Modified Volhard’s Method (Direct Volhard’s Method)

Introduction to Modified Volhard’s Method:

• The Modified Volhard’s method, also called the “direct Volhard’s method,” simplifies the original Volhard’s method by eliminating the back-titration step.
• It combines the addition of excess silver nitrate and the titration with thiocyanate into a single step, making the procedure faster and more efficient.

Principle of Modified Volhard’s Method:

• Like the original method, this technique relies on the selective precipitation of halide ions with silver ions.
• The key difference is that the titration of unreacted silver ions with thiocyanate occurs simultaneously in one step, rather than separately.

Procedure:

1. Pipette a known volume of the halide-containing solution into an Erlenmeyer flask.
2. Add nitric acid (HNO₃) to maintain an acidic pH and prevent the formation of silver hydroxide.
3. Add ferric ammonium sulfate as the indicator (solution turns yellow/pale brown due to ferric ions).
4. Prepare a burette containing a mixture of standardized silver nitrate and thiocyanate solutions. The ratio of AgNO₃ to SCN⁻ matches the ratio of halide to silver ions in the sample.
5. Titrate the sample with the mixed titrant until a permanent red-brown color appears, indicating the endpoint.

Theory:

• The modified method simplifies the titration by combining both steps, reducing the time and potential for measurement errors.
• Halide ions react with silver ions, and any excess silver reacts immediately with thiocyanate present in the same titrant.

Calculation:

Where:

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

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

This arrangement provides a streamlined and clear explanation of the method while keeping it concise.

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