NH₄Cl is a strong electrolyte hence, dissociates completely, NH₄OH is a weak electrolyte. Its dissociation is further suppressed by common ion NH₄⁺ provided by NH₄Cl in the solution.

${\mathrm{NH}}_4{\mathrm{Cl}}_{\mathrm{aq}}\rightleftharpoons {\mathrm{NH}}_{4_{\mathrm{aq}}}^{+}+{\mathrm{Cl}}_{\mathrm{aq}}^{-}$

${\mathrm{NH}}_4{\mathrm{OH}}_{\mathrm{aq}}\rightleftharpoons {\mathrm{NH}}_{4_{\mathrm{aq}}}^{+}+{\mathrm{OH}}_{\mathrm{aq}}^{-}$

This solution acts as a basic buffer and maintains its pH around 9.25. It is capable of resisting the change in pH on addition of small amount of acid or alkali.

To Explain : If a small amount of HCl is added to this solution, H⁺ ions of HCl get neutralized by OH^– ions already present and more of NH₄OH molecules get ionized to compensate loss of OH^– ions. Thus pH practically, remains constant. If a small amount of NaOH is added to this solution, OH^– ions of NaOH combine with NH₄⁺ ions already present in large number forming weakly ionized NH₄OH. Thus, pH of solution remains practically unchanged.