Chapter V — 55 — Mechanism of Osmosis 



namely, a picture of the underlying principles of osmosis upon which the mechanism 

 is based. 



Evaluation of Diffusion Pressure: — If, in considering an osmotic 

 system, one attempts to assign values to the diffusion pressures of both 

 solute and solvent, the latter presents difficulties for in dilute solutions the 

 value approaches infinity. However, in practical problems only differences 

 in diffusion pressure are of significance. Therefore, for convenience in 

 handling such problems, it may be desirable to assign the pure solvent in 

 some standard state an arbitrary value which may be used as a base line 

 for further calculations. We follow Meyer, who states (1945, footnote, 

 page 151) "The diffusion pressure of pure water at atmospheric pressure 

 and at the same temperature as the water, the diffusion pressure of which 

 is to be designated, is conveniently taken as the zero point on the diffusion 

 pressure scale." Using this convention, with a solution having an activity 

 of 1 mol per liter, when TP = 0, OP = DPD = 22.4 atmospheres (neg- 

 lecting dissociation, hydration, association, etc.). 



If solutes in the liquid state followed the gas law one could readily 

 calculate an ideal diffusion pressure for any substance by the formula 



-j^ • 22.4, in which d =: density and M = molecular weight. This would 

 give water a diffusion pressure of 1243.2 atm. at 0° C, the value for ethyl 

 alcohol would be 383.7 atm., glycerine 306.5 atm., sucrose 103.9 atm. The 

 actual deviation from such a law is shown in Figure 16, where calculated 

 and observed values for osmotic pressure are plotted against concentra- 

 tions. Curve C represents the ideal values obtained by equating OP to 



-j^j^ • 22.4 for sucrose. This follows the van't Iloff law. Curve B shows 



the values calculated according to Morse, and curve A the observed values 

 of Frazer and Myrick given in Table 8. 



If water at atmospheric pressure is assigned a diffusion pressure value 

 of zero and water in all aqueous solutions at that pressure negative values 

 (DPD's), solutes may be assigned zero values at zero or any other specified 

 concentration and positive values (TP's) at all higher concentrations. By 

 this convention, solutions may be assigned osmotic pressure values in the 

 common units of pressure. It should be realized, however, that both of 

 these conventions are arbitrary and somewhat illogical for they fail to take 

 into consideration the numerical relations between solute and solvent 

 molecules that have been shown to bear some relationship to their relative 

 diffusion pressures. 



The Diffusion Pressure of the Solvent: — The concept of the diffu- 

 sion pressure of the solvent, i.e., water, is often difficult for the student to 

 grasp. While a solution in a beaker shows no evidence of its potential 

 capacity for work, if placed in an osmometer in contact across the semi- 

 permeable membrane with pure solvent it will Hft a piston. The pure sol- 

 vent in the beaker exhibits no such capacity. 



Referring back to Figure 14, if the solution S is removed and the mem- 

 brane is placed in contact with a very large reservoir of dry air, water will 

 evaporate from the surface of the membrane, the pressure in W will lower 

 and Pi will move to the right if left free. If water is allowed to continue 

 evaporating and the piston Pi is held in a fixed position the pressure in W 

 will lower until a tension of many atmospheres is developed providing the 

 water and apparatus contain no bubbles or unwet surface upon which a 

 vapor phase may be initiated. In fact the cohesion of water, calculated to 



