. JOHN H. NORTHROP 597 



due to increased activity of the hydrogen ions by the salt. In any 

 case, it is not due to any peculiarity of the catalytic reaction since 

 the same discrepancy is found in comparing the hydrogen ion con- 

 centration as determined by the conductivity and e.m.f. methods. 

 The apparent discrepancy between the mass action law and the 

 kinetics of acid catalysis as outhned above is analogous to the case 

 in enzyme reactions where the rate is not proportional to the enzyme 

 concentration. 



Acid hydrolysis, moreover, also shows the same peculiarity in 

 regard to the sugar concentration, i.e. the rate does not increase 

 directly as the sugar concentration, as expressed in grams or molecules 

 per liter. In the case of acid hydrolysis the rate increases more 

 rapidly than the concentration. Arrhenius" has suggested that this 

 beha\^or is due to the fact that the active concentration of sugar 

 is not correctly expressed by the molecular concentration and has 

 shown that very much better results are obtained if the osmotic 

 pressure of the sugar solution is used as a measure of the active 

 concentration. He further assumes that the acid affects the equili- 

 brium between active and inactive sugar molecules and so accounts 

 for the "salt effect." The same mechanism is assumed to account 

 for the effect of temperature, which is much greater than that pre- 

 dicted by the kinetic theory. This hypothesis, of course, fits the 

 facts, but in the absence of independent evidence is really an assump- 

 tion of the law of mass action rather than a proof of the law. Several 

 authors have proposed explanations for catalytic reactions on the 

 same basis; i.e., that the catalyst changes the concentration of certain 

 molecules and so increases the speed of the reaction. Stieglitz^' 

 and his coworkers have been able to verify this hypothesis experi- 

 mentally in the case of the acid hydrolysis of imido esters. This 

 reaction shows the same pecuharities as that found in many enzyme 

 reactions; namely, the rate is not proportional to the total ester 

 concentration. Stieglitz was able to show, however, that the rate 

 was directly proportional to the concentration of ester ions. He 

 considers that the acid causes the formation of imido ester salts and 



^^ Arrhenius, S., Z. physik. Chetn., 1899, xxviii, 317. 



12 Stieglitz, J., and collaborators, Am. Chem. J., 1908, xxxix, 29, 164, 402, 437, 

 586, 719. 



