A simple student laboratory on osmotic flow, osmotic pressure, and the reflection coefficient

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.