GASTRIC JUICE AND CHANGES IN STOMACH. 165 



the change in the indicator salt alone. Assuming in solution a mixture 

 of free hydrochloric acid, protein and hydrochloric acid, and the red 

 methyl orange-hydrochloric acid salt, addition of weak sodium hydroxide 

 would produce a change in color immediately after the neutralization of 

 the last trace of free hydrochloric acid. Any excess of alkali added would 

 separate the protein-acid combination, but the protein would behave itself 

 as a base and furnish hydroxyl ions to decrease the dissociation of the 

 indicator and produce the characteristic yellow. Hence the " neutral " 

 point is reached with the disappearance of the actually uncombined HC1. 

 With a weak acid, like lactic acid, present in small amount the condition 

 would be practically the same. Such an acid is but slightly ionized and 

 not able to form stable salts with the indicator. 



In its behavior methyl orange, or the related bodies, is not quite as sen- 

 sitive as phenol-phthalein. In neutral solution it is slightly dissociated, 

 and shows therefore a mixed shade, due to the undissociated yellow and 

 the red basic ion. This mixed shade is a drawback in titration, as the 

 final change of color is obscured by it. In employing methyl orange it 

 is always best to add but a few drops of the weak solution. With such 

 a trace the excess of acid or alkali required to effect ionization or the 

 reverse is diminished. 



In this explanation of the action of indicators the so-called ionization 

 theory has been followed. It is proper to say that according to a later 

 theory which is finding many adherents, the color changes are not due to 

 any differences in color between unionized and ionized molecules, but to 

 the formation of different forms of the indicator substances. Phenol- 

 phthalein exists according to this notion in a colorless lactone form and 

 as a red carboxyl acid. The action of alkalies is to change the lactone 

 ring into a carboxylic acid group. All indicators must contain a so-called 

 chromophoric group on which the color reaction depends, and the changes 

 of one form into the other must be practically instantaneous. It will not 

 be necessary for our purpose to go into the details of this theory, although 

 it offers a very simple explanation of the various reactions involved. 



Illustration. Before taking up the actual titration of the stomach con- 

 tents a practical illustration of the steps may be found useful. To this 

 end a mixture of about 10 gm. of finely divided coagulated egg albumin 

 with 10 milligrams of powdered pepsin and 100 cc. of 0.4 per cent hydro- 

 chloric acid should be made up to 200 cc. with water. This will give an 

 acid strength at the very outset of 0.2 per cent. 



Immediately after diluting measure out three portions of 25 cc. each of 

 the thoroughly shaken mixture. Filter one portion (A) at once and wash 

 the residue on the filter with water several times, adding the washings 

 to the filtrate. Add a few drops of phenol-phthalein solution and titrate 

 this liquid with A7/IO NaOH, preferably after warming. Warm the second 

 portion (B) of 25 cc. and titrate with the alkali and phenol-phthalein 

 without filtering. In general the result here will be higher than in the 

 first case. It represents the total acidity and corresponds to one-eighth 

 of the acid taken. In the titration of A the result will be lower because 



