Early Theories of Acids and Bases
Modern understanding of acids and bases began with the discovery in 1834 by the English physicist Michael Faraday that acids, bases, and salts are electrolytes. That is, when they are dissolved in water, they produce a solution that contains charged particles, or ions, and can conduct an electric current Ionization. In 1884 the Swedish chemist Svante Arrhenius (and later Wilhelm Ostwald, a German chemist) proposed that an acid be defined as a hydrogen-containing compound that, when dissolved in water, produces a concentration of hydrogen ions, or protons, greater than that of pure water. Similarly, Arrhenius proposed that a base be defined as a substance that, when dissolved in water, produces an excess of hydroxyl ions, OH-. The neutralization reaction then becomes: H+ + OH-⇄H2O
A number of criticisms of the Arrhenius-Ostwald theory have been made. First, acids are restricted to hydrogen-containing species and bases to hydroxyl-containing species. Second, the theory applies to aqueous solutions exclusively, whereas many acid-base reactions are known to take place in the absence of water.