n8 



PHYSIOLOGY OF NUTRITION 



Concentration of Cane Sugar 



Osmotic Value 



Observed 



Calculated 



It is different with electrolytes; from the table given on page 117 it is clear 

 that, of the crystalloids, isosmotic solutions of electrolytes (metallic salts) and 

 non-electrolytes are not equimolecular, the molecular concentrations of the former 

 being much lower. Furthermore, there is no constant relation between the 

 isosmotic concentrations of solutions of electrolytes on the one hand and of non- 

 electrolytes on the other, so that electrolytes do not agree with the gas-pressure 

 theory of osmotic pressure. For example, a 0.1-volume-molecular solution 

 of KNO3 ought, according to this theory, to give a pressure of 0.235 atmos- 

 pheres, but it actually gives one of 0.352 atmospheres. If the value derived 

 directly from the van't Ffoff theory be multiplied by %, the isosmotic coeffi- 

 cient of this salt (considering the coefficient of cane sugar as unity), the value 

 0.352 is obtained, which is the same as that found experimentally. Equimole- 

 cular solutions of potassium nitrate and of organic substances are thus not 

 isosmotic. To obtain a solution of potassium nitrate that shall produce the 

 same osmotic pressure as does a 0.1-molecular cane-sugar solution it is necessary 

 to prepare a Ms (% X Mo) molecular solution of the salt. Salts with other 

 isosmotic coefficients must be employed in corresponding concentrations. Thus, 

 a 0.05-molecular solution of potassium sulphate is isosmotic with a 0.1-molecular 

 solution of cane sugar. The osmotic pressure of a weak solution of an electrolyte 

 is thus equal to the theoretical pressure multiplied by the isosmotic coefficient 

 of the electrolyte in question. This departure from the theory is explained by 

 Arrhenius' hypothesis, which supposes that electrolytes in solution dissociate 

 into ions. In a sodium chloride solution, for example, sodium and chlorine ions 

 are both present as well as molecules of sodium chloride. The more dilute the 

 solution, the greater is the degree of dissociation. 



According to the Arrhenius theory of electrolytic dissociation, the isosmotic 

 coefficient of potassium nitrate indicates that the number of particles in a solu- 

 tion of this salt is increased by dissociation, and if half of the molecules be con- 

 sidered as dissociated the total number of particles ought to be % of what it 

 would be without dissociation, and the osmotic pressure should be correspond- 

 ingly increased. A dissociated molecule of KNO ;i , in the form of two ions, K and 

 NO3, produces twice as much osmotic pressure as does an undissociated molecule. 



