Solution pH and solubility of weakly acidic compounds.

A weakly acidic compound can be present in the nonoionized (HA) or in the ionized form (A-) in aqueous solutions. The extent of ionization depends on the pH of the solution. For example, if the solution pH is very acidic then the nonionized form is predominant. In the other hand, if the solution pH is basic then the nonionized form is predominant. Ionized forms of compounds have larger solubility in water than their nonionized counterparts. Therefore, it is expected that weakly acidic compounds have larger solubility at higher pH values.

The equation  that describes the influence of pH on solubility is derived as the following:

- Assuming excess amount of the acid is present in solution (saturated solution) then the concentration of the nonionized form (HA) is constant and equal to the intrinsic solubility, So:

So = [HA]

In addition, HA dissociates in solution according to the following formula:

HA + H₂O ↔ A^- + H₃O⁺

    equation (1)

Keep in mind that any amount of dissolved HA that dissociates will be substituted from the undissolved excess amount of the acid until equilibrium is reached. Therefore, solubility, S, is the sum of the concentrations of both ionized and nonionized forms of the acid.

S = [HA] + [A^-]     equation (2)

S = So + [A^-]         equation (3)

According to equation (1),  [A^-] can be determined and substituted in equation (3) as the following:

:

     equation (4)

     equation (5)

     equation (6)

     equation (7)

According to equation (7), at Ka = [H₃O⁺] ( pH = pKa) the solubility is equal to 2×So. Moreover, at [H₃O⁺] >> Ka the solubility is equal to So. Finally, at lower [H₃O⁺] values the solubility increases.