CHAPTER X. 



MANNITE. 



DETERMINATIONS OF OSMOTIC PRESSURE. 



According to the very careful determinations of Loomis,* the molec- 

 ular depression of the freezing points of the 0.1 to 0.5 weight-normal 

 solutions of mannite is normal, i. e., 1.85°. For this reason the deter- 

 mination of the osmotic pressure of the substance is of especial interest. 



It was found, as shown in Chapter V, that the osmotic pressure of 

 cane-sugar solutions, up to and including 25°, can be calculated from 

 the observed abnormal depressions of the freezing points, but not for 

 higher temperatures. At and below 25°, the ratio of osmotic to the 

 estimated gas pressure of the solute was the same for each concentra- 

 tion of solution as the ratio of the observed to the theoretical depression 

 of the freezing point. At temperatures above 25° the ratios of osmotic 

 to gas pressure — previously constant but abnormally high — began to 

 decline, and the osmotic pressures of the solutions could, of course, no 

 longer be correctly calculated from the depressions of the freezing 

 points. Stated in another way, the osmotic pressures of cane sugar 

 solutions between 0° and 25° are abnormal to the same degree as the 

 depressions of the freezing points, but not at any higher temperatures. 

 At some temperature above 25°, the ratio of osmotic to gas pressure 

 became unity and constant. The osmotic pressures of the solutions 

 could then, of course, be correctly derived, not from the observed, but 

 from the theoretical, depressions of the freezing points, i. e., from a 

 molecular depression of about 1.85°. 



In view of the relations between freezing points and osmotic pres- 

 sure, which were found to hold in the case of cane sugar, it was to be 

 presumed that the ratio of osmotic to gas pressure in the case of 

 mannite solutions would be found to be unity at all temperatures. 



Unfortunately the solubility of mannite in water is limited, the 0.5 

 weight-normal being the most concentrated solution whose pressures 

 can be measured at low temperatures. Otherwise, it is an excellent 

 substance with which to answer the question whether those compounds 

 which exhibit normal freezing-point depressions may also be expected 

 to exhibit normal osmotic pressures, i. e., pressures which conform to 

 the gas laws. It is readily obtained in sufficient quantity for an 

 extended investigation, and in sufficiently pure condition. 



*Zeitschrift fur physikalische Chemie, 32, 599. 



197 



