8.11 pH and Solubility

Solubility

This relates to the very basic concepts of equilibrium. If a salt such as ${NaF}$ is placed in water, it will reach the equilibrium {NaF ⇆ Na⁺ + F^–}. If this solution is made more acidic, the concentration of H+ will increase. This creates a second equilibrium: {F^– + H⁺ ⇆ HF}. Since HF is a weak acid, it will form. The two ions will not remain dissociated. As a result, the concentration of fluoride ion decreases. According to LeChatelier’s principle, the first system will shift to raise {[F^–]} once again. Therefore, more NaF will dissolve. A salt containing a conjugate base of a weak acid will be more soluble in acidic solutions.