40 DAVID R. BRIGGS 



by the application of mechanical pressure to the solution side of the 

 membrane. This mechanical pressure, which will be exactly equal 

 to and opposite in sign to the difference in the diffusion pressure, or 

 escaping tendency, of the solvent in the two phases, is called the os- 

 motic pressure of the solution. It is a measure of the extent to which 

 the thermodynamic activity of the solvent molecules has been reduced 

 by the presence of the solute molecules in the solution. The osmotic 

 pressure of a solution may be defined as the mechanical pressure that 

 must be applied to the solution in order to bring the solvent in the 

 solution to the same escaping tendency, chemical potential, or partial 

 molar free energy value characteristic of the pure solvent at the same 

 temperature. 



The osmotic pressuie of a solution is one of the so-called colliga- 

 tive properties of the solution, others being the lowering of the freez- 

 ing point, the elevation of the boiling point, and the lowering of the 

 vapor pressure. These properties are all reflections of the lowering 

 of the thermotlynamic activity of the solvent b}" the solute and are, 

 in dilute ideal solutions, proportional in degree to the number of par- 

 ticles, ions, molecules, etc. of solute present per unit number of moles 

 of solution. When the solute concentration is sufficientlj^ dilute and 

 the entire effect of the solute upon the activity of solvent is due to an 

 ideal entropy of mixing, and if the effect upon any one of the colhga- 

 tive properties can be measured for a solution of known weight com- 

 position, the molecular weight of the solvent being known, the num- 

 ber average molecular or particle weight of the solute can be deter- 

 mined. In any case in which the effect upon any one of the colliga- 

 tive properties of the solution can be measured, the corresponding 

 effect upon any other colligative property can be calculated. 



2. Origin of Osmotic Pressure 



Many attempts have been made to arrive at a kinetic explanation 

 of osmotic pressure and the other colligative properties of solutions, 

 but none has proved entirely satisfactory in a quantitative sense. 

 Van't Hoff observed the analogy between the laws describing osmotic 

 pressure versus solute concentration and those describing gas pressure 

 versus gas concentration and conceived the idea that osmotic pressure 

 results from a bombardment of the membrane by the solute molecules 

 in a fashion similar to the pressure of a gas arising from the kinetic 

 bombardment of the walls of the containing vessel by the gas mole- 



