In Equation 2, the quantities in square brackets represent the molar concentrations of the various species, and the γ quantities are the activity coefficients of those species. For an acid capable of ionizing into a univalent anion, γ_H ⁺ and γ_A ^– will be approximately equal, and γ_HA can be approximated as unity, so that the concentration-based equilibrium constant expression can be simply written as:

For weak acids, the magnitude of K _A is very small, and therefore, the resulting H₃O⁺ and A^– ions will be produced in small amounts. Under those conditions, both γH⁺ and γA^– will be approximately equal to one, facilitating the approximation that the thermodynamic equilibrium constant, K, equals the concentration-based ionization constant, K _A . Making use of the Sørensen scale, one can define the pK _A of a weak acid as:

A strong acid is defined as a substance that reacts completely with water so that the acid ionization constant defined in Equation 2 or 3 is very large. This situation can only be achieved if the conjugate base of the strong acid is very weak. A weak acid will be characterized by an acid ionization constant that is considerably less than unity, so that the position of equilibrium in the reaction represented in Equation 1 favors the existence of nonionized free acid. The implication of these properties is that the conjugate base of the weak acid must be moderately strong.

A discussion of the ionic equilibria associated with basic substances exactly parallels that just developed for acidic substances. A base is a substance capable of accepting a proton donated by another substance, such as water:

The basic substance (B) that originally accepted the proton becomes the conjugate acid (BH⁺ ) of that substance, since the conjugate acid could conceivably donate a proton to an even stronger base than the original substance. The concentration-based ionization constant expression corresponding to Equation 5 is:

and pK _B is defined as:

A strong base is a substance that reacts completely with water, so the base ionization constant defined in Equation 6 is effectively infinite. This situation can only be realized if the conjugate acid of the strong base is very weak. Since the conjugate acid of a weak base will be moderately strong, the base ionization constant will be considerably less than one, and the position of equilibrium in the reaction represented in Equation 5 will favor the existence of a nonionized free base.