Model A
2
O
®
Advanced Automated Osmometer User’s Guide
Principles of Freezing-Point Osmometry
When a solute is dissolved in a pure solvent, the following changes in
the solution's properties occur:
• the freezing point is depressed,
• boiling point is raised,
• osmotic pressure is increased,
• vapor pressure is lowered.
These are the so-called "colligative" or concentrative properties of the
solution which, within reasonable limits, change in direct proportion to
the solute concentration (the number of particles in solution).
Of the colligative properties, measurement of the freezing point easily
permits the precise determination of the concentration of an aqueous
solution.
The freezing point of pure H
2
O is precisely +0.010°C. One mole of a
non-dissociating solute (one that does not dissociate into ionic species),
such as glucose dissolved in 1 kilogram (kg) of water, depresses the
freezing point of the water by 1.858°C. This change is known as the
freezing point depression constant for water. The freezing point depres-
sion also depends on the degree of dissociation of the solute. If the
solute is ionic, each ionic species depresses the freezing point by
1.858°C. For example, if one mole of sodium chloride were to com-
pletely dissociate into two ionic species (Na+ and Cl-) in 1 kg of water,
the freezing point would be depressed by 3.716°C. However, dissocia-
tion is never complete. Interference between solute molecules reduces
dissociation by a factor called the osmotic coefficient.
In a simple solution (i.e., glucose or sodium chloride in water), the
freezing point can be measured and the unit concentration easily deter-
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