438 PHYSIOLOGICAL CHEMISTRY. 



organic acids, and acid salts, it is frequently expressed as hydrochloric 

 acid. As 1 c.c. of deci-normal alkali solution corresponds to 0.003637 

 gramme of HC1, the number of c.c. of alkali used multiplied by the 

 factor stated, gives the grammes of HC1 in the 10 c.c. of juice used. 

 Suppose 5.2 c.c. of alkali were required; this would correspond to 

 5.2 X 0.003637, equal to 0.0189 gramme of HC1 in 10 c.c., or to 

 0.189 per cent. 



/. Quantitative determination of free hydrochloric acid. There are 

 numerous methods for the determination of the free hydrochloric 

 acid of the gastric juice. The more important are as follows: 



Determination by means of congo-red. An aqueous solution of 

 congo-red has a bright-red color, which is changed to blue by free 

 acids and restored to red by alkalies. Acid salts, such as acid phos- 

 phates, have no effect on this indicator. If, therefore, a titration of 

 10 c.c. of filtered gastric juice, to which enough of congo-red solution 

 has been added to impart a distinct blue color, is made (as above 

 described for total acidity) then the number of c c. of deci-normal 

 potassium hydroxide solution used to restore the red color indicates 

 the quantity of free acid present. The calculation for hydrochloric 

 acid is made as above mentioned. This method gives not only the 

 quantity of free hydrochloric acid, but also of free organic acids. 

 However, the very small quantities of organic acids which are usually 

 present in the gastric filtrate after a trial breakfast do not materially 

 vitiate the results. If larger quantities of organic acids are present, 

 they must first be removed by shaking 10 c.c. of gastric juice with 

 100 c.c. of ether, in which these acids are soluble. The remaining 

 acidity is due to free hydrochloric acid. 



Determination by means of phloroglucin-vanillin. To 10 c.c. of 

 gastric filtrate, deci-normal potassium hydroxide solution is added 

 until no more free hydrochloric acid is indicated by testing a few 

 drops of the liquid with phloroglucin-vanillin. If, for instance, no 

 reaction occurs after having added 1.3 c.c. of potassium hydroxide 

 solution, while a positive reaction was yet obtained with 1.2 c.c. of 

 alkali solution, then we may say that 1.25 c.c. of deci-normal alkali 

 solution were required for neutralization of 10 c.c. of gastric juice, 

 or 12.5 c.c. alkali for 100 c.c. of juice. Multiplying 12.5 by 0.003637 

 (the factor for hydrochloric acid) we find 0.045 per cent, of hydro- 

 chloric acid as the result of the determination. 



In place of phloroglucin-vanillin, the resorcin-sugar reagent, 

 mentioned before, can be used with equal advantage as an indicator 

 in the above titration. 



