1911.] The Properties of Colloidal Systems. 



249 



The Distribution of Salts between Congo-red Solutions and Water separated by 

 a Membrane. 



The electrical conditions within the membrane are undoubtedly the cause 

 of the peculiar distribution of a foreign salt between the dye inside and the 

 water outside. 



The salt used in my experiments was chiefly sodium chloride, on account 

 of the ease with which chlorine can be estimated by Volhard's method. In 

 order to do this in the presence of the dye, it was found sufficient to precipi- 

 tate the dye acid in a known volume by addition of excess of nitric acid and 

 filtering off the precipitate before the actual Volhard determination. Tests 

 were made in order to see whether any chloride was carried down with the 

 precipitate of the dye acid. This was found not to be the case. 



When sodium chloride is added to either the inner or outer solution in an 

 osmometer there is a considerable fall in osmotic pressure, if sufficient time 

 be allowed for equilibrium to be established. This fact was described in my 

 former paper. If the chlorine content of the two solutions, inside and 

 outside the membrane, be estimated at this time, it is invariably found that 

 the outer fluid, from which dye is absent, contains a larger percentage of 

 chloride than the inner dye solution. The difference is more marked the 

 greater the concentration of the dye salt in relation to that of the sodium 

 chloride. Table V gives a few of the measurements made. The numbers are 

 dilutions in litres. 



Table V. 



Dye. 



Chlorine. 



Inside. 



Outside. 



30 

 30 

 30 

 100 



52 

 465 

 <5500 

 32 -9 



30 



73 -6 

 180 

 29 -5 



I regret that my experiments are not sufficiently numerous to enable a 

 complete theory to be given of this partition, but there are two views which 

 must be referred to. 



Donnan* has investigated the question from the thermodynamic point of 

 view, and comes to the conclusion that, as far as the non-dissociated salt is 

 concerned, there must be equal distribution between the two sides of the 

 membrane. This seems quite what would be expected, since there are no 



* In the Press. Prelim. Communication to the Physiological Society, December, 1910. 



U 2 



