106 PHYSICAL CHEMISTRY 



The experiments of Bartell (1914) are more illuminating, 

 since he used porcelain membranes that could be studied in a 

 more exact manner. He had already observed that porcelain 

 with large pores shows no osmotic effects and that the semi- 

 permeability varies inversely with the size of the pores, seem- 

 ingly directly with the molecular weight of the solute. The 

 membranes showing negative osmose are very leaky membranes 

 with pore diameters of about 0.2 microns. No osmotic effects 

 were observed with MgCl 2 solutions if the pore diameters ex- 

 ceeded 0.4 microns, negative osmose occurring with pore diame- 

 ters from 0.4 to 0.1 microns and positive osmose with pores less 

 than 0.1 micron in diameter. The fact that the same membrane 

 showed positive osmose with KC1 and negative osmose with LiCl 

 indicated a relation of ions to osmose, which will be considered 

 in the next section. 



Electric Polarisation of Membranes and Its Relation to Osmose 

 The electromotive force (emf) of a concentration cell is due 

 to the diffusion of ions, but since ions of the same kind diffuse 

 in all directions in water, at equal rates, one or two other con- 

 ditions must also be present. There must either be a membrane 

 that alters the diffusion rate in one direction or there must be 

 an unequal distribution of other ions. Although the molecules 

 of one non-electrolyte may not affect the diffusion of another, 

 ions exert forces on one another through their electric charges. 

 Take, for example, the forced diffusion of ions in a potential 

 gradient. If an electric current is passed through a dilute solu- 

 tion of HC1, the H ions will carry most of the current because 

 they move nearly five times as fast as the CI ions. Now add KC1 

 until its concentration is twenty-five times as great as the HC1. 

 The K ions move with the same speed as the CI ions, hence they 

 would carry only five times as much current as the H ions al- 

 though twenty-five times as numerous. In a pure HC1 solution 

 the H ions carry % of the current, whereas in the mixture they 

 carry less than 0.1, moving less than 0.1 as fast as in the pure 

 solution. In this way the addition of KC1 reduces the diffusion 

 potential that would be produced at the boundary between solu- 

 tions of HC1 of different concentrations, the more KC1 present 

 at the boundary, the greater the reduction. To take a simple 

 case, consider the following concentration cell: 



