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: Understanding the Concept:
Titration of chloride with silver nitrate using fluorescein is the Fajans method. Fluorescein is an adsorption indicator, and the colour change occurs on the surface of the AgCl precipitate.

Step 2: Before the end point:
Chloride ions are in excess, so AgCl adsorbs \(\text{Cl}^-\) and becomes negatively charged. The fluorescein anion is repelled, so the solution stays pale yellow (yellowish green).

Step 3: At the end point:
After all chloride is used, one extra drop of \(\text{Ag}^+\) makes the precipitate positively charged. The fluorescein anions are now adsorbed on it, forming a reddish pink coloured surface complex.
So the colour changes from pale yellow to reddish pink. A change from colourless would not apply, because the fluorescein solution is already yellow.

Final Answer:
The colour change is pale yellow to reddish pink, option (C). \[ \boxed{\text{Pale yellow to reddish pink}} \]