Solubility of Weak Acids and Dissociation Constant

The aqueous solubility of most ionic surfactants depends on two intensive parameters, or on temperature at atmospheric pressure (see Chapter 6 for a detailed discussion). However, the total solubility of a sparingly soluble organic acid is more sensitive to pH than to temperature. This is because undissociated monomeric acids are in equilibrium with the dissociated forms, and the concentration ratio of dissociated to undissociated acid depends strongly on the solution pH. This pH dependence is represented by the dissociation or acidity constant. 

When an excess solid or liquid phase of a monobasic acid (SH) coexists with an aqueous phase, the total solubility Q, which is the sum of the concentration Cg- of dissociated acid and the concentration Csh of undissociated acid, depends upon the solution pH and can be expressed as 

both precise determination of the total concentration of the acid Q in aqueous phases of different pH and successive measurement of three coefficients are necessary in order to estimate and K 2- The activity coefficients ysH and ys can be estimated by the Debye-Hiickel approximation. The key point of the solubility method is that the concentration of undissociated SH in the aqueous phase remains constant at constant temperature and pressure, regardless of the introduction of any other chemical species into the aqueous phase, so long as the total ionic strength is kept constant and low. This is the case because SH in the aqueous phase is in equilibrium with solid SH , i.e., the chemical potential of SH is kept constant at constant temperature and pressure. 

We now consider the aqueous solubility and aggregation of sparingly soluble organic acids that are hydrophobic enough to form aggregates when the concentration of dissociated acid is increased by raising the pH (Fig. 3.5). The total equivalent concentration of monobasic acid departs from 

The concentration of undissociated monomeric acid is usually so small that it is neglected here. The mean aggregation number n of aggregates between the concentration C, at which aggregation commences and the concentration C. then becomes 

---