Chapter V 



49 — 



Mechanism of Osmosis 



In the following treatment the osmotic system will be considered as 

 operating at atmospheric pressure. If a solution is placed in an osmometer 

 surrounded by pure solvent an unbalance in forces is apparent. The dif- 

 fusion pressure of the solvent on the outside is greater than that on the 

 inside and solvent molecules diffuse inward. This is osmosis. 



As osmosis continues and the mass of liquid inside the osmometer 

 becomes greater, if the osmometer is closed and rigid, pressure is built up 

 on the inside. This pressure, forcing all molecules of the solution closer 

 together, increases the diffusion pressure of both solute and solvent in the 

 osmometer. When the solvent molecules inside the osmometer have in- 

 creased in diffusion pressure until they are in pressure equilibrium with 

 those on the outside, the system is in osmotic equilibrium and the difference 

 in pressure betzveen the liquid inside and the pure solvent outside is com- 

 monly termed the osmotic pressure of the solution. Other terms that have 

 been applied to this equilibrium pressure are hydrostatic pressure and turgor 

 pressure. It should be noted that this is equal to the osmotic pressure of 

 the solution as it exists at the equilibrium concentration, and not that of 

 the original solution. To measure the osmotic pressure of the original 

 solution it would be necessary to impose pressure upon it so that no solvent 

 enters the osmometer. 



Returning to the initial effect of adding a solute to a solvent, many writ- 

 ers have emphasized the resulting reduction in diffusion pressure of the 

 solvent molecules. Haldane (1918) by a rigorous mathematical analysis 

 has shown that the reduction in diffusion pressure of solvent molecules 

 brought about by the addition of the solute is mathematically equal to the 

 osmotic pressure of the solution as defined above. Obviously osmotic pres- 

 sure has been defined in two distinct ways. Can they be reconciled ? 



R 



w 



\ ■•'■ 



M , 



Pz 



Fig. 14. — Apparatus useful in analysing problems of osmosis. M repre- 

 sents a differentially permeable membrane separating a cylinder into two 

 chambers W and S containing pure water and sugar solution, respectively. 

 Pi and P2 are frictionless pistons. 



In Figure 14, M represents a differentially permeable membrane sepa- 

 rating a cylinder into two cells W and S containing pure water and sugar 

 solution respectively. If these two cells are closed by pistons Pi and P2, 

 an osmotic system is provided that may be used in analysing the problem 

 stated above. Starting with the fluids W and S both at atmospheric pres- 

 sure and the same temperature, one can see that a difference in the diffusion 

 pressure across the membrane M exists, water in the solution S having 

 the lower diffusion pressure. 



Obviously, too, the diffusion pressure of water across the membrane 

 can be equalized in two ways : 



1.) If pressure is applied to S by means of P2, W remaining at atmos- 

 pheric pressure, the diffusion pressure of the water molecules in S may be 

 raised to equal that of W — namely, 1 atmosphere. And the hydrostatic 

 pressure (above 1 atmosphere) in S at this state (water equilibrium) 



