4 14 PHYSIOLOGICAL CHEMISTRY. 



In making the calculation note the number of cubic centimeters 

 of -JTF sodium hydroxide required to neutralize 10 c.c. of the gastric 

 juice and multiply it by iato obtain the number of cubic centimeters 

 necessary to neutralize 100 c.c. of the fluid. If it is desired to ex- 

 press the acidity of 100 c.c. of gastric juice in terms of hydro- 

 chloric acid, by weight, multiply the value just obtained by O.OO365. 1 



2. Combined Acidity. 2 Add 3 drops of sodium alizarin sul- 

 phonate solution 3 to the contents of vessel B and titrate with 

 Y^ sodium hydroxide solution until a violet color is produced. In 

 this titration the red color, which appears after the tinge of yellow 

 due to the addition of the indicator has disappeared, must be en- 

 tirely replaced by a distinct violet color. Take the burette reading 

 and calculate the combined acidity. 



Calculation. Since the indicator used reacts to all acidities except 

 combined acidity in order to determine the number of cubic centi- 

 meters of YTF sodium hydroxide necessary to neutralize the combined 

 acidity of 10 c.c. of the gastric juice, we must subtract the burette 

 reading just obtained from the burette reading obtained in the de- 

 termination of the total acidity. The data for 100 c.c. of gastric 

 juice may be calculated according to the directions given under 

 Total Acidity, page 413. 



3. Free Acidity. 4 Add 4 drops of di-methyl-amino-azobenzene 

 (Topfer's reagent) solution 5 to the contents of the vessel C and 

 titrate with -^ sodium hydroxide solution until the initial red color 

 is replaced by lemon yellow? Take the burette reading and calcu- 

 late the free acidity. 



Calculation. The indicator used reacts only to free acid, hence 

 the number of cubic centimeters of -^ sodium hydroxide used indi- 

 cates the volume necessary to neutralize the free acidity of 10 c.c. 

 of gastric juice. To determine the data for 100 c.c. of gastric 

 juice proceed according to the directions given under Total Acidity, 

 page 41 3. 



4. Acidity due to Organic Acids and Acid Salts. This value 

 may be conveniently calculated by subtracting the number of cubic 



1 One c.c. of ^ hydrochloric acid contains 0.00365 gram of hydrochloric acid. 



2 Hydrochloric acid combined with protein material. 



3 One gram of sodium alizarin sulphonate dissolved in 100 c.c. of water. 



4 Hydrochloric acid not combined with protein material. 



5 One-half gram dissolved in 100 c.c. of 95 per cent alcohol. 



6 If the lemon yellow color appears as soon as the indicator is added it denotes 

 the absence of free acid. 



