31/08/2021
Osmolarity and Osmolality
Osmolarity and osmolality are frequently confused and incorrectly interchanged. Osmolarity refers to the number of solute particles per 1 L of solvent, whereas osmolality is the number of solute particles in 1 kg of solvent. For dilute solutions, the difference between osmolarity and osmolality is insignificant. Measurements of osmolarity are temperature dependent because the volume of solvent varies with temperature (i.e., the volume is larger at higher temperatures). In contrast, osmolality, which is based on the mass of the solvent, is temperature independent. For this reason, osmolality is the preferred term for biologic systems and is used throughout this and subsequent chapters. Osmolality has the units of Osm/kg H2O. Because of the dilute nature of physiologic solutions and because water is the solvent, osmolalities are expressed as milliosmoles per kilogram of water (mOsm/kg H2O).
Table 1-1 shows the relationships among molecular weight, equivalence, and osmoles for a number of physiologically significant solutes.
Miscellaneous Physical, Chemical, and Microbiological Test Methods
Vitthal S. Kulkarni Ph.D., Charles Shaw Ph.D., in Essential Chemistry for Formulators of Semisolid and Liquid Dosages, 2016
11.1.4 Osmolality (USP)
Osmolarity and osmolality are units of solute concentration that are often used in reference to biochemistry and body fluids, and are related to the tonicity of the formulation. If the tonicity is too far from isotonic, certain products (e.g., ophthalmic solutions and suspensions) will cause stinging on application. Sodium chloride is often used to adjust the osmolality of a formulation. Osmolality is measured using an osmometer by, for example, freezing point depression of the solution. As with pH, osmolality can be measured and adjusted on the bulk material, and measured and monitored on the finished product at the time of release and on storage.
31/08/2021
13/11/2019