Question

Identify the change in colour when $\mathrm{NaCl}$ solution is titrated against $\mathrm{AgNO}_3$ solution using fluorescein indicator.

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

Hint:

In Fajans precipitation titration using fluorescein, the endpoint color is always characterized by the sudden appearance of a reddish-pink tinge on the white silver halide precipitate.

Answer 1

The Correct Option is B

Step 1: Identify the technique.
Fluorescein is an adsorption indicator, so this is a Fajans-type precipitation titration of $\mathrm{Cl^-}$ (from $\mathrm{NaCl}$) against $\mathrm{Ag^+}$ (from $\mathrm{AgNO_3}$).
Step 2: Note the starting colour.
In the flask, free fluorescein gives the solution a faint greenish or pale yellow tint, so the initial appearance is pale yellow.
Step 3: Track the surface charge before the end point.
While $\mathrm{Cl^-}$ is still in excess, $\mathrm{AgCl}$ adsorbs $\mathrm{Cl^-}$ and the particle surface is negative, written as $(\mathrm{AgCl})\mathrm{Cl^-}$. The fluorescein anion is repelled and stays in solution, keeping the pale yellow colour.
Step 4: Reach the end point.
Once a trace of $\mathrm{Ag^+}$ is in excess, the surface adsorbs $\mathrm{Ag^+}$ and becomes positive, written as $(\mathrm{AgCl})\mathrm{Ag^+}$.
Step 5: Adsorb the dye.
The now-positive precipitate attracts the negative fluorescein anion. On adsorption the dye's effective colour shifts and the precipitate takes on a reddish-pink hue.
Step 6: State the observed change.
The visible transition is therefore pale yellow to reddish pink, which is option (B).
\[ \boxed{\text{Pale yellow} \rightarrow \text{reddish pink}} \]