Step 1: Understanding the Concept:
Fajan's method is a precipitation titration, commonly used to determine halide ions by titrating with silver nitrate, where the endpoint is detected using an adsorption indicator rather than a pH or redox indicator.
Step 2: Key Formula or Approach:
Before the equivalence point, the precipitate surface carries a charge from the excess original ion (for example, excess Cl- keeps the AgCl surface negative). Past the equivalence point, excess Ag+ flips the surface charge positive, which then attracts an oppositely charged dye ion and produces a visible color change right on the precipitate.
Step 3: Detailed Explanation:
Dichlorofluorescein exists as an anion in solution. Once the precipitate surface turns positive after the equivalence point, this anion is adsorbed onto it, and the adsorbed dye shows a distinctly different color (often turning pink) compared to its free form in solution. This surface-charge-driven color change is unique to adsorption indicators and is not something a plain acid-base indicator like methyl red or phenolphthalein, or a metal-chelating indicator like xylenol orange, can do.
Step 4: Final Answer:
Dichlorofluorescein is the indicator used in Fajan's method.