678 PHYSIOLOGICAL CHEMISTRY. 



is placed upon the paper, the presence of free acids is indicated by 

 the change of color from red to blue ; if the blue color is intense, 

 free hydrochloric acid is present. (Neither combined hydrochloric 

 acid nor acid salts, such as acid phosphates, act on congo-red.) 



c. Free hydrochloric acid. There are a number of reagents for the 

 detection of free hydrochloric acid. The more important of these 

 are : tropaeolin 00, phloroglucin-vanillin, resorcin, and dimethyl- 

 amino-azobenzol. 



Tropceolin 00. Dissolved in water, the 1 per cent, brownish -yel- 

 low solution of tropaeolin 00 (diphenylamine-orange) is changed to a 

 brown-red or deep-red color upon the addition of juice containing 

 free hydrochloric acid. Upon gentle evaporation and heating a lilac 

 color is produced. The same reaction may be made with filter-paper, 

 soaked for some time in an alcoholic solution of the reagent, allowed 

 to dry, and used as test-paper. Hydrochloric acid turns this paper 

 brown, and upon heating the brown color changes to blue. (The 

 paper does not keep unchanged over a month.) 



Phloroglucin-vanillin. This reagent is made by dissolving 2 parts 

 of phloroglucin and 1 part of vanillin in 30 parts of alcohol. It is 

 a very sensitive and reliable agent for the detection of free hydro- 

 chloric acid. ^Five drops of the solution mixed with an equal quan- 

 tity of gastric filtrate are gently heated over a Bunsen flame. On 

 complete evaporation a distinct red color or tinge appears in the 

 presence of not less than 0.01 per cent, of hydrochloric acid. The 

 formation of cherry-red crystals indicates the presence of large quan- 

 tities of the acid. Organic acids have no action on this reagent. 



Resorcin. This reagent is equally as sensitive as, and more stable 

 than, phloroglucin-vanillin. The solution is obtained by dissolving 

 5 parts of resublimed resorcin and 3 parts of cane-sugar in 100 parts 

 of dilute alcohol. The manner of testing with this reagent is the 

 same as described above for phloroglucin-vanillin ; a bright-red tinge 

 or color appears, even when very small quantities of free hydrochloric 

 acid are present. 



Dimeihyl-amido-azobeMzol. A 0.5 per cent, solution of this substance 

 in alcohol is mixed with a few drops of the stomach contents, and in 

 the presence of as little as 0.002 percent, of free hydrochloric acid a 

 cherry-red color develops. 



d. Lactic acid. (Use solution C.) Uffelmann's reagent answers 

 best for detecting this acid. It is made by adding 1 or 2 drops of 

 ferric chloride solution to 10 c.c. of a 1 per cent, carbolic acid solu- 

 tion, and diluting this solution with water until it assumes an 



