680 PHYSIOLOGICAL CHEMISTRY. 



solution used to restore the red color indicates the quantity of free 

 acid present. The calculation is made as above mentioned. 



g. Estimation of free hydrochloric acid. The use of dimethyl- 

 ami no-azobenzol as an indicator for free hydrochloric acid has been 

 mentioned above. For quantitative work 10 c.c. of gastric filtrate 

 are mixed with 5 drops of the dimethyl-amino-azobenzol solution, 

 and this mixture is titrated with ^ sodium hydroxide solution. The 

 disappearance of the reddish color indicates when the reaction is 

 completed. The difference between the estimation of total free acids 

 (/) and that of free hydrochloric acid (g) indicates the quantity of 

 organic acids present. 



h. Estimation of combined hydrochloric acid. To 10 c.c. of gas- 

 tric juice add 3 drops of a 1 per cent, alizarin solution and titrate 

 with YQ alkali until the solution assumes a clear violet color. The 

 acidity thus determined is due to free hydrochloric acid, acid salts, 

 and organic acids. The difference between the results of titration 

 with alizarin (A) and with phenol-phthalein (e) shows acidity due to 

 combined hydrochloric acid, while the difference between the titra- 

 tion with alizarin (h) and that with dimethyl-amino-azobenzol shows 

 acidity due to organic acids and acid salts. 



i. The total organic acids, free and combined, may be determined 

 by neutralizing 10 c.c. of gastric juice, using phenolphthalein as an 

 indicator, evaporating the neutral solution to dryness and incinerating 

 the residue. By this operation the organic acids are converted into 

 carbonates, which are titrated with ^ acid, and from the result the 

 quantity of organic acid is calculated, usually as lactic acid. 



j. Pepsin and pepsinogen. In case free acid is present, 10 c.c. of 

 gastric juice are placed in a beaker, and a small bit of dried fibrin 

 or a lamella of blood albumin (Merck), is added, and the beaker 

 placed in a thermostat at a constant temperature of 38 to 40 C. 

 (100 to 104 F.). Pepsin is indicated by the rapid solution of the 

 flake of albumin. If free hydrochloric acid is absent, the juice is 

 rendered acid with a drop of this acid and then tested in the manner 

 described. 



k. Rennin enzyme and rennin zymogen. Rennin is tested for as 

 follows : 10 c.c. of gastric juice are exactly neutralized with deci- 

 normal alkali and mixed with an equal volume of neutral unboiled 

 milk. The mixture is placed in a thermostat at 38 C. (100 F.). 

 If a casein coagulum is formed in ten to fifteen minutes, the coagula- 

 tion is due to the rennin enzyme. 



Rennin zymogen is detected thus: 10 c.c. of gastric juice are 



