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Introduction:
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:
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:
Pipette a known volume of the halide-containing solution into an Erlenmeyer flask.
Add nitric acid (HNO₃) to maintain an acidic pH and prevent the formation of silver hydroxide.
Add ferric ammonium sulfate as the indicator (solution turns yellow/pale brown due to ferric ions).
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.
Titrate the sample with the mixed titrant until a permanent red-brown color appears, indicating the endpoint.
Theory:
The modified Volhard's method streamlines the titration process by combining the addition of excess silver nitrate and the titration with the thiocyanate solution into a single step.
Halide ions react with silver ions, and any excess silver reacts immediately with thiocyanate present in the same titrant.
Calculation:
C_X- = (C_AgNO3 × V_mixed_titrant) / V_sample
Where:
C_X- is the concentration of halide ions (X-) in the sample.
C_AgNO3 is the concentration of the silver nitrate in the mixed titrant.
V_mixed_titrant is the volume of the mixed titrant used at the endpoint.
V_sample is the 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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