Modified Limit Test for Chloride (Volhard Method)

• 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 of Limit Test for Chloride:

Precipitation of Chloride Ions:

• Ag + (aq) + Cl−(aq) → AgCl(s)

Titration of Excess Silver Nitrate:

• Ag+(aq) + SCN−(aq) → AgSCN(s)

Procedure:

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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