CHAPTER III 



OSMOTIC PRESSURE 



There are some membranes that are freely permeable to water 

 but impermeable to many dissolved substances, and are called 

 semipermeable membranes. An example of such is found in one 

 of Traube's (1867) membranes. If a solution of potassium 

 ferrocyanide is allowed to come in contact with a solution of 

 CuS0 4 , a membrane of copper ferrocyanide is formed, which is 

 impermeable to potassium ferrocyanide, CuS0 4 and their ions 

 and to a certain degree to some other electrolytes such as MgS0 4 

 and its ions. If this membrane is made in the form of a bag 

 with CuS0 4 on the inside and placed in pure water, so much 

 pressure will be developed within it that it will burst. This 

 pressure is called the osmotic pressure of the CuS0 4 solution 

 and can be measured if the membrane is strong enough. Mem- 

 branes may be deposited in the pores of strong clay cups and 

 thickened by driving in the mother substances with an electric 

 current (H. N. Morse, 1914). If such a cup is connected with 

 a mercury or nitrogen manometer so that the osmotic pressure 

 can be measured it is called an osmometer. 



It was found by de Vries (1884), Pfeffer (1877) and others 

 that 1 gram molecule (mol) of a non-electrolyte, such as dex- 

 trose, dissolved in water to make 1 liter has an osmotic pressure 

 of 22.4 atmospheres at o°. De Vries found that electrolytes de- 

 parted from this rule, but Arrhenius (1887) showed that this 

 discrepancy could be explained by his theory of electrolytic dis- 

 sociation, since the ions exert osmotic pressure as well as the 

 molecules, and hence ionization would increase osmotic pressure 

 as the total osmotic pressure is the sum of the partial pressure 

 of molecules + ions. Van't Hoff (1887) pointed out that os- 

 motic pressure of ideal dilute solutions is the same quantitatively 

 as gas pressure. 



The formula for osmotic pressure of dilute ideal solutions is 

 pv = RT, where p is the pressure in atmospheres, v is the vol- 



