Relationship between osmolality and osmolarity.

Since the compendia require the osmolarity of certain parenterals to be labeled and since experimentally only the osmolality can be measured, it is necessary to obtain the relationship between these two quantities. This relationship was determined by considering fundamental physical-chemical definitions. The osmolality of a solution was found to be simply related to the osmotic coefficient. The conversion to osmolarity requires the use of the partial molal volume(s) of the solute(s). A single conversion factor is required for a particular solute system; i.e., the conversion factor is independent of the solution concentration.