120 



BIOLOGICAL CHEMISTRY 



TABLE XXII 



is apparently due to a positive ion formed by an amphoteric 

 electrolyte combined with an acid. The considerations on 

 the action of trypsin (p. 63) can be applied to the action of 

 pepsin, only we must substitute positive for negative ions and 

 bear in mind that increase in hydrogen ions increases the 

 activity instead of decreasing it. 



We can show in various ways that the gastric juice contains 

 free inorganic acid. By indicators or by hydrogen electrodes 

 the concentration of hydrogen ions can be shown to be greater 

 than would occur with organic acids. The amount of chlorine 

 is more than sufficient to combine with all the bases present, 

 so the excess must exist as hydrochloric acid. 



The optimum concentration of acid is about 0-2 per cent, of 

 hydrochloric acid. The gastric juice contains when secreted 

 about 0-5 per cent, hydrochloric acid, but some of this is 

 neutralised by the food and if the acidity is still too high 

 alkali passes into the stomach from the duodenum. The 

 concentration of hydrogen ions gives the true acidity and the 

 rate of digestion depends upon the concentration of hydrogen 

 ions. In pathological states the total amount of hydrochloric 

 acid is sometimes important as it indicates the amount of 

 acid secreted by the cells. The total hydrochloric acid can be 

 estimated by incinerating a sample of the gastric juice and 

 also another sample to which excess of alkali has been added. 

 The chlorides in the ash of each sample are estimated and the 



