2 
Walter Stiles 
Determination of Osmotic Pressure 
Osmotic pressure is easily demonstrated by the familiar laboratory 
experiment in which a solution of sucrose is placed in an inverted 
thistle funnel, the mouth of which is closed by a parchment membrane 
and which is then placed in a vessel of pure water. The membrane 
being impermeable to sucrose, water passes through the membrane 
and consequently the level of liquid rises in the tube of the thistle 
funnel. This process continues until the pressure of the height of 
liquid in the funnel is equal to the osmotic pressure forcing water 
into the funnel. The principle involved in this experiment furnishes 
the basis for the direct method of measuring osmotic pressure. For 
a detailed description of this method and of the indirect methods of 
measuring the osmotic pressure of a solution, reference may be made 
to the monograph by Findlay already cited. Herg it will be sufficient 
to record the various methods which are employed. These are as 
follows: 
1. Direct Method. In the direct method of measuring osmotic 
pressure a semi-permeable precipitation membrane of copper ferro- 
cyanide is deposited in the wall of a porous pot into the mouth of 
which is sealed a manometer which serves for the determination of 
the pressure produced. Great difficulties are involved in this method, 
and the original apparatus devised by Pfeffer (1877) has since been 
greatly improved by later workers, especially by Morse and his co¬ 
workers (1901-1912) and by Berkeley and Hartley (1904, 1906 a). 
The indirect methods of determining osmotic pressure depend on 
the relations which exist between osmotic pressure and various 
physical properties. According to the particular property measured 
the indirect methods may be called the vapour pressure method, the 
freezing point method, the boiling point method, and the critical 
solution temperature method. 
2. Vapour Pressure Method. The vapour pressure of a liquid is 
lowered by the presence of a dissolved substance. A number of 
equations have been obtained by different investigators for the rela¬ 
tion between vapour pressure and osmotic pressure. The formula of 
Spens (1906) is probably an exact enough approximation for most 
purposes. This equation is 
p 
Pv 8 = sp log, p, 
where P is the osmotic pressure, p and p' are the vapour pressures 
