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DrX Whiz Niraj
Precipitation Titrations: Mohr, Volhard & Fajans
Comprehensive exam notes on Argentometric titrations, indicators used, and the estimation of Sodium Chloride.
A titration in which the reaction between the titrant and the analyte results in the formation of a sparingly soluble substance (precipitate) is called a precipitation titration. The most important precipitating agent used in pharmacy is Silver Nitrate (AgNO₃). Hence, these titrations are largely known as Argentometric Titrations.
Solubility Product Principle:
The entire precipitation depends on the Solubility Product (Ksp). Precipitation occurs only when the ionic product of the reacting ions exceeds the solubility product of the substance.
Principle & Indicator
Used for determining Chlorides (Cl⁻) and Bromides (Br⁻). The titrant is Silver Nitrate (AgNO₃) and the indicator is Potassium Chromate (K₂CrO₄).
- Titrant: AgNO₃ (Standard)
- Indicator: 5% Potassium Chromate
- End Point: Appearance of a permanent Brick-Red precipitate of Silver Chromate (Ag₂CrO₄).
pH Condition (Very Important)
The titration must be performed in a Neutral or Weakly Alkaline solution (pH 6.5 to 9.0).
- If too acidic (pH < 6.5): The chromate ion gets converted to dichromate, which delays the end point.
- If too basic (pH > 9.0): Silver reacts with OH⁻ to form a black precipitate of Silver Oxide (Ag₂O), ruining the titration.
End Point: 2AgNO₃ + K₂CrO₄ → Ag₂CrO₄↓ (Brick-Red Ppt) + 2KNO₃
Unlike Mohr's method which is direct, Volhard's method is an Indirect (Back) Titration. It is performed in a strongly acidic medium (Nitric acid).
| Feature | Details |
|---|---|
| Reagents Used | Excess standard AgNO₃ is added to the sample. The unreacted AgNO₃ is back-titrated with standard Ammonium Thiocyanate (NH₄SCN). |
| Indicator | Ferric Alum [FeNH₄(SO₄)₂] |
| End Point | Formation of a Reddish-Brown complex of Ferric Thiocyanate [Fe(SCN)²⁺]. |
| pH Condition | Strongly Acidic (Dil. HNO₃). This prevents precipitation of iron as ferric hydroxide. |
Why Modified Volhard's Method?
When titrating Chlorides using Volhard's method, a problem occurs. Silver Chloride (AgCl) is more soluble than Silver Thiocyanate (AgSCN). Therefore, the thiocyanate titrant reacts with the already formed AgCl, leading to a fading end point and inaccurate results.
The Modification: To prevent this, AgCl is either filtered out, or a coagulating agent like Nitrobenzene is added. Nitrobenzene coats the AgCl precipitate with an oily layer, protecting it from reacting with the thiocyanate titrant.
Fajans method is a direct titration where the end point is indicated by a color change on the surface of the precipitate, not in the solution. This is done using an Adsorption Indicator (e.g., Fluorescein, Eosin).
The estimation of Sodium Chloride is typically performed using Mohr's Method (Direct Argentometric Titration).
Assay Procedure Overview:
- Sample: Accurately weighed NaCl dissolved in water.
- Titrant: Standardized 0.1 N Silver Nitrate (AgNO₃).
- Indicator: 5% w/v Potassium Chromate solution.
- Process: Titrate the NaCl solution with AgNO₃ until the first appearance of a permanent faint brick-red color of Silver Chromate.
- Factor: Each ml of 0.1 N AgNO₃ is equivalent to 0.005844 g of NaCl.