36 MOLECULAR WEIGHT AND POLYMERISM 



B. Methods for Molecular Weight Determination 

 of Dissolved Bodies 1 . 



As in gases a measurement of molecular weight involves 

 a knowledge of the weight, volume, pressure, and tempera- 

 ture in the gaseous or vaporous condition, so in solutions 

 the same four data suffice, the osmotic pressure replacing 

 the ordinary pressure. 



From the theoretical point of view, the first thing to 

 consider is the osmotic pressure itself; but in practice 

 indirect methods involving quantities related to that 

 pressure occur ; we shall therefore discuss in turn the 

 direct and indirect methods of molecular weight deter- 

 mination for dissolved bodies. 



i. Direct Methods for Molecular Weight Deter- 

 mination of Dissolved Bodies. 



(a) Comparison of the osmotic pressure of different 

 solutions (Isotony). Whilst for gases the determination 

 of molecular weight is an easy operation, for solutions 

 a direct application of the corresponding laws meets with 

 the difficulty of measuring osmotic pressure a difficulty 

 due to the necessary use of a semipermeable membrane, 

 i. e. a wall or boundary permeable to the solvent but not 

 to the dissolved body. If the absolute value of the osmotic 

 pressure is to be measured, the ' membrane ' has, in addition, 

 to stand a mechanical pressure which may be great, which 

 makes the experiment still more difficult ; we shall, there- 

 fore, first describe the methods in which this difficulty, 

 due to pressure, is avoided by observing, instead of the 

 actual osmotic pressure of a solution, equality between 

 two solutions of equal osmotic pressure ; afterwards the 



1 Verschaffelt, Le poids mole'culaire de I'eau el de Tiode ; Acad. des Sciences de 

 Belgique. Bruxelles, 1896. Biltz. Praxis der Holekulargewichtsbestimmung, 

 Berlin, 1898. 



