Osmosis process is the movement of a solvent from a solution of lower solute concentration (higher solvent concentration) to one of the higher solute concentration (lower solvent concentration). In osmotic pressure measurements a semi-permeable membrane. It is permeable only to solvent (as it have holes that are small enough to prevent large solute molecules passing through) separates two liquids. This means that this technique is used for relatively large molecules, often polymers or biological macromolecules. Typically, one liquid is pure water and the other is the solute solutions of interest. This produces a flow of water from the solvent to the solute solutions. The experimental apparatus (Fig. 1) incorporates one for each liquid, two identical vertical columns, and the height of the liquid in the solution columns increase relative to that in the pure solvent column due to this net flow.
Fig.1. Schematic diagram of the apparatus for measuring osmotic pressure. (a) Apparatus at the start of the measurement; (b) apparatus at equilibrium