GASTRIC DIGESTION. 123 



after testing for free hydrochloride acid neutralize the filtrate with 

 potassium hydroxide solution. If any of the acid metaprotein (acid 

 albuminate) is still untrans formed into proteoses it will precipitate 

 upon neutralization. If any precipitate forms heat the mixture 

 to boiling, and filter. If no precipitate forms proceed without fil- 

 tering. 



We now have a solution containing a mixture consisting princi- 

 pally of proteoses and peptones. Separate and identify the pro- 

 teoses and peptones according to the directions given on pages 114 

 and 115. 



Tests for Free and Combined HC1. 



These tests are made with a class of reagents known as indicators, 

 so-called because they serve to indicate the nature of the reaction of 

 a solution. These indicators are weak acids or bases and are but 

 slightly dissociable. The dissociation, with the formation of the 

 colored ion, forms the basis for the color reaction. 



Examine each of the following solutions by means of the tests 

 given below and report the results in a form similar to the chart 

 given on page 125: (i) 0.2 per cent free hydrochloric acid. (2) 

 0.05 per cent free hydrochloric acid. (3) o.oi per cent free hy- 

 drochloric acid. (4) 0.05 per cent combined hydrochloric acid. 

 (5) i per cent lactic acid. (6) Equal volumes of 0.2 per cent free 

 hydrochloric acid and i per cent lactic acid. (7) i per cent potas- 

 sium hydroxide. 



1. Dimethyl-amino-azobenzene (or Topfer's Reagent), 1 



N(CH 3 ) 2 -C 6 H 4 -N = N-C 6 H 5 . 



Place 1-2 drops of the reagent in the solution to be tested. Free 

 mineral acid (hydrochloric acid) is indicated by the production of a 

 pinkish-red color. If free acid is absent a yellow color ordinarily 

 results. 



2. Giinzberg's Reagent. 2 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 rod through the dried reagent. 

 Warm again gently and note the production of a purplish-red 

 color in the presence of free hydrochloric acid. 



1 To prepare Topfer's reagent dissolve 0.5 gram of di-methyl-amino-azobenzene 

 in 100 c.c. of 95 per cent alcohol. 



2 Giinzberg's reagent is prepared by dissolving 2 grams of phloroglucin and i 

 gram of vanillin in 100 c.c. of 95 per cent alcohol. 



