GASTRIC ANALYSIS 155 



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

 and that Boas' reagent and Tropaeolin 00 are also delicate reagents for 

 free mineral acid. 



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

 outlined on page 154 and tabulate the results. 



1. 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 

 rod through the dried reagent. Warm again gently and note the production of a 

 purplish-red color hi the presence of free hydrochloric acid. 



2. Boas' Reagent. 2 Perform this test in the same manner as i, above. 

 Free hydrochloric acid is indicated by the production of a rose-red color which 

 becomes less pronounced on cooling. 



3. Tropaeolin OO, 3 



NH (C 6 H 5 ) -C 6 H 4 -N = N-C 6 H 4 -SO 3 Na. 



Place 2 drops of the solution to be tested and i drop of the indicator in an evapo- 

 rating dish and evaporate to dryness over a low flame. The formation of a red- 

 dish-violet color indicates free hydrochloric acid. 



This test may also be conducted in the same manner as i, above. 



HYDROGEN ION CONCENTRATION AND TITRATABLE ACIDITY 



The acidity of a solution may be determined in two different ways 

 by means of indicators. One method is by titration with standard 

 alkali using the indicator to determine the end point of the titration. 

 For this purpose the indicator should be one which gives a sharp color 

 change which is sensitive to the form of acidity which is to be deter- 

 mined, and which is not destroyed by any substance contained in the 

 titration mixture. Thus phenolphthalein can be used for the titration 

 of strong bases and nearly all weak acids, but cannot be used for weak 

 bases, and is unsatisfactory in the presence of ammonium salts. Methyl 

 orange on the other hand is useful for strong acids and weak bases such 

 as ammonia and for the soluble carbonates but cannot be used for weak 

 acids such as carbonic acid or the organic acids. Almost any indicator 

 may be used in the titration of mineral acids against strong bases such 

 as KOH inasmuch as under these conditions i drop of the standard 

 solution will throw the hydrogen ion concentration so far beyond that of 

 neutrality that the turning point of any common indicator will be 

 passed. 



Titration does not, however, enable us to determine in all cases the 



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

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



2 Boas' reagent is prepared by dissolving 5 grams of resorcinol and 3 grams of sucrose in 

 100 c.c. of 50 per cent alcohol. 



3 Prepared by dissolving 0.05 gram of tropaeolin OO in 100 c.c. of 50 per cent alcohol. 



