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Advanced
®
Micro-Osmometer Model 3320 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, and
• 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; in other words, the number of particles in solu-
tion.
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 H
2
O is precisely +0.010°C. One mole of a
non-dissociating solute such as glucose (where the solute does not dis-
sociate into ionic species, but remains intact), when dissolved in 1 kilo-
gram (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 dissoci-
ation 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 dissociate 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.
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