DIGESTION. 679 



amethyst-blue color. To 2 c.c. of this solution an equal volume of 

 gastric juice is added. In the presence of at least 0.01 per cent, of 

 lactic acid the liquid assumes a pure yellow color. As the presence 

 of too much hydrochloric acid (or even of some other substances) 

 prevents the change, it is well to shake (in doubtful cases) 10 c.c. 

 of juice with 50 c.c. of ether, evaporating the ethereal solution 

 to dryness, dissolving the residue in a few drops of water, and 

 adding to this solution, which contains the lactic acid, the above 

 reagent. 



Butyric acid changes Uffelmann's reagent to brownish yellow. 

 Butyric and acetic acids may be recognized by their odor. 



It has been mentioned that the total acidity of gastric juice is due to acid 

 salts, organic acids, free and combined hydrochloric acid. Clinically it is 

 sometimes necessary to estimate the acidity due to each. This is done by the 

 following method. 



e. Total acidity. This is best determined by titration with an 

 alkali ; the estimation is conducted as follows : To 10 c.c. of the 

 filtered liquid a few drops of phenolphthalein solution are added, and 

 to the mixture deci-normal potassium hydroxide solution is slowly 

 added from a burette until the liquid assumes a slight reddish tint, 

 which does not disappear on stirring. 



It is customary to express the acidity in percentages, according to 

 the quantity of deci-normal potassium hydroxide used. Thus, 52 

 per cent, acidity would indicate that every 100 c.c. of gastric fil- 

 trate are exactly neutralized by 52 c.c. of deci-normal potassium 

 hydroxide. 



Though the total acidity is due to a mixture of free and combined 

 hydrochloric acid, organic acids, and acid salts, it is frequently 

 expressed as hydrochloric acid. As 1 c.c. of deci-normal alkali 

 solution corresponds to 0.003618 gramme of HC1, the number of 

 c.c. of alkali used multiplied by the factor stated, gives the grammes 

 of HC1 in the 10 c.c. of juice used. Suppose 5.2 c.c. of alkali were 

 required; this would correspond to 5.2 X 0.003618, equal to 0.0188 

 gramme of HC1 in 10 c.c., or to 0.188 per cent. 



/. Estimation of free acids. Both free hydrochloric and organic 

 acids change the bright-red color of congo-red to blue, while alkalies 

 restore it to red. Acid salts, such as acid phosphates, have no effect 

 on this indicator. If, therefore, a titration of 10 c.c. of filtered gas- 

 tric juice, to which enough of congo-red solution has been added to 

 impart a distinct blue color, is made (as above described for total 

 acidity), then the number of c.c. of deci-normal potassium hydroxide 



