OsmoPRO® Multi-Sample Micro-Osmometer User’s Guide
21
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
pre0.010 °C. One mole of a non-dissociating solute (a substance which 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 depression 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 (NaCl) 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.
In a simple solution (i.e., 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 solution, all ionized and
non-dissociated species contribute to the freezing point depression. The concentration of each
solute cannot be easily determined.
Each of the colligative properties has a similar problem. Though each of the colligative
properties changes in direct proportion to the solute concentration, each requires a different
mode and unit of measurement. Osmolality is a common unit of concentration measurement
that relates all the colligative properties to each other, and to other concentration units. Because
of its universality, most osmometry applications regularly use osmolality, expressed as
"mOsm/kg H
2
O", as the common unit of concentration rather than applying further conversion
factors.
2.2.1 Instrumentation
Advanced Instruments Osmometers measure the freezing point of an aqueous solution to
determine solute concentration. Advanced Instruments Osmometers utilize high precision
thermistors to sense the sample temperature, to control the degree of super cooling and freeze
induction, and to measure the freezing point of the sample. The thermistor can determine
differences of ±1 mOsm/kg H
2
O.
2.2.2 Freezing-point thermodynamics
The quickest and most precise way to measure the freezing point of a solution is to supercool it
several degrees below its freezing point and inducing crystallization of the solution via
mechanical agitation. The sudden liberation of energy (heat of fusion) causes the sample
temperature to rise toward a plateau temperature, where a liquid/solid equilibrium occurs. The
equilibrium temperature is the freezing point of the solution. Managing the plateau temperature
for precise measurement is the basis for several patents issued to Augustus Fiske.
The duration of the liquid/solid equilibrium phase is a function of the speed at which the heat of
fusion is liberated versus the speed at which it is dissipated to the surrounding environment.
This ratio can be slowed to prolong the equilibrium time, giving a distinct plateau measurable to
0.001 °C.
Sensitive thermistor probes monitor the sample temperature and control the thermoelectric
cooling element. Microprocessor control and automated operation minimize imprecision
introduced by operator technique.
