Titration of a Weak Base with a Strong Acid

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.

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.

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.

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.

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.

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.

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.