OsmoPRO® Multi-Sample Micro-Osmometer User’s Guide
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The standard freezing curve shown below (Figure 2.1) illustrates the temperature of a sample
as it progresses through the freezing cycle and shows the action of the osmometer at each
stage of the cycle.
Start cooling
SAMPLE THERMODYNAMICS
0°C
SAMPLE
FREEZING
POINT
Heat of fusion released
Super-cooled
Freezing initiated
TIME
Figure 2.1: Standard Freezing Curve
2.3 Definitions
The following are key terms relevant to the science of osmolality.
Avogadro’s number
The number of molecules in one mole (gram molecular weight) of a substance. One mole of
non-ionic solute (such as sucrose) dissolved in one kilogram of water will yield Avogadro’s
number (6.022 x 10
23
) of molecules. One mole of ionic solute dissolved in one kilogram of water
will yield almost twice Avogadro’s number of particles.
Colligative properties
The properties of solutions depend on the number of particles in each volume of solvent, not the
mass of the particles. Colligative properties include: vapor pressure, boiling point, freezing point
and osmotic pressure (see also Concentrative Properties).
Concentration
The relative amount of solute in a solution. This can be expressed in many ways: solute to
solvent, solute to solution, mass to mass, mass to volume, etc.
Concentrative properties
When a solute is dissolved in solvent, certain properties of the solvent ― freezing point, boiling
point, vapor pressure and osmotic pressure ― are changed nearly in proportion to the
concentration of the solute, expressed in dissolved particles. Avogadro’s number of particles,
regardless of their size or shape, when dissolved in a kilogram of water, will change each of the
concentrative properties a specific amount.
Freezing point osmometers
Instruments which measure the osmotic strength of solution by using freezing point depression.
Freezing point depression
The phenomenon where the freezing point of a liquid (a solvent) is depressed when another
compound is added, meaning a solution has a lower freezing point than a pure solvent.
Measurement of temperature at plateau
TEM
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