[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 TABLE NOTE: 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). [140] ‹Z. phys. Chem.›, «2», 289 (1888). Note the remarks in the footnote following.