Question

An indicator changes colour over pH 3 to 4 is suitable for __________ titration.

• A. strong acid–strong base
• B. weak acid–strong base
• C. strong base–weak acid
• D. weak base–strong acid

Hint:

The "strong" component always pulls the equivalence point toward its side: - Strong Acid + Weak Base \(\rightarrow\) Equivalence pH is Acidic (< 7) \(\rightarrow\) Use acidic range indicators (e.g., Methyl Orange, pH 3.1–4.4). - Weak Acid + Strong Base \(\rightarrow\) Equivalence pH is Basic (> 7) \(\rightarrow\) Use basic range indicators (e.g., Phenolphthalein, pH 8.3–10.0).

Answer 1

The Correct Option is D

Concept: An acid-base indicator is a weak organic acid or base whose ionized and unionized forms display distinctly different colors. The pH range over which an indicator changes color depends on its dissociation constant (\(\text{p}K_a \pm 1\)). For a titration to be accurate, the rapid pH inflection jump at the equivalence point must fully encompass the indicator's transition interval.

Step 1: Analyze the neutralization profiles
• Weak base titrated with a Strong Acid: At the equivalence point, all the weak base is converted into its conjugate acid. This conjugate acid undergoes partial hydrolysis with water, generating hydronium ions (\(\text{H}_3\text{O}^+\)). Consequently, the solution at the equivalence point is distinctly acidic, usually falling within a pH range of 3 to 6.
• Weak acid titrated with a Strong Base: The equivalence point features a basic salt whose anion hydrolyzes to produce hydroxyl ions (\(\text{OH}^-\)). The pH at equivalence is basic, typically between 8 and 10 (ideal for phenolphthalein).
• Strong acid titrated with a Strong Base: The equivalence point is neutral (pH \(\approx 7\)), but the inflection curve is broad, spanning a steep jump from pH 3 to 11.

Step 2: Match the indicator range An indicator that shifts color in the narrow acidic range of pH 3 to 4 (such as methyl orange or bromophenol blue) will cleanly catch the sharp downward drop of a weak base–strong acid titration where the equivalence point sits in the acidic domain. It would be entirely useless for a weak acid–strong base setup, as the color change would finish long before reaching the true stoichiometric endpoint.