POTENTIALS AT PHASE BOUNDARIES 



215 



60. Potentials at precipitation membranes 



The theoiy of cell membrane potentials is based upon the analogy 

 existing between an "oil' '-phase and a cell membrane. But there is 

 still an older model for cell membranes, which reproduced very 

 well indeed its properties of permeabihty, namely M. Traube's 

 precipitation membranes. Traube's membranes were adopted by 

 Pfeffer in his well known cells for the qualitative and quantitative 

 study of osmotic pressure and semipermeabiHty. The fact that 

 these copper ferrocyanide membranes showed the same impermea- 

 bility for many dissolved substances as was assumed in the explana- 

 tion of shrinking and swelling phenomena in cells placed into non- 

 isotonic solutions, gave a great stimulus to the entire theory of 

 semipermeabiHty. And without this we could scarcely ever have 

 arrived at the idea of a membrane which 

 is permeable for cations but not for 

 anions. The questions arise naturally 

 whether such precipitation membranes 

 can also be the seat of potential differ- 

 ences, whether these are analogous to the 

 cell membrane potentials, and whether 

 Beutner's theory is apphcable to them, 

 or, whether in this case the older idea of 

 a specific permeability for definite ions 

 is effective instead. 



Disregarding the older investigations 

 studied such chains. 



- Secfrvmeter 



Fig. 27 



of Bruning,^* Beutner 

 His arrangement was the following (fig. 27): 

 A ilf/40 warm solution of potassium ferrocyanide in 10 per cent 

 gelatin solution was poured into a glass tube open at both ends, 

 permitted to gel, and immersed in a beaker containing M/20 CuS04 

 solution. At the zone of contact a brown precipitation membrane 

 of copper ferrocyanide was immediately formed, barely perceptible 

 at first, but increasing in thickness with time. The upper end of 

 the gelatin layer and the outer copper solution were led off by means 

 of calomel electrodes. An E.M.F. of 0.10-0.12 volts was observed, 

 the CuS04 solution being positive. Then the composition of the 

 outer solution was varied, keeping its osmotic pressure constant 

 throughout by the addition of sugar in order not to destroy the 



21 



Briining, Pflugers Arch. f. d. ges. Physiol. 117, 409 (1907). 



