APPENDIX
C
Theory of Operation
Osmolality is an expression of the total concentration of dissolved
particles in a solution without regard for particle size, density, con-
figuration, or electrical charge. Indirect means for the measurement
of osmolality are afforded by the fact that the addition of solute par-
ticles to a solvent changes the free energy of the solvent molecules.
This results in a modification of the cardinal properties of the sol-
vent, i.e., vapor pressure, freezing point, and boiling point.
Compared with pure solvent, the vapor pressure and freezing point
of a solution are lowered, while its boiling point is elevated, provided
that a single solvent is present in the solution. Solutions containing
more than one solvent generally behave in more complex ways.
In single-solvent solutions, the relative changes in solution proper-
ties are linearly related to the number of particles added to the sol-
vent, although not necessarily linearly related to the weight of
solute, since solute molecules may dissociate into two or more ionic
components. Since these properties all change linearly in proportion
to the concentration of solute particles, they are known as "colliga-
tive" properties.
Osmotic pressure is also a colligative property of a solution, but
unlike the other three, it is not a cardinal property of the solvent.
Solution osmotic pressure can be measured directly using a semi-
permeable membrane apparatus, but only with respect to those
solute particles that are impermeable, since smaller solute particles
freely transude the membrane and do not directly contribute to
osmotic pressure. Such a measurement is referred to as "colloid
osmotic pressure" or "oncotic pressure." It is expressed in terms of
pressure, in mmHg or kPa. Total osmotic pressure, i.e., that which
can be calculated on the basis of total solute concentration, is a the-
oretical concept only.
The measurement of total solution concentration, or osmolality, can
only be made indirectly by comparing one of the solution colligative
properties with the corresponding cardinal property of the pure sol-
vent. The first practical laboratory instruments developed for routine
measurement of osmolality were based upon depression of the
freezing point and, until recent years, all osmometers for large-scale
testing were based upon this methodology.
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