TOPFER'S METHOD OF GASTRIC ANALYSIS 227 



2. Free Acidity (Organic and Inorganic). Add 3 drops of 

 sodium alizarin sulphonate solution l to the contents of vessel B 

 and titrate with N/10 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 entirely replaced by a distinct violet color. 

 Take the burette reading and calculate the free acidity due to 

 organic and inorganic acids. 



Calculation. Since the indicator used reacts to both organic 

 and inorganic acids, the number of cubic centimeters of N/10 

 sodium hydroxide used indicates the free acidity of 10 c.c. of 

 gastric juice. The data for 100 c.c. of gastric juice may be calcu- 

 lated according to the directions given under Total Acidity, page 

 226. 



3. Free Hydrochloric Acid. 2 Add 4 drops of di-methyl- 

 aminoazobenzene (Topfer's reagent) solution 3 to the contents 

 of the vessel C and titrate with N/10 sodium hydroxide solution 

 until the initial red color is replaced by orange yellow. 41 



Calculation. The indicator used reacts only to free hydro- 

 chloric acid, hence the number of cubic centimeters of N/10 sodium 

 hydroxide used indicates the volume necessary to neutralize 

 the free hydrochloric acid 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 226. 



4. Combined Acidity. This value may be obtained by sub- 

 tracting the number of cubic centimeters of N/10 sodium hydroxide 

 used in neutralizing the contents of vessel B from the number of 

 cubic centimeters of N/10 sodium hydroxide used in neutralizing 

 A. The data for 100 c.c. of gastric juice may be calculated accord- 

 ing to directions given under Total Acidity, page 226. 



5. Acidity Due to Organic Acids and Acid "Salts. This value 

 may be conveniently calculated by subtracting the number of 

 cubic centimeters of N/10 sodium hydroxide used in neutralizing 

 the contents of vessel C from the number of cubic centimeters of 

 N/10 sodium hydroxide solution used in neutralizing the contents 



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



2 Hydrochloric acid not combined with protein material. 



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



4 If the orange yellow color appears as soon as the indicator is added it 

 denotes the absence of free acid. 



