INDICATORS 7 



color changes in dilute solutions with indicators of the last-men- 

 tioned 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 solu- 

 tions for their total acidity. 



The hydrogen ion concentration of pure water or a neutral 

 solution is approximately 1 X 10~ 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 expressed for the sake of brevity as 

 their logarithms with the sign reversed. For example the logar- 

 ithm of 1 X 10~ 7 would be 7.0 and according to this notation 

 the H ion concentration would be expressed as P# = 7.0. The 

 product of the hydrogen ion concentration (H+) by the hydroxyl 

 ion concentration (OH~) is constant at about 1 X 10~ 14 so that as 

 (H+) increases from 1X1Q- 7 (P* = 7.0) to 1X1Q- 4 (P*=4.0) the 

 (OH~) falls to 1X10~ 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 concentration at which 

 certain indicators commonly used in titration work change color, 

 are indicated below. 



Tests with Indicators. Prepare a series of solutions of varying 

 acidities as outlined below. Introduce 5 or 10 c.c. portions of 

 each of these into a series of test-tubes and add to each a few drops 

 of a solution of Tropaeolin 00. Make a note of the colors produced, 

 in the spaces left for this purpose. In the same way test out the 

 other indicators mentioned, in order, using in each case a few drops 

 of the indicator solution. The tests using the last three men- 

 tioned indicators, Giinzberg's, Boas' and Tropaeolin (evaporation 

 test), are carried out differently, as indicated below. 



