Advances in Physiology Education, Vol 268, Issue 6 10-S20, Copyright © 1995 by American Physiological Society
A simple student laboratory on osmotic flow, osmotic pressure, and the reflection coefficient
J. J. Feher and G. D. Ford
Department of Physiology, Medical College of Virginia, Richmond 23298-0551, USA.
Osmosis is usually taught from the point of view of the osmotic pressure
developed when solutions of different concentrations of solute are
separated by an ideal semipermeable membrane. The osmotic pressure is
defined at equilibrium when there is no net flow, and it takes some time to
reach this equilibrium. Although the osmotic pressure is certainly
important, teaching only this point of view implicitly diminishes the
importance of osmotic flow, which begins almost instantaneously across a
membrane. A device was constructed with which students could measure the
flow across a model membrane (dialysis tubing) as a function of
concentration for solutes of different sizes. The device produced flows
that were linearly proportional to the concentration, providing a
confirmation of van't Hoff's law. Separate student groups repeated these
experiments using both different solutes and different dialysis membranes.
The combined results of four student groups showed that the flow across
these nonideal membranes depends on the solute and membrane as well as the
concentration of solute. Given a value for area times filtration
coefficient (A x Lp) for the membranes (determined beforehand by their
instructor), the students could calculate the reflection coefficient
(sigma) for three solutes and two membranes. The results showed that large
solutes had large sigma and that less porous membranes had larger sigma. A
concurrent demonstration using this device and membranes showed that the
osmotic flow can generate large pressures. These experiments and
demonstration provide a balanced view of osmotic flow and pressure.
