VELOCITY OF REACTION 63 



In the case of dilute acids, acting upon esters or upon di- 

 saccharides, the reaction is mono-molecular, and the curve show- 

 ing the ratio of percentage conversion to time of reaction is as 

 the formula indicates, a logarithmic curve. 1 



The velocity is proportional in each case to the dissociation 

 of the acid, that is to say, to the concentration of the hydrogen 

 ions, but the proportionality is by no means exact. . Thus a 

 0-5 normal hydrochloric acid inverts cane-sugar at 6'07 times the 

 rate of a Ol normal solution, although its concentration in hydrogen 

 ions is only 4-64 times as great. Hence the negative ions of the 

 acid must also possess a certain secondary action in increasing 

 the power of the hydrogen ions as catalysts, as suggested by 

 Arrhenius. That this is the cause of the discrepancy is further 

 shown by the action of the ions of the neutral salt when present 

 along with the acid, for although the neutral salt alone does not 

 catalyse, the presence of an equivalent amount of the neutral 

 potassium salt of the acid used increases the rate of catalysis by 

 the acid alone by about 10 per cent. 



An application of this inverting power of the acid or hydrogen 

 ion, as a test for free hydrogen ions or acidity, is of great import- 

 ance to the biological chemist. The method is of highest value 

 when dealing with a secretion of acid reaction, 2 where it is im- 

 portant to determine whether and in how far the reaction may be 

 due to the presence of a feebly dissociated acid, such as carbonic, 

 acetic, lactic, &c., or to a strongly dissociated acid such as 

 hydrochloric. 



Here the usual methods of titration of the acid entirely fail ; 

 for example, a deci-normal solution of acetic acid will give on 

 titration in presence of an indicator the same acidity as a deci- 

 normal solution of hydrochloric acid. But the real effective acidity 

 of the two solutions is entirely different, the acetic acid only 

 possessing about 3 per cent, of the hydrogen ion concentration 

 of the hydrochloric acid, and being in consequence for most 

 physiological purposes correspondingly weak in its action. 



1 See, however, Armstrong and Caldwell. These authors find with very dilute 

 acid that there is an initial period, in which the amount converted increases 

 more than proportionally to the logarithmic law, indicating an approach to a 

 linear law. 



2 A similar application can be made in the case of alkaline solutions, in de- 

 termining by inversion the concentration of the alkali, or hydroxyl ion. 



