# pH

## Key Questions

• pH is a measure of hydrogen ion concentration. The pH scale was originated by Sorensen. He set 0 pH as the hydrogen ion concentration of a 1.0 molar solution of a strong monoprotic acid (like HCl). 14 on the pH scale uses its standard as the hydrogen ion concentration in a 1.0 molar concentration of a strong base (like NaOH). pH is actually a negative logarithmic scale. To determine pH, take the negative logarithm of the hydrogen ion concentration.

Decreasing the pH increases the solubility of sparingly soluble bases and basic salts.

#### Explanation:

Increasing the pH has the opposite effect.

Many sparingly soluble compounds have solubilities that depend on pH.

It all involves the application of Le Châtelier's Principle.

Here are two common examples.

Bases

${\text{Zn(OH)}}_{2}$ is a sparingly soluble base.

$\text{Zn(OH)"_2"(s)" ⇌ "Zn"^"2+""(aq)" + "2OH"^"-""(aq)}$

If you increase the pH by adding $\text{OH"^"-}$ ions, Le Châtelier's Principle says that the position of equilibrium will move to the left.

The solubility of the ${\text{Zn(OH)}}_{2}$ decreases.

If you decrease the pH by adding ${\text{H"_3"O}}^{+}$ ions, the added ${\text{H"_3"O}}^{+}$ ions will react with the $\text{OH"^"-}$ ions and form water.

$\text{OH"^"-}$ ions are removed from solution.

According to Le Châtelier's Principle, the position of equilibrium moves to the right to replace the $\text{OH"^"-}$ ions.

The solubility of the ${\text{Zn(OH)}}_{2}$ increases.

Salts of Strong Bases and Weak Acids

${\text{CaCO}}_{3}$ is the sparingly soluble salt of the strong base ${\text{Ca(OH)}}_{2}$ and the weak acid ${\text{H"_2"CO}}_{3}$.

$\text{CaCO"_3"(s)" ⇌ "Ca"^"2+""(aq)" + "CO"_3^"2-""(aq)}$

$\text{CO"_3^"2-}$ is a base.

$\text{CO"_3^"2-""(aq)" + "H"_2"O(l)" ⇌ "HCO"_3^"-""(aq)" + "OH"^"-""(aq)}$

The overall equilibrium is

$\text{CaCO"_3"(s)" + "H"_2"O(l)" ⇌ "Ca"^"2+""(aq)" + "HCO"_3^"-""(aq)" + "OH"^"-""(aq)}$

If you increase the pH by adding $\text{OH"^"-}$ ions, Le Châtelier's Principle says that the position of equilibrium will move to the left.

The solubility of the ${\text{CaCO}}_{3}$ decreases.

If you decrease the pH by adding ${\text{H"_3"O}}^{+}$ ions, the added ${\text{H"_3"O}}^{+}$ ions will react with the $\text{OH"^"-}$ ions and form water.

$\text{OH"^"-}$ ions are removed from solution.

According to Le Châtelier's Principle, the position of equilibrium moves to the right to replace the $\text{OH"^"-}$ ions.

The solubility of the ${\text{CaCO}}_{3}$ increases.

Here's a video that discusses the effect of pH on solubility.