296 PROTOPLASM 



dissociation in a normal solution of acetic acid is but 0.04). 

 When hydrochloric acid, on the other hand, dissociates, it does 

 so fully (so we now believe) into H+ and CI" ions; consequently, 

 all of its hydrogen contributes to the ionic acidity (and therefore 

 to the electrical conductivity, catalytic power, and poisonous 

 effect) of the solution. In other words, in hydrochloric acid, 

 all of the hydrogen is available as H+ ions, which make for 

 acidity; while in a solution of acetic acid, there are three kinds of 

 hydrogen present — the bound hydrogen of the CH3 radical, which 

 contributes nothing to the acid properties of the solution; the 

 hydrogen of undissociated COOH radicals, which is available for 

 exchange with the metal of a base and thus contributes to the 

 titratable acidity, or normality, of the solution; and the free 

 hydrogen ions from the dissociated carboxyl radicals, which alone 

 represent the hydrogen-ion concentration, i.e., the ionic, catalytic, 

 and physiological acidity. 



Living organisms are very sensitive to hydrogen ions. The 

 biologist is, therefore, interested in ionic acidity rather than 

 normality. The stimulus resulting from this interest has led to 

 a great advance in knowledge of the properties of acidity in 

 terms of hydrogen ions. This newer knowledge has come 

 primarily from the Dane S. P. L. S0rensen, whose work in 

 hydrogen-ion concentration was the first and most outstanding. 

 Important also are the contributions made in Germany by 

 Leonor Michaelis and in America by W. Mansfield Clark. 



Dissociation. — The Dutch physical chemist van't Hoff became, 

 as a result of his long and intimate friendship with his fellow 

 countryman, the botanist Hugo de Vries, interested in the osmotic 

 properties of solutions. This interest led to the discovery that 

 the pressure of a substance in solution is the same as it would be 

 if the substance were in a gaseous state occupying the same 

 volume as does the solution. Thus do the gas laws of Boyle, 

 Gay-Lussac, and Avogadro hold true for (dilute) solutions. 

 Van't Hoff found, however, that when the dissolved substance 

 is an electrolyte, e.g., a salt, the gas laws do not hold strictly. 

 Such solutions are osmotically more active than they should be; 

 that is to say, they exert more pressure than does the same 

 concentration of a nonelectrolyte {e.g., sugar), though both 

 contain the same number of molecules. Van't Hoff added a 

 factor i to his formula for the behavior of solutions to account for 



