3.4:
Titration of a Weak Base with a Strong Acid
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.
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 with a strong base.
Using the ICE table and substituting the Kb value, we calculate the initial pH of 50 mL of 0.1 M ammonia to be 11.11. Addition of 25 mL of 0.1 M hydrochloric acid to this solution of ammonia results in a buffer with an equal concentration of ammonia and ammonium ions. The pH of this buffer can be calculated by substituting these values into the Henderson-Hasselbalch equation, which shows that at this point, also known as the half-equivalence point, the pH is equal to pKa. Continuing the titration to the equivalence point would mean that a total of 50 mL of 0.1 M hydrochloric acid would have been added, converting all the ammonia molecules to ammonium ions. At this point, the ammonium ions undergo hydrolysis to generate hydronium ions, making the solution acidic, and an ICE table calculation shows that the pH is 5.28. After the equivalence point, as the hydronium ions predominate the solution, the pH further drops to 1.6. The titration endpoint is detected using the indicator methyl red, which exhibits a color change in the desired pH range.