158 PHYSIOLOGICAL CHEMISTRY 



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

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



2. Boas' Reagent. 1 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, 2 



(C6H 5 )NH-C 6 H4-N = N-C 6 H 4 -S0 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 

 true acidity of a solution, that is, its hydrogen ion concentration. In the 

 case of strong acids and bases very accurate results for the true acidity 

 may be obtained in this way. In the case of weak acids or bases the 

 titration values may give but slight information as to the true acidity. 

 Thus in the case of a slightly dissociated acid, such as acetic acid, as 

 fast as the acidity due to its dissociated hydrogen ions is neutralized 

 the undissociated acid ionizes further and the titration value finally 

 obtained represents the total acid present at the beginning both ionized 

 and unionized. Salts of strong acids and very weak bases and vice 



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

 100 c.c. of 50 per cent alcohol. 



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



