Introduction:
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:
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:
Pipette a known volume of the halide-containing solution into an Erlenmeyer flask.
Add a few milliliters of distilled water to the flask to dilute the sample.
Add 2-3 drops of an adsorption indicator, such as fluorescein, to the flask. The solution will have a yellow or pale color due to the presence of the indicator.
Slowly titrate with standardized AgNO₃, forming a white or pale precipitate of silver halide.
Continue titrating until the appearance of the first permanent faint pink or reddish color, which indicates the endpoint of the titration.
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:
C_X- = (C_AgNO3 * V_AgNO3) / V_sample
Where:
C_X- is the concentration of halide ions (X-) in the sample.
C_AgNO3 is the concentration of the silver nitrate titrant.
V_AgNO3 is the volume of silver nitrate titrant used at the endpoint.
V_sample is the volume of the halide-containing sample.
This version maintains clarity and conciseness while covering the key elements of Fajans method.