Advanced Instruments 3320, User Manual

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xvii
In a simple solution such as glucose or sodium chloride in water, the
freezing point can be measured and the unit concentration easily deter-
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 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 measure-
ment. 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 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 conver-
sion factors.
Instrumentation
Advanced Osmometers are devices for the determination of the concen-
tration of solutions by means of freezing-point measurement.
Advanced Osmometers utilize high-precision thermisters 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. It is
unstable in this state, and a mechanical agitation induces crystallization.
The heat of fusion suddenly liberated causes the sample temperature to
rise toward a plateau temperature, where a liquid/solid equilibrium
occurs. The equilibrium temperature is, by definition, the freezing point
Foreword