OSMOSIS AND OSMOTIC PRESSURE
When a semi-permeable membrane is placed between a solution and a solvent, it is observed that solvent molecules enter the solution and the volume of solution increases. It is also observed that if two solutions of unequal concentrations are separated by a semipermeable membrane, the solvent molecules from a solution of lower concentration move towards a solution of higher concentration. This phenomenon was first observed by Abbe Nollet (1748) and termed as Osmosis (reek, osmos = to push). Osmosis is defined as the spontaneous flow of solvent molecules through semipermeable membrane from a pure solvent to a solution or from a dilute to a concentrated solution.
The phenomenon of osmosis can be demonstrated by the following experiment
Two eggs of same size are taken and their outer hard shell is removed by dissolving in dilute hydrochloric acid. One of the eggs is placed in distilled water and the other in saturated salt solution.
After sufficient time, it is noticed that the egg placed in water swells up and that placed in salt solution shrinks. In the first case water enters the concentrated egg fluid while in the second case water comes out of the egg as salt solution is more concentrated thatn egg fluid.
Difference between osmosis and diffusion
The two processes, diffusion and osmosis, can be distinguished in terms of the following aspects:
(i) In diffusion, solute as well as solvent molecules flow in opposite directions while in osmosis the flow of solvent molecules occurs in one direction only.
(ii) For osmosis, a semipermeable membrane is required while for diffusion it is not required.
A porous pot is taken and a semipermeable membrane of copper ferrocyanide is deposited in its walls. It is fitted with a long glass with the help of a rubber stopper. It is filled with concentrated aqueous sugar solution and placed is distilled water. Osmosis occurs and the level of the solution in glass tube rises over a period of time. After a few days, the level becomes stationery. At this equilibrium state the hydrostatic pressure of the liquid column exactly balances the tendency which enables the water molecules to pass through semipermeable membrane.
The hydrostatic pressure developed as a result of osmosis is a measure of osmotic pressure of the solution. Osmotic pressure is also defined as the hydrostatic pressure built up on the solution which just stops the osmosis.
Osmosis pressure = hydrostatic pressure
π = hdg
where h = increase in level in the tube of unit cross section, d = density of solution and g = acceleration due to gravity.
Actually, this will not be an exact measure of osmotic pressure of the solution originally taken because sufficient dilution has taken place with time.
An apparatus as shown in Fig. 5.10 consists of water-tight chamber which is divided into two halves by a semipermeable membrane and fitted with solution and the other compartment is filled with pure solvent. Water (solvent) tries to flow into the solution side. To check this tendency a certain pressure shall have to be applied by the piston. This external pressure is this a measure of osmotic pressure of the solution. The external pressure which must be applied on the solution in order to stop the flow of the solvent into the solution through semipermeable membrane is equal to osmotic pressure.