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The modified limit test for chloride is based on the Volhard titration method, where chloride ions (Cl⁻) are precipitated with silver nitrate (AgNO₃) and the excess silver nitrate is titrated with ammonium thiocyanate (NH₄SCN).
The process involves two key reactions:
Chemical Reactions
a) Precipitation of chloride ions
Ag⁺ (aq) + Cl⁻ (aq) → AgCl (s)
b) Titration of excess silver nitrate
Ag⁺ (aq) + SCN⁻ (aq) → AgSCN (s)
Procedure of Volhard Method
1) Prepare Test Solution
Prepare a solution from the sample, ensuring the chloride ions are in a soluble form.
2) Add Silver Nitrate (AgNO₃)
Add a known excess of AgNO₃ to the test solution to precipitate chloride ions as silver chloride (AgCl).
3) Acidify with Nitric Acid (HNO₃)
Add a small amount of nitric acid to prevent the precipitation of other silver salts, ensuring the reaction proceeds correctly.
4) Titrate Excess AgNO₃ with NH₄SCN
Titrate the excess silver nitrate with ammonium thiocyanate (NH₄SCN) using ferric ammonium sulfate as an indicator.
The endpoint is reached when a red complex forms between ferric ions and thiocyanate (after all excess silver ions have reacted).
5) Calculate Chloride Ion Concentration
Calculate the concentration of chloride ions in the sample based on the volume of NH₄SCN used in the titration.
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