Volhard’s method

Introduction to Volhard’s method:

• Volhard’s method is an indirect precipitation titration used to determine halide ions (Cl⁻, Br⁻, I⁻) and thiocyanate (SCN⁻) in solution.
• It involves two steps: adding an excess of silver nitrate to precipitate the halide, followed by a back-titration of unreacted silver ions with a standardized thiocyanate solution.
• The endpoint is indicated by a red-brown ferric thiocyanate complex, using ferric ammonium sulfate as the indicator.

Principle of Volhard’s method:

• Halide ions react with silver nitrate to form a precipitate:
• The excess Ag⁺ is titrated with thiocyanate (SCN⁻), forming silver thiocyanate:
• The endpoint occurs when the first excess SCN⁻ forms a red-brown complex with ferric ions:

Procedure:

1. Pipette a known volume of the halide-containing solution into a flask.
2. Add an excess of standardized AgNO₃ to precipitate all halide ions.
3. Add nitric acid (HNO₃) to maintain an acidic environment and prevent silver hydroxide formation.
4. Add ferric ammonium sulfate as the indicator (solution turns pale yellow).
5. Titrate with standardized thiocyanate (NH₄SCN) until the first permanent red-brown color appears, indicating the endpoint.

Theory:

• Volhard’s method relies on the selective precipitation of silver halides and the clear color change when the ferric thiocyanate complex forms.
• The concentration of halide ions is calculated using:

Where:

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

• CX−​ = concentration of halide ions,
• CAgNO3​​ = concentration of AgNO₃,
• VAgNO3​​ = volume of AgNO₃ added,
• CSCN−​ = concentration of SCN⁻ titrant,
• VSCN−​ = volume of SCN⁻ used,
• V_sample​ = volume of the halide-containing sample.

This streamlined version maintains the key concepts while reducing redundancy.

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