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Advanced
™
Osmometer Model 3250 User’s Guide
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 and unit
of measurement. 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 H₂O, as the common unit of concentration rather than
applying further conversion factors.
Instrumentation
Advanced
™
Osmometers are devices for the determination of the
concentration of solutions by means of freezing-point measurement.
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 HO.
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 of the solution. Managing the plateau temperature for precise
measurement is the basis for several patents issued to Augustus Fiske.
The time over which liquid/solid equilibrium develops and is maintained,
is a function of the speed with which the heat-of-fusion is liberated
vs. the speed it is transferred away, or absorbed, by the surrounding
environment. This ratio can be slowed and the equilibrium time
stretched, to give a distinct plateau height measurable to 0.001°C.
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