- 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.
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)
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