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.