62 DAVID R. BRIGGS 



or organic solvents. From the standpoint of the time required to 

 make the measurements, it is advantageous to use a membrane with 

 the highest obtainable degree of porosity for the solvent, so long as it 

 is completely impermeable to the solute under consideration. 



The great profusion of osmometers described in the literature can 

 be classified broadly into two types. Those that measure, at equilib- 

 rium, the pressure difference developed across the membrane in a 

 liquid column or head resulting from an influx of solvent into the 

 solution are called static elevation osmometers. Those that measure 

 the instantaneous diffusion pressure of the solvent through the appli- 

 cation of an external pressure to the solution side (or a negative pres- 

 sure to the solvent side) of the membrane which is just sufficient to 

 prevent flow of solvent into the solution are called dynamic equilib- 

 rium osmometers. Further variations in the construction and mode 

 of operation of the osmometers employed by various investigators 

 are due primarily to (a) the shape of membrane employed, (b) the 

 magnitude of the pressures that they are designed to measure and the 

 devices used to measure the pressures, (c) the volumes of solution and 

 solvent required to fill them, and (d) the manner in which surface 

 tension effects are eliminated or corrected for. Arrangements for 

 stirring the solutions to hasten equilibrium and attachments de- 

 signed to minimize evaporation are often included in the design of 

 the osmometers, particularly in the static elevation type. 



The static elevation osmometer has the advantage of simplicity 

 of construction and operation but the disadvantage that the length 

 of time required for making a measurement is relatively great. 

 Since the pressure difference is developed by a movement of solvent 

 into the solution, the concentration of the latter will be changed dur- 

 ing the course of equilibration. This is usually minimized by start- 

 ing the determination with a pressure on the solution approximately 

 equal to the equilibrium pressure expected (determined usually by 

 a preliminary run) and by using a capillary tube in which the column 

 of liquid giving rise to the pressure is allowed to develop. Measurement 

 or calculation of the surface tension pull exerted on the liquid in the 

 capillary must be taken into account in obtaining the effective pres- 

 sure at the membrane. Allowance for density differences between 

 the solution and solvent must likewise be made. The use of a flat 

 membrane leads to a somewhat greater degree of accuracy in esti- 

 mating the active pressure than is the case with the saclike membrane, 

 especially where very small osmotic pressures are being measured. 



