PHYSIOLOGICAL CHEMISTRY 377 



hydrogen ions, one or other of three general causes may be responsible, 

 viz. the presence of free mineral acid, free organic acid, and acid salt. 

 The acidity in each case is in direct proportion to the dissociation of 

 the acid in watery solution, being greatest for mineral acid. One of 

 the first questions, therefore, which confronts us in connection with the 

 acidity of gastric juice is : to which of the above causes is the presence 

 of hydrogen ions due 1 The question is most simply answered by the 

 use of indicators, for it has been found that the behaviour of these 

 varies with the nature and cause of the acidity. 



Let us see, first of all, whether the acidity is due to a free acid or to 

 an acid salt. Congo red is the most useful indicator for this purpose. 



EXPERIMENT I. To a 0-2 per cent. HC1 solution add a few drops of 

 congo red solution: 1 the red turns to blue. Repeat with a dilute 

 solution of acid sodium (NaH 2 P0 4 ) phosphate no blue colour develops. 

 Show that the latter solution reacts acid towards litmus or phenol- 

 phthalein. Repeat this experiment, using, instead of a congo red 

 solution, pieces of congo red paper prepared by dipping filter paper 

 in a congo red solution and drying. 



The result with congo red indicates that the acidity is due to free 

 acid, but it does not tell us whether this is a mineral or an organic 

 acid, for both of these turn it blue, although the mineral acid does 

 so much more readily, i.e. in much greater dilution than the organic. 

 (Indeed, if a solution of the acidity of gastric juice as determined 

 by the method described on p. 381 gives the reaction with congo 

 red distinctly, especially when congo red papers are used, it is almost 

 certain evidence that free mineral acid, and not organic acid, is the cause 

 of the acidity.) 



To further trace the cause of the acidity, use is made of several 

 indicators whose behaviour towards dilute organic and combined 

 mineral acids is quite different from that occurring in the presence of free 

 mineral acid. The most important of these indicators are employed in 

 the following experiments which should be performed with 0'2 per cent. 



N 



hydrochloric acid solution, -r^ hydrochloric acid solution (0'0365 per 



N N 



cent.), Y^T lactic acid solution (O9 per cent.) and -^r lactic acid solution 



(0-09 per cent.). 



EXPERIMENT II. Giinzberg's Test. Place a few drops of the reagent 

 (a solution of 2 parts phloroglucin and 1 part vanillin in 30 parts 

 95 per cent, alcohol) in an evaporating basin, and add a few drops of 



1 Congo red solution dissolve 0'5 gm. of congo red in 100 c.c. of 10 per cent, 

 alcohol. 



