GASTRIC DIGESTION 265 



If it is desired to know the hydrogen-ion concentration of 

 gastric contents, the determination may be made by electrolytic 

 methods, or approximately by the use of certain series of 

 indicators. 



For most clinical work the only data required are total 

 acidity as determined by titration, using phenolphthalein as. 

 indicator, and free hydrochloric acid as determined by titration 

 using Toepfer's reagent as indicator or as determined with 

 Guenzberg's reagent. 



1. Study the following indicators, by observing their color 

 in faintly acid and faintly alkaline solution. 



(a) Congo red gives a blue color with free HC1, violet with 

 an organic acid and brown with combined HC1. Add a few 

 drops of congo red to 0.2% HC1 and to 0.5% acetic acid. Neu- 

 tralize one with NaOH. Add 2-3 drops of congo to a few c.c. 

 of your hydrochloric acid extract of pig's stomach. 



(b) Guenzb erg's reagent (2 g. phloroglucin, 1 g. vanillin, 

 100 c.c. alcohol) produces a purplish red color on evaporating 

 with free HC1. Place 2 drops, of the reagent in an evaporating 

 dish. Evaporate to dryness on the water bath. A yellow spot 

 results. Add 2 drops of 0.2% HC1 and replace on the water 

 bath. Observe the red spot. Repeat with 2 drops of your HC1 

 extract of stomach mucosa. Repeat with 2 drops of lactic acid. 

 Guenzberg's test may be made roughly quantitative by dilut- 

 ing the acid solution until it will just respond with a faint 

 positive reaction. At this dilution the HC1 is about 1/2500 

 normal. 



(c) Uffelmann's Test for Lactic Acid. In each of 3 test tubes 

 place 5 c.c. of 1% phenol and two or three drops of ferric 

 chloride solution. The resulting amethyst liquid is Uffelmann's 

 reagent. To one tube add 2 c.c. dilute lactic acid. To the second 

 2 c.c. 0.2% HC1 and to the third 2 c.c. acetic. 



(d) To a small amount of 0.2% HC1 add Toepfer's reagent 

 (dimethyl-amino-azobenzene). Make slightly alkaline and observe 

 the change in color. 



2. Determination of Acidity of Gastric Contents. This may 



