[136] The apparatus described by A. A. Noyes and Blanchard for comparing acids is used [J. Am. Chem. Soc., 22, 737 (1900)].
[137] The conductivity depends in this case chiefly on the migration of the fast moving hydroxide ions (p. [56]), common to the three bases. There is little difference in the rates at which the cations move.
[138] The water should be free from carbonic acid.
[139] The various indicators show different, specific degrees of sensitiveness to acids (to hydrogen ions) and to bases (to hydroxide ions). That is, different concentrations of hydrogen-ion or of hydroxide-ion are required to change their colors. As they are particularly useful in demonstrating varying concentrations of these ions, they will frequently be used in illustrating conclusions reached in the course of our work, just as they are used extensively in practical analysis. The following tables are intended to give some definite information on this valuable quality. The fourth column of the first table shows the concentration of hydrogen-ion, required to change the color of the indicator from the tint given in the second column to the tint given in the third column. The second table gives, similarly, the concentrations of hydroxide-ion required to produce the changes of tint indicated. The tables refer to results obtained when 0.1 c.c. (about two drops) of a 0.1 to 0.15% solution of the indicator is added to 10 c.c. of the solution examined.
Table of Sensitiveness to Acids (to Hydrogen-ion)
| Indicator. | From | to | Concentration H+. |
|---|---|---|---|
| Phenolphthaleïn | Pink | Colorless | 10E−9 |
| Azolitmin[A] (litmus) | Violet | Violet pink | 1E−6 |
| Methyl orange | Yellow | Reddish orange | 1E−3 – 0.1E−3 |
[A] Azolitmin is an important component of litmus.
Table of Sensitiveness to Bases (to Hydroxide-ion)
| Indicator. | From | to | Concentration HO−. |
|---|---|---|---|
| Phenolphthaleïn | Colorless | Pink | 10E−6 |
| Azolitmin | Violet | Violet blue | 1E−6 – 0.1E−6 |
| Methyl orange | Orange | Yellow | 1E−9 |
It is clear that of the three indicators given in the table, phenolphthaleïn is the most sensitive to acids, methyl orange the most sensitive to bases. An extended table of the sensitiveness of many indicators, on which the above tables are based, is given by Salm, Z. phys. Chem., 57, 471 (1907). The theories (of Ostwald, Bernthsen, and others) regarding the color changes, and the theory (of Ostwald) concerning the sensitiveness of indicators, are discussed (with references to the literature) by Stieglitz, J. Am. Chem. Soc., 25, 1117 (1903). Later modifications of the views on color changes are discussed in papers by Stieglitz, Am. Chem. J. 39, (1908), and by Acree, ibid., 37, 39, 42, and in these papers references to the literature will be found. For investigations on the sensitiveness of indicators, see McCoy, ibid., 31, 508 (1904), Salm, loc. cit., and A. A. Noyes, J. Am. Chem. Soc., 32, 815 (1910).