ELECTROLYTES AND THEIR ACTION 



187 



agents. Means will be indicated later by which changes in acidity due to the 

 products of their activity may be neutralised, and their activity kept constant, in 

 so far as it is affected by this change of acidity, 



Even enzymes such as emulsin, which do not, like pepsin or trypsin, require fairly strong 

 acid or alkaline reaction, are greatly affected in their rate of action by changes such as are 

 brought about by the addition of blood serum. This is not generally recognised in testing for 

 the presence of " anti-enzymes," and has led to the belief in their existence when the result 

 obtained was due merely to reduction of H* ion concentration (see Bayliss, 1912, 2, pp. 

 460-462). 



The heart of the frog is affected by so small a change of H- ion concentration 

 as that from neutrality (10~~ 77 ) to one of 10~ 6 ' 5 , and is killed by one of 10~ 6 

 (Fig. 55). On the alkaline side, an H* ion concentration of 10~ 10 is fatal. The 

 addition of 0'036 mgm. of hydrochloric acid to 1 litre of distilled water 

 would raise its H* ion concentration from 10~~ " to lO" 6 . 



The respiratory centre is extremely sensitive to very minute changes in the 

 carbonic acid pressure of the blood, i.e., in all probability, to changes in H' ion 

 concentration from dissociation of H 2 CO 3 . 



FIG. 55. EFFECT OF ACID ON THE ISOLATED HEART OF THE FROG. 



A, Perfused with normal Ringer's solution, with H' ion concentration of 10-7-7. 



B, After perfusion for twenty minutes with faintly acid Ringer's solution, H' ion concentration, lO- 11 ' 8 . 



C, After perfusion of the acid solution for eighty minutes. The upper curve in each is that of the auricle. The 



lower one, that of the ventricle. The signal gives time in seconds. As shown by the time signal, the 

 rate of movement of the surface was quickened on two occasions in order to show details of the curves better. 



(Clark, 1913, 1.) 



These facts will suffice to show the importance of two things to which we 

 must devote some attention. The first is the means of exact measurement of the 

 H- ion concentration of a solution, the second is the capacity possessed by the 

 blood and the cells to neutralise even considerable addition of acids or alkalies, 

 in .order to maintain the state of nearly complete neutrality which is essential. 



If we were dealing with distilled water only, the addition of one-millionth of a gram- 

 molecule of hydrochloric acid to a litre would raise its H' ion concentration from lO" 7 ' 7 to 1Q- 8 , 

 that is more than ten times, a change that would be fatal to many delicate protoplasmic 

 processes. The mechanism which prevents such a result will be described later. 



Measurement of Hydrogen Ion Concentration. Very brief consideration will 

 suffice to show that, in the case of weak acids and bases, or even strong 

 ones in concentrated solutions, the ordinary methods of titration by 

 adding a standard solution of acid or base until a certain change in a 

 coloured indicator is produced, although giving valuable information as to the 

 total concentration of free acid or base (i.e., dissociated plus undissociated), 

 are not sufficient to afford the desired data as regards the H' ions of the 

 dissociated fraction. Different acids, as we have seen, vary considerably in 

 their degree of dissociation in equimolar solutions. This dissociated part is 



