Question:
Which is used as Fajan's method indicator
Which is used as Fajan's method indicator
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Dichlorofluorescein acts as an adsorption indicator in Fajan’s method by color change at the precipitate surface.
Updated On: Jul 14, 2026
- Dichlorofluorescein
- Methyl red
- Phenolphthalein
- Xylene orange
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The Correct Option is A
Approach Solution - 1
- Fajan's method is a type of adsorption indicator method used in precipitation titrations, particularly for determining halides by titration with silver nitrate.
- Dichlorofluorescein is commonly used as the indicator in Fajan's method because it adsorbs onto the surface of the precipitate, changing color at the endpoint.
- This color change indicates the completion of the reaction, enabling accurate determination of the endpoint.
- Other indicators like methyl red, phenolphthalein, and xylene orange are used in different types of titrations and are not suitable for Fajan's method.
- Dichlorofluorescein is commonly used as the indicator in Fajan's method because it adsorbs onto the surface of the precipitate, changing color at the endpoint.
- This color change indicates the completion of the reaction, enabling accurate determination of the endpoint.
- Other indicators like methyl red, phenolphthalein, and xylene orange are used in different types of titrations and are not suitable for Fajan's method.
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Approach Solution -2
The question asks which compound is used as the indicator in Fajan's method, an adsorption-indicator technique for precipitation titrations. Let's check what type of indicator each option actually is.
- Dichlorofluorescein: This is a classic adsorption indicator. Its anion adsorbs onto the surface of the silver halide precipitate once excess Ag+ is present past the equivalence point, producing a visible color change directly on the precipitate surface, which is the operating principle of Fajan's method.
- Methyl red: This is a pH (acid-base) indicator that changes color over a defined pH range. It responds to hydrogen ion concentration, not to surface adsorption on a precipitate, so it does not fit Fajan's method.
- Phenolphthalein: This is also an acid-base indicator, used in neutralization titrations because its color depends on pH, not on precipitate surface charge.
- Xylene orange: This dye is used as a metal-ion (complexometric) indicator, forming colored chelates with metal ions during EDTA titrations, not as a surface-adsorption indicator for halide precipitation.
Only dichlorofluorescein works by adsorbing onto a precipitate surface and changing color as the surface charge reverses at the endpoint, which is exactly the mechanism Fajan's method depends on.
Therefore, the correct answer is Dichlorofluorescein.
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