3.4
Titration of a Weak Base with a Strong Acid
The titration curve of a weak base like ammonia with a strong acid like hydrochloric acid is the mirror image of the titration curve of a weak acid wi…
The weak base versus strong acid titration curve is the reverse of a weak acid-strong base titration curve.
Aqueous ammonia forms ammonium and hydroxide ions. As calculated from the ICE table and Kb expression, the initial pH of 50 mL of 0.1 M ammonia solution is basic with an 11.11 pH.
On adding 25 mL of 0.1 M hydrochloric acid to the ammonia solution, the resulting buffer contains an equal amount of ammonia and ammonium ions. The buffer's pH is calculated by substituting these values into the Henderson–Hasselbalch equation. At this stage, the pH equals pKa.
At the equivalence point, the 50 mL of hydrochloric acid added converts all the ammonia to form ammonium ions. They hydrolyze to produce hydronium ions making the solution acidic with a 5.28 pH.
After the equivalence point, as the hydronium ion species predominate, the pH drops to 1.6.
For the endpoint detection, a methyl red indicator can be used.
Q1: What is the initial pH of a weak base solution before titration begins?
The initial pH of 50 mL of 0.1 M ammonia solution is 11.11, calculated using the ICE table and Kb expression. This basic pH reflects the weak base's partial ionization in water, forming ammonium and hydroxide ions. The calculation demonstrates how weak bases establish equilibrium between molecular and ionic forms.
Q2: How does the titration curve of a weak base with strong acid compare to other titration types?
The weak base versus strong acid titration curve is the mirror image of a weak acid with strong base titration curve. This inverse relationship reflects the opposite charge behavior: weak bases start basic and end acidic, while weak acids start acidic and end basic. Both curves show characteristic buffer regions and equivalence points.
Q3: What happens at the half-equivalence point during weak base titration?
At the half-equivalence point, equal amounts of ammonia and ammonium ions form a buffer solution. Substituting these values into the Henderson-Hasselbalch equation shows that pH equals pKa at this stage. This point occurs after adding 25 mL of 0.1 M hydrochloric acid to the original 50 mL ammonia solution.
Q4: Why does the solution become acidic at the equivalence point?
At the equivalence point, all ammonia converts to ammonium ions after adding 50 mL of 0.1 M hydrochloric acid. Ammonium ions then hydrolyze to produce hydronium ions, making the solution acidic with a pH of 5.28. This hydrolysis occurs because ammonium acts as a weak acid in water.
Q5: What pH changes occur after passing the equivalence point?
After the equivalence point, hydronium ions predominate in the solution, and pH drops sharply to 1.6. This steep decline reflects the excess strong acid present beyond what was needed to neutralize the weak base. The post-equivalence region shows behavior dominated by the strong acid titrant.
Q6: Which indicator is used to detect the endpoint in weak base-strong acid titration?
Methyl red indicator is used to detect the titration endpoint. This indicator exhibits a color change in the desired pH range, signaling when the equivalence point has been reached. The color transition occurs within the acidic region where ammonium hydrolysis dominates.
Q7: How does solution composition change during the titration process?
Solution composition during acid base titrations evolves through distinct stages: initial weak base, buffer formation with mixed ammonia and ammonium, complete conversion to ammonium at equivalence, and excess strong acid afterward. Each stage has unique pH and ionic species ratios that determine the titration curve shape.