Conversion Between Osmolality and Osmolarity of Infusion Solutions

In order to be applied into the body, intravenous fluids should be formulated to have an osmotic pressure similar to that of biological fluids. Physiologic acceptability of large-volume parenteral formulations in clinical practice is largely determined due to the osmotic effect of the dissolved solutes. Therefore, it is generally recommended that the labelling should list osmolarity. However, the declared osmolarity is not consistent with the principle of osmometry which measures osmolality. Then, the conversion of the measured osmolality to declared osmolarity is necessary. The conversion factor, defined as the ratio of molarity and molality of the solution, necessitates the measurement of the solution density and the expression of water content in solution. In this work, the relationship between molarity and molality of the aqueous solutions of nine parenteral solutes is studied. The equations for the interconversion between molarity and molality, employing the experimentally obtained mean molal volume of the dissolved solute, are proposed. The original equation allowing the estimation of the molal volume of the solute in aqueous solution from its true powder density is introduced and recommended for nonelectrolytes as well as the hydrates of electrolytes.