108 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The methods which are always cited in this connection are three 

 in number — namely, the freezing-point, the boiling-point, and the vapor- 

 tension methods. It is known that the depression of the freezing-point 

 of an aqueous solution will give us approximately its osmotic pressure 

 within a very limited region of temperature which includes the freezing- 

 point itself. So much has been experimentally proved. It is probably 

 true also — though it has not yet been demonstrated by direct measure- 

 ments — that the osmotic pressure of an aqueous solution in the imme- 

 diate vicinity of its boiling-point can be derived from the elevation of 

 the boiling-point. But how about the osmotic pressures of the solution 

 throughout that relatively much larger temperature area, between the 

 freezing and boiling points, within which a variable hydration may exist, 

 or other molecular influences than those concerned in the formation 

 of hydrates may come into play and modify osmotic pressure? 



The osmotic pressures given for cane sugar in Table 9 have been 

 calculated from the depressions of the freezing-points of the solutions 

 (Table 8) , and they are there compared with the pressures which were 

 obtained by direct measurement. As seen in Table 9, the agreement is 

 satisfactory at all temperatures not exceeding 25° ; but the two sets of 

 values are thoroughly and increasingly discordant at all higher tempera- 

 tures. It appears, therefore, that the osmotic pressure of cane-sugar 

 solutions can be calculated up to 25° from the depressions of the freezing- 



