152 PHYSIOLOGICAL CHEMISTRY 



THE USE or INDICATORS IN DETERMINING THE REACTION OF GASTRIC 

 JUICE AND OTHER FLUIDS 



The reaction of the gastric juice and other body fluids is most 

 readily tested by means of indicators, so-called because they show 

 changes of color with differing degrees of acidity or alkalinity of the 

 solution. They behave as though they were weak acids or bases whose 

 ions and unionized molecules have different colors. Modern theories 

 of color in organic compounds however class them as tautomeric 

 substances. 



A neutral solution is one in which there are equal numbers of hy- 

 drogen and hydroxyl ions. An acid solution has a preponderance of 

 hydrogen ion and an alkaline solution an excess of hydroxyl ion. All 

 indicators do not show changes of color at the true neutral point, but 

 at some fixed degree of acidity (or alkalinity), i.e., at a definite hydrogen 

 or hydroxyl ion concentration. Indicators which change color at the 

 approximate true neutral point are litmus and rosolic acid, while phenol- 

 phthalein changes color in a slightly alkaline solution. Congo red, 

 sodium alizarin sulphonate and tropaeolin OO are examples of indicators 

 which change color in an acid solution. 



Organic acids in general are not sufficiently strong; i.e., do not dis- 

 sociate with the production of enough hydrogen ion to cause color 

 changes in dilute solution with indicators of the last-mentioned class. 

 Litmus, rosolic acid and phenolphthalein, however, change at so 

 low a hydrogen ion concentration that they are affected by dilute 

 solutions of organic acids and may be used for their titration. Even 

 very dilute solutions of mineral acids are sufficiently acid to produce 

 color changes with Congo red, alizarin, etc., and hence these indicators 

 may be used in the titration of mineral acid. Phenolphthalein which 

 changes color in a weakly alkaline solution indicates the presence of acid 

 combined with weakly alkaline substances (as protein) as well as other 

 types of acid such as acid salts, and hence, is used in the titration of 

 solutions for their total acidity. 



The hydrogen ion concentration of pure water or a neutral solution 

 is approximately iXio~ 7 , being expressed as approximate moles of 

 hydrogen ion per liter. That is water is a 1/10,000,000 N solution of 

 hydrogen ions. The concentration of hydroxyl ions in pure water or a 

 neutral solution is exactly equal to that of the hydrogen ions, so that 

 water may be considered to be an N/ 10,000,000 alkali as well as an 

 N/io,ooo,ooo acid. Hydrogen ion concentrations are often ex- 

 pressed for the sake of brevity as their logarithms with the sign re- 

 versed. For example the logarithm of iXio~ 7 would be -7.0 and 



