Determination of Osmotic Pressure by Pfeffer 's Method - QS Study
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Determination of Osmotic Pressure by Pfeffer ‘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. It was the botanist Pfeffer who successfully developed the technique for preparing good semi-permeable membranes and made measurements of osmotic pressure.

Pfeffer ‘s method: Pfeffer development a technique of depositing gelatinous copper ferrocyanide in the pores of an earthenware pot and used this pot to act as a semi-permeable membrane. His apparatus for the measurement of osmotic pressure is shown in Figure. The porous pot, P, having the deposit of copper ferrocyanide in its pores is connected to a manometer, M, is shown. The pot is filled with the experimental solution and then placed in a vessel containing the solvent. The entire assembly is then placed in a thermostat and allowed to stand until equilibrium is reached. At equilibrium the difference in the heights of mercury columns in the two tubes gave the osmotic pressure.

Figure A1: Pfeffer‘s apparatus for measurement of osmotic pressure.

The method is slow because it might take couple of days to reach equilibrium. Also, if the pressure is high, the membrane may break and cannot be used. Pfeffer’s method was improved by Berkeley and Hartley.