Salt Hydrolysis and Aqueous pH

Salt hydrolysis is the reaction of the ions of a salt with water that changes the pH: the anion of a weak acid makes the solution basic, the cation of a weak base (or a small, highly charged metal cation) makes it acidic, and ions of strong acids and strong bases do not affect the pH. In brief: a 0.100 mol/L solution of sodium acetate has pH 8.87, ammonium chloride has pH 5.13, and sodium chloride has pH 7.00.

When a salt dissolves, its ions can act as weak acids or weak bases. This article gives rules to predict whether a salt solution is acidic, basic or neutral, shows how to calculate the pH using K_w, tabulates typical results, explains the case of salts with both ions hydrolyzing, works through examples and gives some history.

What are the rules?

• Cations of strong bases (Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺) and anions of strong acids (Cl⁻, Br⁻, I⁻, NO₃⁻, ClO₄⁻, SO₄²⁻) are neutral spectators.
• The anion of a weak acid is a weak base: A⁻ + H₂O ⇌ HA + OH⁻, K_b = K_w ÷ K_a.
• The cation of a weak base is a weak acid: BH⁺ + H₂O ⇌ B + H₃O⁺, K_a = K_w ÷ K_b.
• Small, highly charged metal ions (Al³⁺, Fe³⁺, Cr³⁺, Cu²⁺) are acidic: [Al(H₂O)₆]³⁺ + H₂O ⇌ [Al(H₂O)₅OH]²⁺ + H₃O⁺.

How do anions hydrolyze?

The acetate ion takes a proton from water: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻. Because K_a of acetic acid is 1.8 × 10⁻⁵, K_b = 5.6 × 10⁻¹⁰. The weaker the parent acid, the more basic its anion. Carbonate (K_b = 2.1 × 10⁻⁴) is much more basic than acetate.

How do cations hydrolyze?

The ammonium ion gives up a proton: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, K_a = 5.6 × 10⁻¹⁰. Hydrated metal ions are acidic since the metal polarizes the O–H bonds of coordinated water.

pH of salt solutions

What if both ions hydrolyze?

Compare K_a of the cation with K_b of the anion. If K_a > K_b, the solution is acidic; if K_b > K_a, basic; if equal, neutral. Ammonium acetate: K_a(NH₄⁺) = 5.6 × 10⁻¹⁰ and K_b(CH₃COO⁻) = 5.6 × 10⁻¹⁰, so pH ≈ 7. Ammonium cyanide is basic (CN⁻ is a stronger base than NH₄⁺ is an acid), and ammonium fluoride is acidic.

Worked examples

1. Sodium acetate, 0.100 mol/L. K_b = 5.6 × 10⁻¹⁰; [OH⁻] = √(5.6 × 10⁻¹⁰ × 0.100) = 7.5 × 10⁻⁶ mol/L; pOH 5.13; pH 8.87.
2. Ammonium chloride, 0.100 mol/L. [H⁺] = √(5.6 × 10⁻¹⁰ × 0.100) = 7.5 × 10⁻⁶ mol/L; pH 5.13.
3. Sodium carbonate, 0.100 mol/L. K_b1 = 1.0 × 10⁻¹⁴ ÷ 4.7 × 10⁻¹¹ = 2.1 × 10⁻⁴. Solve x² + 2.1 × 10⁻⁴x − 2.1 × 10⁻⁵ = 0: x = 4.5 × 10⁻³ mol/L; pOH 2.35; pH 11.65.
4. Predict. KNO₃ neutral; FeCl₃ acidic; Na₃PO₄ basic (phosphate is the conjugate base of the weak acid HPO₄²⁻).
5. K_a from K_b. For methylammonium, K_a = 1.0 × 10⁻¹⁴ ÷ 4.4 × 10⁻⁴ = 2.3 × 10⁻¹¹.
6. Percent hydrolysis. In example 1: 7.5 × 10⁻⁶ ÷ 0.100 × 100% = 0.0075%. Hydrolysis is small, but it shifts the pH by almost two units.

Where does hydrolysis matter?

• Soaps: sodium salts of fatty acids give basic solutions.
• Baking: baking soda gives a weakly basic solution.
• Water treatment: aluminum salts hydrolyze to give aluminum hydroxide flocs that remove particles.
• Buffers: the hydrolysis equilibria are the basis of buffer behavior.

The term hydrolysis was used in the 19th century for the reaction of salts with water, and Arrhenius and Ostwald gave a quantitative treatment in the 1880s and 1890s.

Which mistakes are common?

Mistake 1: Saying that all salts are neutral

Salts of weak acids or bases are not.

Mistake 2: Using K_a of the acid for the anion’s pH

Use K_b = K_w ÷ K_a.

Mistake 3: Hydrolyzing Cl⁻ or Na⁺

They come from a strong acid and a strong base and do not react with water appreciably.

Mistake 4: Forgetting the highly charged metal ions

Fe³⁺ and Al³⁺ make solutions acidic.