CHAPTER VI 



MEASUREMENT AND CALCULATION OF OSMOTIC 

 PRESSURE 



I. MEASUEEMENT OF OSMOTIO PEESSUEE 



a) Direct method. — Tha direct method of measurement 

 of osmotic pressure is very difficult of operation, and deter- 

 minations thus made are exceedingly tedious processes. 

 Nor is this method susceptible of sufficient accuracy to 

 recommend it to physiologists. But since it is the classical 

 method used by Pfeffer' in his original investigation of 

 the subject, and since it has been used since that time by 

 physical chemists in establishing the principles by which 

 indirect methods become available, it will be described here 

 at some length. 



A membrane of copper ferrocyanid is precipitated within 

 the walls of a cup or bulb of porous clay (a filter bulb 

 serves admirably) by filling the bulb with a solution of po- 

 tassium ferrocyanid and surrounding it with one of copper 

 sulphate. The bulb should first be thoroughly cleaned and 

 freed from air by boiling for some time in water. When 

 the membrane is well formed (which occurs after fifteen to 

 forty hours), the cup is filled with the solution to be tested, 

 closed with a rubber stopper bearing a mercury manometer, 

 and immersed in water. The osmotic pressure rises for a 

 number of hours, being indicated by the rise in the mercury 

 column, and at last, when the membrane has been ruptured 

 somewhere, begins to descend again. The maximum read- 

 ing of the mercury column is taken as the osmotic pressure 

 of the solution. The difficulty of the method lies iu getting 



1 w. Pfefpee, Osmofische tjntersuchungen, Leipzig, 1877, 



35 



