156 PHYSIOLOGICAL CHEMISTRY 



Tests with Indicators. Prepare a series of solutions of varying acidities as 

 outlined in the following table, page 157. Introduce 5 or 10 c.c. portions of each 

 of these into a series of test-tubes and add to each a few drops of a solution of 

 Tropaeolin OO. Make a note of the colors produced, in the spaces left for this 

 purpose. In the same way test out the other indicators mentioned, in order, 

 using in each case a few drops of the indicator solution. The tests using the last 

 three mentioned indicators: Giinzberg's, Boas' and Tropaeolin (evaporation 

 test) are carried out diff erently as indicated below. 



Are the following assumptions, on which the use of certain of these 

 indicators in gastric analysis is based, borne out by your findings? 



1. That Topfer's reagent (Dimethyl-amino-azo-benzene) gives its 

 characteristic pinkish-red color only in the presence of free HC1. 



2. That a blue color with Congo red indicates free hydrochloric (or 

 other mineral acid), a violet color indicates an organic acid, and a brown 

 color indicates combined hydrochloric acid. 



3. That Tropaeolin 00 and methyl orange are indicators for free 

 mineral acid. 



4. That alizarin reacts to free mineral acid, organic acids and acid 

 salts but not to combined HC1. 



5. That phenolphthalein can be used in titrating total acidity, that 

 is, acidity due to mineral and organic acids, acid salts and combined 

 acid. 



6. That iodine is liberated from KI KIOs to a relatively slight ex- 

 tent by other than free mineral acid. 



7. That Giinzberg's test is the most satisfactory one for free HC1 

 and that Boas' reagent and Tropaeolin OO are also delicate reagents for 

 free mineral acid. 



Special Tests for Free HC1. Perform the following tests on the solutions as 

 outlined above and tabulate the results. 



i. Giinzberg's Reagent. 1 Place 1-2 drops of the reagent in a small porcelain 

 evaporating dish and carefully evaporate to dryness over a low flame. Insert 

 a glass stirring rod into the mixture to be tested and draw the moist end of the 



1 Giinzberg's reagent is prepared by dissolving 2 grams of phloroglucinol and i gram of 

 vanillin in 100 c.c. of QS per cent alcohol. 



