Fajan’s Method

Introduction

Fajan’s Method is a precipitation titration technique that uses adsorption indicators to detect the endpoint. Use it primarily for the determination of halide ions (Cl⁻, Br⁻, I⁻) and other anions by direct titration with silver nitrate (AgNO₃). This method is unique because it relies on the adsorption of an indicator onto the surface of a precipitate, leading to a visible color change at the endpoint. Unlike Mohr’s or Volhard’s methods, Fajan’s method does not require an external indicator but instead utilizes surface chemistry for endpoint detection.

Principle of Fajan’s Method

Fajan’s titration is based on the concept of adsorption of indicators onto the precipitate surface. When an anion (e.g., Cl⁻) is titrated with AgNO₃, a precipitate of AgCl forms. Initially, the precipitate carries a negative charge due to excess Cl⁻ ions.

At the equivalence point, all Cl⁻ ions react with Ag⁺, and the precipitate becomes positively charged due to excess Ag⁺. The negatively charged adsorption indicator (a dye) attracts the positively charged precipitate, causing a distinct color change.

Indicators Used in Fajan’s Method

Fajan’s method uses adsorption indicators, which are dyes that change color upon adsorption onto the precipitate surface. These dyes are usually negatively charged and change color as they adsorb onto the positively charged silver halide precipitate.

Common Adsorption Indicators

Among these, fluorescein is the most commonly used indicator for chloride titration.

Procedure of Fajan’s Method

1. Preparation of Solutions

• Standard Silver Nitrate (AgNO₃) Solution (0.1 N): Used as the titrant.
• Sample Solution (Containing Cl⁻, Br⁻, or I⁻): The analyte.
• Adsorption Indicator (Fluorescein/Eosin/Dichlorofluorescein): Detects the endpoint.

2. Titration Steps

• Take a known volume of the sample solution containing chloride, bromide, or iodide ions.
• Add a few drops of adsorption indicator (e.g., fluorescein).
• Titrate with standard AgNO₃ solution while stirring continuously.
• Observe the color change at the endpoint: 1. Before the endpoint: The indicator is in solution and exhibits its original color. 2. At the equivalence point: The charge on the precipitate reverses and the adsorption of the dye causes a distinct color change (e.g., yellow-green to pink for fluorescein).

Chemical Reactions in Fajan’s Method

Step 1: Precipitation Reaction

Step 2: Charge Reversal at Equivalence Point

Initially, excess chloride ions keep the AgCl precipitate negatively charged. As Ag⁺ is added, the Cl⁻ ions are consumed, and at the equivalence point, the precipitate becomes positively charged.

Step 3: Indicator Adsorption and Color Change

The negatively charged adsorption indicator (e.g., fluorescein) gets adsorbed onto the positively charged AgCl precipitate, leading to a distinct color change, marking the endpoint.

Calculation

The concentration of halide ions is calculated using the standard titration formula:

The mass of the halide ion can be determined using:

Advantages of Fajan’s Method

• No Need for External Indicators: Adsorption indicators provide direct visual detection.
• Highly sensitive: Detects very low concentrations of halide ions.
• Works Well in Aqueous Solutions: No requirement for additional stabilizing agents.
• Simple and Quick: Provides a sharp color change, making endpoint detection easy.

Limitations of Fajan’s Method

• Only Works with Specific Anions: Mainly applicable for Cl⁻, Br⁻, I⁻, and some other anions.
• Colloidal Stability Issues: If the precipitate is not properly formed, endpoint detection may be difficult.
• pH Sensitivity: Some indicators change color based on pH, so the pH of the solution must be carefully controlled.
• Not Suitable for Very Dilute Solutions: Requires a minimum concentration of precipitate for visible adsorption.

Applications of Fajan’s Method

• Water Analysis: Determination of chloride content in drinking water, wastewater, and seawater.
• Pharmaceutical Analysis: Quantification of halides in drugs and saline solutions.
• Food and Beverage Industry: Measurement of salt content in food products.
• Biological Fluids: Chloride determination in blood serum and urine.
• Industrial Applications: Quality control of halides in chemical manufacturing.

Comparison: Mohr’s vs. Volhard’s vs. Fajan’s Method

Conclusion

Fajan’s Method is an efficient and sensitive precipitation titration technique for halide determination, particularly chloride ions. By utilizing adsorption indicators, it offers a direct and visual method for detecting the endpoint without additional reagents like chromate (Mohr’s method) or ferric ions (Volhard’s method). Its high sensitivity and sharp color change make it an essential tool in pharmaceutical, environmental, and industrial analyses.