QS Study

Determination of Osmotic Pressure by Berkeley and Hartley’s method

Abbe Nollet (1748) was the first to observe the phenomenon of osmosis and made measurements of osmotic pressure. His measurements with pig’s bladder as the semi-permeable membrane gave only semi-quantitative results as the pig’s bladder is not a good semi-permeable membrane.

Berkeley and Hartley’s method: The principle of the method is illustrated in Figure. The apparatus consists of two concentric cylindrical tubes, A and B. The porous tube, A, carries in it the semi-permeable membrane of copper ferrocyanide. The inner tube is surrounded in the outer tube, B. The outer tube has an opening at C through which pressure could be applied from outside. The porous tube A has two side tubes, D and E. The solvent is added to A through E. The side tube is a capillary tulle so that any small change in pressure in A could be easily detected by the movement of the liquid meniscus in the tube.

Fig: Berkeley and Hartley’s apparatus pressure

Due to osmosis the water from tube A flows to the solution in B and causes a movement in the liquid meniscus in D. Berkeley and Hartley prevented the inflow of water on tube B by applying an external pressure through C and maintained the liquid meniscus in at the position until equilibrium. The excess pressure thus applied to the solution side to prevent water inflow gave the osmotic pressure at the temperature of the experiment. The apparatus is capable of giving good results in a relatively short time compared to other methods where the solvent is allowed to flow into the solution. In this method, there is no change in the concentration of the solution because no solvent is allowed to enter the solution tube. Pressure up to 150 atmospheres can be measured by this method. Some results of Berkeley and Hartley are given in Table.

Osmotic pressure of Sucrose solutions at 300 C