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 K_b 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 pK_a. 

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.