl dissociates into two particles, contributing about 2 osmoles per mole if fully dissociated. Non-electrolytes like glucose contribute one osmole per mole. In biological contexts, ions typically dominate osmolarity.

Physiological relevance and measurement: Osmolarity influences water movement across cell membranes and helps determine osmotic pressure. In medicine, the term osmolality is often used interchangeably with osmolarity, though tonicity (the effective osmolality for cells) depends on membrane permeability to solutes. Normal plasma osmolarity is roughly 275–295 mOsm/L. Clinically, plasma osmolality can be estimated with formulae such as Osm ≈ 2[Na+] + glucose/18 + BUN/2.8 (in mOsm/kg). Osmolarity can be measured directly by methods like freezing point depression or vapor pressure osmometry, though these are more common in laboratory settings.