The Plot of pH -metric titration of weak base NH 4 OH vs strong acid HCl looks like

Titration curve of ${\mathrm{NH}}_4\mathrm{OH}$ vs $\mathrm{HCl}\left(\mathrm{WB}\right)\left(+\mathrm{SA}\right)$

At the very beginning of the curve, the pH starts by falling quite quickly as the acid is added, but the curve very soon gets less steep. This is because a buffer solution is being set up composed of the excess ammonium hydroxide and the ammonium chloride being formed.

Notice that the equivalence point is now somewhat acidic ( a bit less than pH 5), because pure ammonium chloride isn't neutral. However, the equivalence point still falls on the steepest bit of the curve. That will turn out to be important in choosing a suitable indicator for the titration.

For the above titration the indicator suitable is methyl orange