HYDROGEN-ION CONCENTRATION 23 



Actual Degree of Acidity or Alkalinity. According to the foregoing 

 method of titration a normal solution of a powerful mineral acid, such 

 as hydrochloric, is no stronger than a normal solution of a weak acid, 

 such as acetic or lactic. It requires no fewer c.c. of N alkali to neutralize 

 it. But the normal solution of the powerful acid tastes more acid, is 

 more toxic, dissolves metals more readily, and in all its other chemical 

 and physiological properties acts much more quickly than the weak acid, 

 so that the titrable, acidity or alkalinity can not express the real strength 

 of the acid or alkali, or the actual degree of acidity or alkalinity. It is 

 in this connection that the dissociation hypothesis aids us, for it suggests 

 that the degree to which the acid becomes dissociated into H- and the 

 remainder of the molecule will ' determine its real strength (see page 16). 

 The question is, how are we to measure the latter ? One action of H ions 

 which we may measure is that known as catalytic that is, the power to 

 accelerate reactions, such as the splitting of cane sugar (G^Ti^O^) into 

 glucose and levulose, which otherwise would proceed very slowly (see 

 page 75). If then the real strength of an acid depends on the degree 

 of dissociation which it undergoes, figures representing the catalytic 

 power should correspond with those representing the relative conductivi- 

 ties of the acids in equivalent concentration (see page 19). That this is 

 actually the case is shown in the following table, in which the above values 

 of various acids are given compared with HC1, which is taken as 100. 



ACID CATALYTIC POWER RELATIVE CONDUCTIVITY 



HC1 100 100 



Dichloracetic 27 25 



Monochloracetic 4.8 4.9 



Formic 1.5 1.7 



Acetic 0.40 0.42 



It will be evident that, if we could measure the concentration of free 

 H ions in a solution that is, of H ions that are not matched by OH ions 

 we should have a faithful index of its real acidity. This measurement 

 has been rendered possible by the application of two other physico- 

 chemical principles namely, those of mass action and electromotive 

 force. Since the object of this volume is to present the scientific basis 

 for the various methods that are used in modern medicine, it will be nec- 

 essary for us to review the main principles of these two actions. We shall 

 see that they apply, not only in the measurement of H-ion concentration, 

 but in many other physiological processes. 



Mass Action 



When materials take part in a reaction, some molecules are decom- 

 posing while others are being formed. After some time, however, a 



