CHAPTER II 



OSMOTIC PRESSURE MEASUREMENTS 



David R. BriGGS, University of Minnesota 



A. General 39 



1 . Definition of Osmotic Pressure 39 



2. Origin of Osmotic Pressure 40 



3. Importance of Osmotic Pressure Determinations 42 



B. Theoretical Considerations 45 



1 . In Dilute "Ideal" Solutions 46 



2. In Actual Solutions 49 



3 . Donnan Membrane Equilibria 52 



C. Methods for Measuring Osmotic Pressure 55 



1 . Indirect Methods 55 



Determination of Freezing Point Depression 56 



Vapor Pressure Methods 58 



2. Direct Methods 59 



Plasmolysis Method 59 



Osmometers 61 



References 64 



A. GENERAL 



1. Definition of Osmotic Pressure 



When two phases, one consisting of a pure solv^ent and the other 

 consisting of a solute dissolved in that solvent, are placed in contact 

 with the opposite sides of a semipermeable membrane, that is, a mem- 

 brane permeable to the molecules of the solvent but impermeable to 

 the solute molecules, it is observed that a movement of the solvent 

 (osmosis) occurs across the membrane. The direction of movement 

 (if the membrane plays no part other than that of a barrier to the 

 solute) is from the side of the pure solvent toward the side in contact 

 with the solution. The net movement of the solvent molecules can 

 be stopped (i.e., brought to an equilibrium state wherein the passage 

 of solvent molecules in each direction across the membrane is equal) 



39 



