Foreword
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mined 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 regular-
ly use osmolality, expressed as "mOsm/kg H
2
O", as the common unit of
concentration rather than applying further conversion factors.
Instrumentation
Advanced Osmometers measure the freezing point of an aqueous solu-
tion to determine solute concentration. Advanced Osmometers utilize
high precision thermistors to sense the sample temperature, to control
the degree of supercooling and freeze induction, and to measure the
freezing point of the sample. They can routinely determine differences
of ±1 mOsm/kg H
2
O.
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. The
solution is unstable in this state. Thus, a mechanical agitation induces
crystallization. The sudden liberation of the heat of fusion causes the
sample temperature to rise toward a plateau temperature, where a liq-
uid/solid equilibrium occurs. The equilibrium temperature is, by defini-
tion, the freezing point of the solution. Managing the plateau tempera-
ture 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 vs. the speed at which it is
transferred away, or dissipated to the surrounding environment. This
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