Chapter IV 

 OSMOSIS AND OSMOTIC PRESSURE 



Introduction: — The Abbe Nollet (1748) is credited with performing 

 the first recorded experiments on the phenomenon of osmosis. Using an 

 animal bladder, he found that water would pass through but that alcohol 

 would not. Traube (1867) discovered the copper ferrocyanide membrane, 

 and Pfeffer (1877), by precipitating such a membrane in a porous pot, 

 was able to measure osmotic pressures up to several atmospheres. Exam- 

 ples of Pfeffer's results are given in Tables 5 and 6. 



Table 5. — Osmotic pressures of sucrose solutions at constant temperature : — 



Because the concentration of a solution in mols per liter equals the 

 reciprocal of the volume V in which one mol of solute is dissolved, it fol- 

 lows from Table 5 that PqV = a constant. This is analogous to Boyle's 

 law for gases. The proportionality shown in Table 6 between osmotic 



pressure and absolute temperature shows that a relation analogous to Gay 



p 

 Lussac's law for gases also applies to solutions ; that is -^ = a constant. 



By combining these equations one obtains the relation PqV = RT 

 where R is a proportionality constant. For dilute solutions van't Hoff 

 gave the equation 



P„V = nRT (i) 



where n = the mols of solute in V liters of solution. 



By a comparison of the osmotic pressure of a cane sugar solution with 

 gas pressure exerted by hydrogen gas at the same temperature and con- 

 centration van't Hoff showed that the constant R in the equation above 

 had the same value as the gas constant. 



Table 7 from van't Hoff (1888) shows the results of such a com- 

 parison. The data are osmotic pressures of a sucrose solution containing 

 1 gram in 100.6 cc. of solution compared with pressures of an ideal gas 

 at the same concentration. 



