Fajans Method (Adsorption Indicator Method)

Introduction to Fajans Method:

• Fajans method, also known as the adsorption indicator method, is used for determining halide ions (Cl⁻, Br⁻, I⁻) and other anions like thiocyanate (SCN⁻).
• It uses silver nitrate (AgNO₃) as the titrant and an adsorption indicator, such as fluorescein, to detect the endpoint based on color change.

Principle of Fajans Method:

• Halide ions precipitate with silver ions to form silver halide: Ag+ + X− → AgX(s)
• After all halide ions react with silver, the excess silver ions adsorb the indicator onto the surface of the precipitate, causing a color change that signals the endpoint.

Procedure:

1. Pipette a known volume of the halide-containing solution into an Erlenmeyer flask.
2. Add distilled water to dilute the sample.
3. Add 2-3 drops of an adsorption indicator (e.g., fluorescein), which gives the solution a yellow or pale color.
4. Slowly titrate with standardized AgNO₃, forming a white or pale precipitate of silver halide.
5. Continue titrating until a faint pink or reddish color appears, indicating the endpoint.

Theory:

• The endpoint is determined by the adsorption of the indicator on the precipitate surface, which causes a visible color change.
• As halide concentration decreases, excess silver ions adsorb the indicator, leading to the color change that signals the endpoint.

Calculation:

• Where:

• $C_{X^-} = \frac{C_{\mathrm{AgNO_3}} \times V_{\mathrm{AgNO_3}}}{V_{\text{sample}}}$

• CX−​ = concentration of halide ions,
• CAgNO3​​ = concentration of AgNO₃ titrant,
• VAgNO3​​ = volume of AgNO₃ used at the endpoint,
• V_sample​ = volume of the halide-containing sample.

This version maintains clarity and conciseness while covering the key elements of Fajans method.

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