POLARISATION OF MEMBRANES 145 



follows that potential differences due to the Donnan equililH-iuni 

 are common occurrences in the body. 



Differential permeability docs not afford a sufficient explanation 

 for all bioelectric phenomena. Even when the membrane is freely 

 permeable to both ions of the salt and when the anion is the faster 

 of the two, the dilute side of the parchment becomes electro- 

 positive. This brings us again to the charge on the membrane. 

 Parchment has a negative charge in water, and in dilute solutions 

 of neutral salts — so has baked clay, wood, bone, charcoal, natural 

 gelatin, etc., and all these cause a positive charge to develop on 

 the dilute side. That is, the generation of a potential difference 

 is just the reverse of electrical endosmose (Fig. 35). This may 

 be confirmed by altering the charge on the membrane. Gelatin 

 may be induced to take up a positive charge. Mines found that 

 when a dilute solution N/80 was separated by a gelatin membrane 

 from a more concentrated N/8 solution of sodium chloride, the 

 dilute solution became electropositive. Gelatin is made positive 

 by treatment with the ions of polyvalent metals. When an 

 electropositive gelatin membrane was used, the dilute solution 

 became negative. 



A slight alteration in hydrogen ion concentration occurring on 

 one side of a membrane will cause the development of E.M.F. 

 If two solutions, one of pH 7 and the other of pH 8, are separated 

 by a membrane more permeable to H^ ions, an E.M.F. of about 

 30 millivolts may be obtained. The living cell has a pH of about 

 7-4< and its E.M.F. is not usually greater than 30 millivolts. 



Polarisation. When a current is passed between two electrodes 

 immersed in an aqueous solution, the potential difference between 

 the electrodes tends to decrease and will in time fall off altogether 

 on account of the deposition of ions of the opposite sign on the 

 surface of the electrode. This polarisation of the electrode may 

 be prevented by physical or chemical means (cf. various types of 

 concentration cells). A similar ionic layer forms on membranes 

 when a current is passed through them for some time (see also 

 Chap. XII., Polarisation Current). 



Selective permeability of membranes has often been noticed in 

 electrical transference experiments. The classical experiments of 

 Hittorff are now known to be, in some cases, vitiated by his 

 use of ox-gut membranes to prevent convection currents. For 

 instance, such membranes are nmch more permeable to SO4 ions 

 than to Cu ions. A large error is thus introduced into electrical 

 diffusion experiments with CUSO4 due to the adsorption of the 

 copper ions on the substance of the membrane. 



Till more is known of the physical state of the cell and its 



B, 10 



