15.5 pH and Solubility

Changing the pH of the solution can also change the solubility of different compounds in the solution.

Let’s take the example of the dissolution of AgF in the solution. How will lowering the pH affect the solubility of this compound?

AgF (s)                           Ag⁺ (aq) + F^–(aq)

To answer this question, we can go back to the meaning of reducing pH. Reducing pH is same as increasing the H⁺ ion concentration in solution. As the H⁺ concentration increases, these H⁺ ions will combine with F^– ion coming from AgF dissolution and produce HF, a weak acid in solution.

H ⁺ (aq) + F^– (aq)                                 HF(aq)

Since HF is a weak acid, it will not dissociate back, and will mostly stay as HF in solution. As a result, we can say as the concentration of H⁺ ion will increase in solution, more HF will form and will result in removal of more F^– ion from the solution.

Now have a look at AgF dissolution equilibrium again, and see how the removal of F^– ions will affect this equilibrium.

AgF (s)                           Ag⁺ (aq) + F^–(aq

As per the Le Châtelier’s principle, as we remove F^– ions from the solution, the equilibrium for dissolution of AgF will shift to the right side and will result in increased solubility of AgF in solution. So overall effect of decreasing pH will be an increase in solubility of AgF in solution.

However, this will not be the case with all silver salt. If we take the example of dissolution of AgCl in solution.

AgCl (s)                      Ag⁺ (aq) + Cl^–(aq)

In this case, when we reduce the pH of the solution, added H⁺ ions would combine with Cl^– ions to produce HCl. Since HCl is a strong acid it will again ionize completely to produce H⁺ and Cl^– ion. In other words, we can say that lowering the pH will not affect the concentration of Cl^– ions in the solution; hence the solubility of AgCl will not get affected by lowering the pH of the solution.

In some cases, solubility can be directly affected by pH change due to the common ion effect. Let’s discuss the example of Pb(OH)₂. The dissolution of this compound can be shown with the reaction below.

Pb(OH)₂ (s)                        Pb⁺² (aq) + 2 OH^– (aq)

Here, if we increase the pH of the solution, it will result in increasing the concentration of OH^– ions in the solution. As per the Le Châtelier’s principle, as the OH^– ion concentration will increase, it will shift the equilibrium to the left side and dissolution of Pb(OH)₂ will go down; or we can say solubility of Pb(OH)₂ will decrease with increase in pH.

Look at the video below to see more examples of the effects of pH on the solubility of ionic compounds.

The impact of an increase in acidity of ocean water is often blamed for the depletion of coral reefs. Coral reefs are mainly made up of CaCO₃. Dissolution of CaCO₃ produces Ca⁺² and CO₃^-2 in solution.

CaCO₃ (s)                          Ca⁺² (aq) + CO₃^-2 (aq)

As the acidity of water increases, the concentration of H⁺ ions increases in water. These H⁺ ions combine with CO₃^-2 ions to produce HCO₃ ^–, a very weak acid in solution.

H⁺ (aq) + CO₃^-2 (aq)                             HCO₃^– (aq)

HCO₃^–  a weak acid, does not dissociate back.  As the concentration of H⁺ ions increase, more and more carbonate ions will be removed from the solution in the form of HCO₃^– .

This removal of CO₃^-2 ions from the ocean water will shift the equilibrium of dissolution of CaCO₃ to the right side and will result in the increased solubility of CaCO₃ in water.

CaCO₃ (s)                          Ca⁺² (aq) + CO₃^-2 (aq)

Learning Check

1. What will be the effect of decreasing the pH on the solubility of MgCO₃?

Answer: Solubility will increase. Decreasing the pH will increase the number of H⁺ ions in solution. These H⁺ ions will combine with CO₃^-2 ions to produce HCO₃^– a weak acid and will shift the equilibrium to the right. Hence, solubility will increase.

MgCO₃ (s)                          Mg⁺² (aq) + CO₃^-2 (aq)

H⁺ (aq) + CO₃^-2 (aq)                             HCO₃^– (aq)