Clinical Significance

Of the colligative properties, measurement of the freezing point allows the concentration of an aqueous solution to be easily determined with great precision.

The freezing point of pure H₂O is precisely +0.010°C. One mole of a non-dissociating solute such as glucose (where the solute does not dissociate into ionic species, but remaints intact), when dissolved in 1 kg of water will depress the freezing point by 1.858°C. This change is known as the freezing point depression constant for water. The freezing point depression also depends upon the degree of dissociation of the solute. If the solute is ionic, the freezing point is depressed by 1.858°C for each ionic species. For example, if one mole of sodium chloride were to completely disssociate into two ionic species (Na⁺ and Cl^-) in 1 kg of water, the freezing point would be depressed by 3.716°C. However, dissociation is never complete. Interference between solute molecules reduces dissociation by a factor called the osmotic coefficient.

In a simple solution such as glucose or sodium chloride in water, the freezing point can be measured and the unit concentration easily determined from an equation or a reference table. However, the equation is unique for each solute. In a more complex situation, all ionized and non-dissociative species contribute to the freezing-point depression and the concentration of each solute cannot be easily determined. Each of the colligative properties has a similar problem, and though each of the colligative properties changes in direct proportion to the solute concentration, each requires a different mode & unit of measurement. Osmolality is a common unit of concentration measurement that can be used to relate all the colligative properties to each other, and to other concentration units. Because of is universality, most osmometry applications regularly use osmolality, expressed as "mOsm/kg H₂O", as the common unit of concentration rather than applying further conversion factors.

_{Source: The Advanced Micro-Osmometer Model 3300 User's Guide, Advanced Instruments, part no. 3305 Rev6 121301. 2001.}