14-2 



MEMBRANES (PLASMAHAUT) 



ELECTRICAL 



OSMOSIS 



ANODE 



4- 



+ + 

 — + + 





CATHODE. 



•5-04 



ef 



falls abruptly to reach a minimum at a concentration of M/16. 

 This fall is caused by the increasing prevalence of the repelling 

 action of the cation on the positively charged particles of water. 

 Further increase in concentration causes an increase of rate of 

 diffusion. This final passage of water into the solution is due to 

 true osmotic pressure. At the concentrations where the 7-ate of diffu- 

 sion is decreased, i.e. where the curve falls (M/256 — MjlQ in the case 

 of N a^JlPO ,^) water passes from the solution through the collodion 

 membrane to the pure solvent. That is, negative osmosis takes place. 

 Negative osmosis is a particular instance of electrical osmosis. 



In Part II., p. 534, will be found 

 details of an experiment which 

 shows that water can be drawn 

 through certain colloidal membranes 

 by direct electrical means. If the 

 water is acidulated the attraction 

 is towards the anode, but if alkali 

 is added the water rises in the tube 

 containing the cathode. To obtain 

 this result, the membrane used must 

 be of material capable of combining 

 either with anions or with cations — 

 e.g. of protein. Collodion does not 

 form such compounds and so cannot 

 form a membrane suitable for ex- 

 periments on electrical osmosis until 

 it has adsorbed an amphoteric col- 

 loid. In Fig. 35 is represented a 

 gelatin - collodion membrane in 

 acidulated water — i.e. in water with 

 a slight excess of hydrogen ions. 

 The membrane adsorbs some of these excess ions, interacts 

 chemically with them to form gelatin hydrochloride, and so acquires 

 a positive electrical charge. The passage of a current through 

 the membrane and water depends on the carriage of the charge by 

 ions — in this case H^ and 0H~. The negative ions are attracted 

 to the positively charged membrane till the charge on it is equalised. 

 The positive ions attracted by the negative potential pressure at 

 the cathode pass through the membrane, and raise the hydrostatic 

 pressure on the cathodal side. It is obvious that the hydrogen ion 

 concentration must increase at the cathode and decrease at the 

 anode. (Pole finding paper is blotting paper soaked in phenol- 

 phthalein — an indicator which while colourless in neutral solutions 

 becomes red in distinctly alkaline solutions.) 



1+ + 

 K + 



Fig. 35. — Diagrammatic section tlirough 

 a gelatin-collodion membrane showing a 

 single large pore to demonstrate electrical 

 osmosis. 



