Chloride Test in Water - Determine the Concentration of Chlorides by Volhard's Method

Theoretical Background / Context

• Precipitimetry: It is a volumetric method of analysis that involves the formation of a practically insoluble salt using a precipitating agent and is one of the chloride analysis methods. 
• Solubility Product (Ksp): In a saturated solution of a sparingly soluble electrolyte, the product of the molar concentration of ions each raised to a power equal to the number of ions produced is constant at room temperature and pressure.

Ksp of AnBm =[A]n[B]m

 N.B: Substance with low Ksp precipitate first.

Conditions required for Preciptimetric reactions:

1)   The precipitate must be practically insoluble

2)   Rapid precipitation

3)   Ease of detection of the endpoint , which is essential for accurate chloride test in water. 

 Endpoint detection in Precipitimetry.

Principle Work of Chloride Test in Water

Volhard Method for Chloride Determination (Formation of the Colored Complex):

• q  Direct: Ag+ ions titrated by thiocyanate using ferric alum as an indicator, Endpoint: First faint red color.
• q Indirect: Known excess standard AgNO3 precipitate halides, cyanide, phosphate then back titrate excess unreacted AgNO3 by thiocyanate using ferric alum as an indicator, Endpoint: First faint red color.

Here we are using the indirect Volhard’s method (back titration):

• In Acidic medium (pH 1-3)
• Known excess standard AgNO3 precipitate chloride, then back titrate excess unreacted AgNO3 by thiocyanate using ferric alum as an indicator, Endpoint: First faint red color.
• This chloride test in water procedure involves chloride measurement in water through precipitation reactions.

Ag+ +Cl-  → AgCl

Fe3+ +2SCN- → [Fe(SCN)2]+  (blood red color)