GASTRIC ANALYSIS 155 



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 th^y 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 i X jo" 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/ 10,000,000 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 i X io~~ 7 would be 7.0 and 

 according to this notation the H ion concentration would be expressed 

 as P H = 7-o. The product of the hydrogen ion concentration (H + ) 

 by the hydroxyl ion concentration (OH~) is constant at about i X io~ 14 

 so that as (H + ) increases from i X io~ 7 (P H = 7.0) to i X io~~ 4 

 (P H = 4.0) the (OH~) falls to i X io~ 10 , and vice versa. It must be 

 borne in mind that higher figures for the logarithmic notation indicate 

 lower figures for (H + ). The hydrogen ion concentrations at which 

 certain indicators commonly used in titration work change color, are 

 indicated below. 



