160 



Chemistry. — "On a veto [winciple for the direct determination 

 0/ osmotic pressure'. By Prof. Eunst (buEN and G. de Bruin. 



Direct determination of osmotic pressure in very dilute solutions 

 i. e. solutions which obey van 't Hoff's law, are of great interest 

 both to physico-chemists and biologists. As such measurements however 

 are attended with great difficulties, until 

 now physical chemists have generally 

 employed indirect methods, such as 

 lowering of the freezing point, eleva- 

 tion of the boiling point etc. Consi- 

 dering that in a dilute solution in 

 water a lowering of the freezing point 

 of Viooo c>f ii degree corresponds to an 

 osmotic pressure of 124 mm. water, 

 the superior accuracy of the direct 

 determination becomes evident. We 

 describe here a principle which avoids 

 the above mentioned difficultiec. The 

 vessel B in the figure is to receive 



^1 -"^"^^ 





.*_>__ 



Cj! 



IB :! 



a dilute solution in water (for instance 



of cane-sugar). Its bottom is formed 



by a membrane which issemi-perme- ^ 



able with respect to this solution. 



Copper ferrocyanide may be used 

 for the purpose. The vessel A con- 

 tains water. The top of B is tightly 

 closed with a stopper through which is passed a fine capillary tube 

 and a wire which carries the half of a reversible electrode. A cop- 

 perplate will serve. The other part of the electrode is put into the 

 water in A. 



Tf B is new immersed in the vrater in A the liquid rises, as is 

 ^ji^ell known, in the capillary. When it has reached a certain height, 

 say Ai, which is noted on a millimetrescale, the rise of the liquid is 

 stopped by introducing between the electrodes C and D (at P^ 

 and Pi) a certain potential-difference (E^). According to the researches 

 of G. Wiedemann and those of von Helmholtz ^), we can write then: 



P—h^=zkE, ....... (1) 



where P is the osmotic pressure to be measured and ^ is a constant. 



1) Gomp. G. Wiedemann, Die Lehie von der Elektrizitat; 1, 993 seq. Braunschweig 

 1893 Ghwolson, Lelirbucli der Physik. 4, (2) 583 Braunschweig 1903. 



