Hi, Ode, It does seem strange to use something like parchment to accompish the separation. But this is the principle of diffusion, osmosis and dialysis. This transport mechanism can be quite strong and specific to the ion or particle. Parchment was the membrane of choice from the time of Graham (1860s) to the heyday of colloidal chemistry led by Freundlich (1920s.) Here is what Freundlich had to say about colloid fractionation by parchment [Colloid And Capillary Chemistry, 3rd ed., 1922, page 377]:
"Dialysis depends, as has already been said, upon the impermeability of membranes, such as parchment and collodion, to the colloid particles and their permeability to liquids and truly dissolved substances. The process has retained its importance for colloid chemistry undiminished since the time of Graham (i.e. since 1861.) It is still aways employed to recognize a colloidal solution as such and to free it from dissolved impurities... For the purification of sols the old Graham dialyser is still frequenty employed; the dialysing membrane is attached to a bottle in place of the bottom, removed by cracking it off; the sol is put into the bottle, which dips into a dish containing the pure dispersing medium (i.e. DW in the case of EIS.) The dialysis is more rapid and thorough when the surface of the dialysing membrane is as large as possible compared to the volume of the colloidal solution... A very effective method for purifying colloidal solutions, which is also employed technically...consists in combining dialysis with electrosmosis and electrolysis [Ruppel, Ber. d. deutsch. pharm. Ges. 30, 314 (1920).] The sol is contained in a vessel between diaphragm walls which prevent the passage of colloid particles, while the solvent containing electrolytes is forced through the pores by electrosmosis, and ions are forced through by electrolysis...The correct choice of the diaphragm is of importance; those which assume a negative charge hinder the passage of the anions, those with the positive charge that of the cations. It is therefore necessary to take for the middle chamber diaphragms of different kinds, negative at the side towards the cathode, and positive at the anode side..." Freundlich also describes the successful use of treated filter papers by Bechhold, Martin and others for the dialysis of colloids. Best regards, Matthew
