|
In order to give strength to the copper ferrocyanide membrane, so as to make it capable of withstanding considerable pressures, Pfeffer in 1877 deposited it in the walls of an unglazed earthenware cell, such as is used for the porous pots in galvanic batteries.
The pot is immersed in copper sulphate solution, and placed under the receiver of an air-pump. The air in the pores is then removed, and on admitting air to the receiver, the copper solution is forced into the pores of the pot. The latter is removed from the solution, quickly washed out, and filled up with a 3 per cent, solution of potassium ferrocyanide. The pot is then allowed to stand for several hours in copper sulphate solution. The two salts diffuse through the porous wall, meeting somewhere inside, and producing a coherent film of copper ferrocyanide in the wall of the pot. The latter is now washed out, filled up with a solution, say of sugar in water, and fitted with a manometer cemented into the open top as shown in
 | Fig: Pfeffers apparatus Pfeffers apparatus for measurement of osmotic pressures. |
When the pot is plunged into water, there is a gradual rise of pressure in the manometer, until a steady value is finally reached. This is the osmotic pressure of the solution.
The preparation of a good semipermeable pot is a matter of no little difficulty; most of the results are failures, and many precautions must be taken which cannot be described here. Better results are said to be obtained by driving the ions, Cuยทยท and FeCy6'''', by electrolysis into the pot.
|
|
