Arrhenius concept


Savente  Arrhenius (1884) proposed his concept of acid bases. According to this concept, an acid is a compound that releases H⁺ ions in water, and the base is a compound that releases OH⁻ ions in water.


 Limitation of Arrhenius: Arrhenius concepts of acids and bases proved to be very useful in the study of chemical reactions. However, it has the following limitations.

 1. Free H⁺ and OH⁻ ions do not exist in water. The H⁺ and OH⁻  ions produce by acids and bases respectively do not exist in water in the free state. They are associated with water molecules to form complex ions through hydrogen bonding. The H⁺ ion forms a hydronium ion.

Similarly OH⁻ ion forms the complex H3O2⁻
Although the hydrogen and hydroxyl ion are associated with water molecules for simplicity we shall generally write them  H⁺ and OH⁻.



2.Limited to water only: Arrhenius defined  acids and bases as compounds producing 
    H⁺ and OH⁻. ions in water only. But a truly general concept of acids and bases should be appropriate to other solvents as well.

3. Some bases do not contain  OH⁻.: Arrhenius ion is that produce  OH⁻. ion in water. Yet there are compounds like ammonia (NH3), and calcium oxide(CaO) that base but contain no  OH⁻. ion in their original formulation. 

Arrhenius's model of acids and bases has no doubt, proved very helpful in interpreting their actions. However, on account of its limitation, the Arrhenius concept needed to be modified.