De defined as logarithm of the reciprocal of H0 ton concentration. 7.25.2. pH scale Theoretically, a solution may have any H,O" ion concentration and hence may have any pH value. However, practically, the solutions having HO" ion or OH ion concentration more than 10 (1 M) and hence OH ion or H0t ion conc. less than 10" M are not common. This is because such solutions are not easy to obtain and moreover as the concentration increases, the degree of dissociation of the acid or the base decreases enormously. Hence, the pH range is taken as O to 14 for most of the practical purposes. The acidity or the alkalinity of a solution may be represented on the pH scale as follows [HO 100 10 10 10 10 10 102 10-1 10-7 10 10-9 10 10 10-11 10-12 10-13 10-14 ACIDIC CHARACTER INCREASES BASIC CHARACTER INCREASES NEUTRAL pH 5 10 11 14 Strictly seaking, in place or hmolar concentrations, activities of H* ions (a.) are used. However, at tow 12 3 concentrations, the two are equal.