ELEMENTARY CHEMICAL PHYSIOLOGY 229 



colour develops. Show that the latter solution reacts acid towards 

 litmus or phenolphthalein. 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, and that it is almost certainly mineral acid as organic 

 acids, except in very strong solution, do not affect congo red. 



To trace further 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, hydrochloric acid solution, hydrochloric acid 



100 



N 

 solution (0-0365 per cent.), -- lactic acid solution (0-9 per cent.) and 



N 



lactic acid solution (0-09 per cent.). 



EXPERIMENT II. Gunzberg's Te^. Place a few drops of the re- 

 agent (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 the liquid to be tested. Slowly evaporate to dryness. 

 With dilute hydrochloric acid a red colour develops, with lactic 

 acid no colour. 



EXPERIMENT III. Topfer's Test. Add 1-2 drops of the di- 

 methylaminoazo-benzol reagent 1 to some of the solution to be 

 tested. If this contain free mineral acid a pinkish red colour 

 develops. Organic acids, even when quite dilute, will also give a 

 faint red colour with this reagent. 



It will be found, as a result of these experiments, that the reactions 

 obtained with the hydrochloric acid solutions resemble those of the 

 stronger lactic acid solution, except in the case of Giinzberg's 

 reaction. This reagent gives a positive result with hydrochloric 

 acid diluted to 1 in 10,000 parts. The Topfer reaction with 0-2 

 HC1 is also quite distinguishable from those given by lactic acid 

 solutions of the above strengths, but in greater dilutions of HC1 

 the distinction is by no means so definite. 



If the contents of the stomach (removed by a stomach tube, 

 Einhorn method or through a fistula) be tested with any of the above 

 reagents some three hours after an ordinary meal, results like those 

 obtained with the HC1 solutions will be observed. This is taken as 

 evidence of the presence of free hydrochloric acid. 



Although in certain diseases where there is deficient secretion of 

 hydrochloric acid, if the reaction of the stomach contents be 

 tested, it will be found strongly acid to litmus, and if , moreover, 

 the degree of this acidity be estimated (by the method described 



1 Dissolve 0-5 grm. dimethylaminoazo -benzol in 100 c.c. 95 per cent. 

 alcohol. 



