ELECTRICAL OSMOSIS 



115 



OSMOSIS 



CATHODE 



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 rate of 

 diffusion is decreased, i.e. where the curve falls (M/256 M/IQ in 

 the case of Na 2 HPO 4 ) water passes from the solution through the 

 collodion membrane to the pure solvent. That is, negative osmosis 

 takes place. 



Negative osmosis is a particular ELECTRICAL 



instance of electrical osmosis. 



In Part II. p. 399 will be found ANODE. 

 details of an experiment which shows 

 that water can be drawn through 

 certain colloidal membranes by 

 direct electrical means. If the pH 

 of the water is greater than 7 (i.e. 

 acidulated water) the attraction is 

 towards the anode, but if the pll is 

 less than 7, 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. proteins. 

 Collodion does not form such 



i j r> FIG. 20. Diagrammatic section through 



COmpOUlKtS and SO Cannot torm a a gelatine - collodion membrane showin- a 



membrane suitable for experiments SSSdJ!* 8 p01 

 on electrical osmosis until it has 



adsorbed an amphoteric colloid. In Fig. 20 is represented 

 a gelatine-collodion membrane in acidulated water --i.e. in 

 water with a slight excess of hydrogen ions. The mem- 

 brane adsorbs some of these excess ions, interacts chemically 

 with them to form gelatine 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 OH~. 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 



