Reactivity 3.1.12 — The pH of a Salt Solution

• When an acid reacts with a base, a salt and water are formed in what is called a neutralization reaction.
• However, the term “neutralization” refers only to the stoichiometry of H⁺ and OH^– reacting in equal amounts, and does not guarantee that the resulting solution is neutral in pH.
• In fact, the resulting salt solution can be acidic, basic, or neutral, depending on the nature of the acid and base from which the salt is derived.

Understanding how salts affect pH requires knowledge of conjugate acid-base pairs and hydrolysis.

• In aqueous solution, the ions from a salt may interact with water molecules in a process called hydrolysis. During hydrolysis, ions may react with water to either produce H⁺ (or H₃O⁺) or OH^–, thereby altering the pH of the solution.
• Whether hydrolysis occurs depends on the strength of the parent acid and base that formed the salt. More precisely, it depends on whether the resulting ions (from salt dissociation) are weak enough to act as Bronsted-Lowry acids or bases and react with water.

Key Concept:

Only the conjugate base of a weak acid and the conjugate acid of a weak base undergo significant hydrolysis in water. The conjugates of strong acids or strong bases are extremely weak and do not react appreciably with water.

Therefore, the pH of a salt solution depends on:

1. Whether the cation of the salt is a conjugate acid of a weak base

2. Whether the anion of the salt is a conjugate base of a weak acid

3. The relative strengths of these conjugate species (Ka and Kb values) if both are weak

Case-by-Case Explanation:

1. Salt of a strong acid and a strong base (e.g., NaCl):

HCl is a strong acid and NaOH is a strong base. The salt formed, NaCl, dissociates completely into Na⁺ and Cl^–. These ions are spectator ions. Neither ion reacts with water to change the pH. Therefore, the solution remains neutral, with pH ≈ 7.

2. Salt of a strong acid and a weak base (e.g., NH₄Cl):

HCl is a strong acid and NH₃ is a weak base. The salt formed, NH₄Cl, dissociates into NH₄⁺ and Cl^–. Cl^– does not hydrolyze, but NH₄⁺ is the conjugate acid of NH₃. It hydrolyzes as follows:

NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺

This increases [H₃O⁺], lowering the pH, making the solution acidic.

3. Salt of a weak acid and a strong base (e.g., CH₃COONa):

CH₃COOH is a weak acid and NaOH is a strong base. The salt CH₃COONa dissociates into CH₃COO^– and Na⁺. Na⁺ does not hydrolyze, but CH₃COO^– is the conjugate base of CH₃COOH. It hydrolyzes as follows:

CH₃COO^– + H₂O ⇌ CH₃COOH + OH^–

This increases [OH^–], raising the pH, making the solution basic.

4. Salt of a weak acid and a weak base (e.g., NH₄CH₃COO):

NH₄⁺ is the conjugate acid of NH₃ (weak base) and CH₃COO^– is the conjugate base of CH₃COOH (weak acid). Both ions hydrolyze in water:

NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺

CH₃COO^– + H₂O ⇌ CH₃COOH + OH^–

The final pH of the solution depends on the relative values of Ka for NH₄⁺ and Kb for CH₃COO^–. If Ka > Kb, solution is acidic; if Ka < Kb, solution is basic; if Ka ≈ Kb, solution is neutral.

Table — How Parent Acid and Base Strength Influence Salt Solution pH

How a salt affects the pH of its solution, we must analyze how its constituent ions behave in water. This involves:

• Dissociating the salt into its ions
• Identifying if the ions are conjugates of weak acids or weak bases
• Constructing the hydrolysis equations for the ions that react with water
• Predicting whether the ion produces H₃O⁺ (acidic), OH^– (basic), or neither (neutral)

Only ions that are conjugates of weak acids or weak bases undergo hydrolysis.

Hydrolysis Behavior of Common Ions:

Step-by-Step Process for Determining pH Effect of a Salt:

1. Write the formula of the salt and dissociate it into its ions.
2. Identify the origin (parent acid/base) of each ion.
3. Determine if the ion is a conjugate of a weak acid or base.
4. If so, write the hydrolysis equation and identify if it produces H₃O⁺ or OH^–.
5. Conclude whether the solution is acidic, basic, or neutral.

Amphiprotic Ions (e.g., HCO₃^–):

HCO₃^– can both donate and accept protons, leading to two possible hydrolysis reactions:

• As an acid: \( \text{HCO}_3^- + \text{H}_2\text{O} \rightleftharpoons \text{CO}_3^{2-} + \text{H}_3\text{O}^+ \)
• As a base: \( \text{HCO}_3^- + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 + \text{OH}^- \)

The net pH depends on the relative Ka and Kb values. In most cases, the basic nature slightly dominates, and the solution is weakly basic.

Conclusion:

• The pH of a salt solution is determined by whether its ions hydrolyze in water.
• Conjugates of weak acids produce OH^– (basic), while conjugates of weak bases produce H₃O⁺ (acidic).
• Spectator ions from strong acids/bases do not affect pH.
• If both ions hydrolyze, compare \( K_a \) and \( K_b \) to predict whether the solution is acidic or basic.