Question

Identify colour change when NaCl solution is titrated against \(\text{AgNO}_3\) solution using fluorescein indicator.

• A. Colourless to pale yellow
• B. Reddish pink to pale yellow
• C. Pale yellow to reddish pink
• D. Colourless to reddish pink

Hint:

Fluorescein is an adsorption indicator, so the colour change happens on the precipitate surface.

Answer 1

The Correct Option is C

Step 1: Adsorption indicator idea:
The indicator does not react in solution. It is adsorbed by the precipitate once the precipitate carries a positive charge.

Step 2: Sequence:
Start: solution has free fluorescein, pale yellow. Excess $\text{Ag}^+$ at the end point charges the AgCl surface positively, which attracts the fluorescein anion. The surface becomes pink. Answer: pale yellow to reddish pink (C).

Final Answer:
Pale yellow to reddish pink. \[ \boxed{\text{(C)}} \]