Pfeffer's Method
The apparatus used by Pfeffer (1877) for determination of osmotic pressure is shown in Figure. It consists of a porous pot with copper ferrocyanide membrane deposited in its walls and having a glass top cemented to it. The glass top has an open tube and a closed mercury manometer in the side.
The apparatus is filled with a solution under examination through the tube which is then sealed off. The pot is now placed in pure water maintained at constant temperature. The water passes across the membrane into the solution and develops a pressure on the manometer. The highest pressure registered by the manometer gives the osmotic pressure of the solution.
The Pfeffer’s method suffers from two disadvantages:
(a) It is slow and it takes a few days before the highest pressure is reached.
(b) It cannot be used for measuring high osmotic pressures as the ferrocyanide membrane being weak ruptures.
Morse and Frazer (1910) used essentially the same method but deposited the copper ferrocyanide membrane electrolytically. The membrane so produced being more stout, they were able to measure osmotic pressures up to about 300 atmospheres.
