Relative Strength of Acids and Bases

According to Arrhenius concept, an acid is a substance which furnishes H⁺ ions when dissolved in water. All the acid properties on an acid are due to H⁺ ions present in the solution.

The extent to which an acid property is given by an acid is a measure of its strength. The strength of the acid solution does not depend on its concentration but on the number of H⁺ ions present. The concentration of H⁺ ions depends on the ionisation of an acid in solution. On dilution, the ionisation increases and more of H⁺ ions come to solution with the result that the strength of the acid increases. Thus, strength of the acid increases on dilution while its concentration decreases.

At infinite dilution the dissociation of an acid is nearly complete and all acids are equally strong at infinite dilution.

The concentration of H⁺ ions at all other dilutions of equimolar solutions of the acids may not be equal and depends on their degree of dissociation. Thus, to measure the relative strength of the two acids, the measurements of hydrogen ion concentration, i.e., degree of dissociation is made of equinormal solutions of the two acids. Various methods are used for this purpose. Some are described below.