Solvent System Theory:
An acid is a substance which increases the concentration of the cation obtained by autoionisation of the solvent, and bases are the substances which increase the concentration of the anion of the solvent.

If the solvent is represented by AB which gives A+ and B- ions hten , like water , the ionic product KAB may be written as

Limitations :
(i) In order to understand the acidic or basic behavior of a substance, presence of solvent is a must. There is no place for the reactions taking place in the absence of solvent.
(ii) This definition over emphasizes the combination of ions in ionization and neutralization reactions. A number of reactions are known in which acid and base are not ionized. For example, thionyl chloride (SOCl2) is assumed to be an acid in liquid SO2 because according to definition, it gives SO2+ ion in solvent . In fact, ionization tendency of SOCl2 is negligible.
(iii) Theoretical explanation of most of the reactions may be given by solvent system definition , however , it has no practical utility .