GASTRIC DIGESTION. 47 



Test each of the following solutions with all the indicators 

 mentioned below, and tabulate the results whether positive or 

 negative. 



(a) 0.3 per cent HC1; (6) 0.05 per cent HC1; (c) 0.8 per 

 cent lactic acid; (d) a mixture containing equal volumes of 

 "a" and "c"; (e) a mixture containing equal volumes of "b" 

 and a 2 per cent albumose solution. Before using, warm the 

 last mixture for a few minutes at 40 C. 



Indicators. 



An indicator is a substance which possesses a different 

 color when dissolved in an alkaline solution from what it 

 does in an acid one. They are slightly dissociable acids or 

 bases, and the change in color is due to the appearance or 

 disappearance of colored ions. 



1. Dimethylaminoazobenzene, N(CH 3 )2-C6H4-N=N-C 6 H5. 

 Add one or two drops directly to the solution to be tested. 

 Free mineral acid is indicated by a carmine-red color. 



2. TropceolinOO, NH(C 6 H 5 ) - C 6 H 4 - N = N - C 6 H 4 - 

 S0 3 Na. Add one or two drops directly to the solution to 

 be tested. Free acid is indicated by a red or reddish- violet 

 color. The reaction becomes more delicate when performed 

 in a similar manner to that suggested for Giinzburg's reagent. 



3. Congo-red. Use Congo-red paper, prepared by dipping 

 filter-paper into the alkaline indicator solution and drying. 

 Free acid is indicated by the blue color. 



4. Gunzburg's Reagent. Evaporate 2 drops of the solution 

 to be tested with 1 drop of the indicator, in a porcelain dish, 

 carefully, over a water-bath. Upon dryness the presence of 

 free HC1 is indicated by the development of a rose-red color. 



5. Boas' Reagent. Mix 3 drops of the solution to be tested 

 with the same amount of indicator and evaporate cautiously 



