If the concentration of the common ions in the solution of two electrolytes , for example ion concentration in and or ion concentration in and is same, then on mixing them there is no change in the degree of dissociation of either of the electrolytes. Such solutions are called isohydric solutions.
Consider two isohydric solutions of acids and. Let and be their dilutions and and be their degree of dissociation at the respective dilution. Then,
Above equation is useful for calculating the relative dilution of two acids at which they would be isohydric.