GASTRIC ANALYSIS 153 



according to this notation the H ion concentration would be expressed 

 as P H = y.o. The product of the hydrogen ion concentration (H + ) 

 by the hydroxyl ion concentration (OH~) is constant at about i X io~ 14 

 so that as (H + ) increases from iXio~ 7 (P H = 7-o) to iXio~ 4 

 (P H = 4.0) the (OH~) falls to iXio~ 10 , and vice versa. It must be 

 borne in mind that higher figures for the logarithmic notation indicate 

 lower figures for (H + ). The hydrogen ion concentrations at which 

 certain indicators commonly used in titration work change color, are 

 indicated below. 



True nature 



Indicator Hydrogen ion concentration ^herTthe 1 



color changes 



Phenolphthalein Between iXio~ 8 and iXio~ 9 Alkaline. 



Neutral red i X io~ 7 Neutral. 



Rosolic acid . i X io~ 7 . . . Neutral. 



Litmus Between i X io~ 6 and i X io~ 



Sodium alizarin sulphonate Between iXicr 5 and iXicr 



Congo red. . . Between i X io~ 5 and i X io~ 



Dimethyl-amino-azobenzene Between iXio~ 3 and 1X10" 



Methyl orange Between i X io~ 2 and i X icf 



Neutral. 



Acid. 



Acid. 



Acid. 



Acid. 



Tropaeolin OO i X io~ 2 Acid. 



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

 outlined in the following table, page 154. 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: Gunzberg's, Boas' and Tropaeolin (evaporation 

 test) are carried out differently as indicated on page 155. 



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 Tb'pfer'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 OO 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-KIO 3 to a relatively slight ex- 

 tent by other than free mineral acid. 



