Q.What are the limitations of Arrhenius concept ?

The Arrhenius concept is limited because it is valid only in water and cannot explain acids/bases that do not release $H^+$ or $OH^-$ ions.

The Arrhenius concept (1887) defines an acid as a substance that ionises in water to give $H^+$ (more correctly $H_3O^+$) ions, and a base as a substance that ionises in water to give $OH^-$ ions; neutralisation is simply $H^+ + OH^- \rightarrow H_2O$. While useful, this concept has several important limitations:

1. Restricted to aqueous solution only. The concept defines acids and bases only in terms of their behaviour in water, so it cannot describe or compare acid-base behaviour in non-aqueous solvents (such as liquid ammonia, glacial acetic acid, or benzene) or in the gas phase.

2. Cannot explain basicity of substances without $OH^-$. Compounds such as $NH_3$, amines, and $Na_2CO_3$ behave as bases (they can accept a proton or neutralise acids) even though they do not contain an $OH^-$ ion in their formula; the Arrhenius concept has no way to classify them as bases.

3. Cannot explain acidity of substances without ionisable $H$. Species such as $CO_2$, $SO_2$, $BF_3$ and $AlCl_3$ behave as acids (they accept electron pairs / react with bases) although they possess no ionisable hydrogen at all; Arrhenius's definition cannot account for this.

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