When phosphoric acid is neutralized by one equivalent of a base, say of sodium hydroxide, the salt formed, sodium dihydrogen-phosphate, NaH_{2}PO_{4}, yields sodium-ion and dihydrogen-phosphate-ion, H_{2}PO_{4}^{−}. The latter is ionized ‹somewhat› into H^{+} and the bivalent hydrogen-phosphate-ion, HPO_{4}^{2−}. The ionization of the ion H_{2}PO_{4}^{−} is now the chief source of supply of hydrogen-ion (the further ionization of HPO_{4}^{2−} is practically negligible here) and it is ionized so little that the solution of NaH_{2}PO_{4} no longer changes the color of methyl orange (see EXP. below). The solution is, however, acid to the indicator phenolphthaleïn, which is much more sensitive to the hydrogen-ion and will show the presence of much smaller concentrations of it than will methyl orange. The addition of a second equivalent of sodium hydroxide to the solution converts NaH_{2}PO_{4} into Na_{2}HPO_{4}. This salt gives sodium-ion and the hydrogen-phosphate-ion HPO_{4}^{2−}, which, in turn, is ionized only very slightly, producing phosphate-ion PO_{4}^{3−}, and again hydrogen-ion. The ionization of HPO_{4}^{2−} is so slight, however, and the concentration of the hydrogen-ion, therefore, so minute, that the solution does not react acid even to the sensitive indicator phenolphthaleïn. EXP. Methyl orange (very little) is added to 10 c.c. of a 0.1 molar solution of phosphoric acid and 10 c.c. of 0.1 molar sodium hydroxide solution is added to the mixture; the color will be found to change from the acid to the neutral tint just as the last drop or two of the alkali are added. Phenolphthaleïn is then added to the mixture and 10 c.c. more of the 0.1 molar sodium hydroxide solution are required to change the color of the mixture to a pronounced pink (alkaline) tint. Even sulphuric acid, although its two hydrogen atoms are ionized very easily, making sulphuric acid a strong acid, shows a difference in the ease of ionization of the two hydrogen atoms. Since ionization, in general, is favored by dilution, we find that in the case of such a strong acid the difference is most marked in more concentrated solutions, the smaller amount of water starting the ionization in the more favored direction and producing first, chiefly, hydrogen-sulphate ions, HSO_{4}^{−}. When the solution is diluted, the hydrogen-sulphate ions are to a very considerable extent dissociated into sulphate ions and hydrogen ions. The described change in ionization can be roughly followed with the aid of an insoluble sulphate like barium sulphate. Barium sulphate, while very insoluble in water, dissolves in rather strong sulphuric acid to form the acid sulphate, Ba(HSO_{4})_{2}, the SO_{4}^{2−} ion of the sulphate being more or less suppressed by uniting with hydrogen-ion. We have the action BaSO_{4} ⇄ Ba^{2+} + SO_{4}^{2−} and Ba^{2+} + SO_{4}^{2−} + H^{+} + HSO_{4}^{−} ⇄ Ba^{2+} + 2 HSO_{4}^{−}.