A semipermeable membrane is a thin, sheetlike material that allows the passage of a solvent (such as water) but not larger dissolved solutes (such as salt and sugar). The property of permeability depends on the molecular or ionic diameter of the dissolved substance and the nature of the membrane. Although various semipermeable membranes are available from natural sources they are very susceptible to damage once they have been removed from the parent plant or animal. One of the few exceptions is parchment – a paper-like writing material prepared from the skin of sheep or goats. Parchment is, therefore, used in some experiments in osmosis, but artificially prepared membranes are preferred.
For many of his experiments Pfeffers, and other investigators after him, used a semipermeable membrane made of copper (II) hexacyanoferrate (Cu2[Fe(CN)6]}. This is made by the action of dilute copper sulfate on a dilute solution of potassium ferrocyanide. As this membrane is very thin and fragile it has to be supported on a strong porous framework, and a porous pot was found to be suitable. To obtain the membrane in the walls of the pot, the pot itself is filled with dilute copper sulfate solution. The whole pot is then immersed for a few days in a dilute solution of potassium ferrocyanide. To remove air bubbles from the pores, the pot must first be put in a vacuum vessel for a while.