Liebig's hydrogen theory of acids
Circa 1838 Justus von Liebig proposed[3] that an acid is a hydrogen-containing substance in which the hydrogen could be replaced by a metal.[4] This redefinition was based on his extensive work on the chemical composition of organic acids, finishing the doctrinal shift from oxygen-based acids to hydrogen-based acids started by Davy. Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition.[5]