Question

Explain why indicators cannot be satisfactorily used, when dilute sulfurous acid is titrated against 0.10M ammonium hydroxide

Answer 1

dilute sulfurous acid is weak and ammonium hydroxide is weak base .

An indicator works when the solution's pH is in its range. It's not necessarily the equivalence point. The point is called the end point where it actually changes color. If the difference in volume is small between the equivalence and end points, then the indicator can be used. Since a gradual change implies more volume must be added to obtain the same pH change in other acid base titration, the difference between equivalence point and end point(if you use indicator) will be more and therefore indicator will not be suitable for it.

there is no suitable indicator for weak acid and weak base titration