VELOCITY OF KEACTION 183 



The results of experiment may be stated either in the form of 

 a curve m which the co-ordinates represent time from the com- 

 mencement of the experiment and percentage of substance con- 

 verted, or by calculating the values of the constant of reaction k 

 at each period at which a determination is made. 



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

 rides, the reaction is mono-molecular, and the curve showing the 

 ratio of percentage conversion to time of reaction is, as the formula 

 indicates, a logarithmic curve. 



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 

 0-1 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 importance 

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

 dealing with a secretion of acid reaction, 1 where it is important 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, 

 etc., or to a strongly dissociated acid, such as hydrochloric. 



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

 for example, a decinormal 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 possess- 

 ing 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 A similar application can be made in the case of alkaline solutions, in 

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



