BP106T • Unit 5Topic 2 of 3⏱ 5 Min Read
2. Precipitation Titrations: Mohr's, Volhard's & Fajan's Methods
💡 Core Concept & Clinical Context:
Precipitation Titration is a titrimetric method based on the formation of a sparingly soluble precipitate of low solubility product ($K_{sp}$) upon addition of standard titrant (typically Silver Nitrate, $AgNO_3$; hence termed Argentometric Titrations). Argentometric end-points are determined using three distinct classical techniques: Mohr's Method (Direct titration forming colored secondary precipitate), Volhard's Method (Indirect back-titration in acidic medium), and Fajan's Method (Direct titration using adsorption indicators).
Clinical Scenarios & Pharmacopoeial Examples:
1Comparison of the Three Classic Argentometric Titration Methods
Reactions, conditions, and official pharmacopoeial applications.
Step-by-Step Breakdown:
•A. MOHR'S METHOD (Direct Method):
•• Analyte: Chloride (Cl-) or Bromide (Br-) in Neutral / Slightly Alkaline pH (6.5 - 9.0).
•• Titrant: Standard Silver Nitrate (AgNO3).
•• Indicator: Potassium Chromate (K2CrO4, 5% w/v).
•• Reactions:
•1. Primary: Ag+ + Cl- -> AgCl (White precipitate) v [Ksp = 1.8 x 10^-10]
•2. End Point: 2 Ag+ + CrO4(2-) -> Ag2CrO4 (Brick-Red precipitate) v [Ksp = 1.1 x 10^-12]
•• Crucial Condition: pH must be 6.5-9.0. If acidic (pH < 6.5), chromate converts to dichromate (HCrO4-), failing to precipitate; if alkaline (pH > 10), silver precipitates as black silver oxide (Ag2O).
•B. VOLHARD'S METHOD (Indirect Back-Titration):
•• Analyte: Halides (Cl-, Br-, I-) in Strongly Acidic Medium (Dilute HNO3).
•• Principle: Excess known standard AgNO3 is added to halide. Excess unreacted Ag+ is back-titrated with standard Ammonium Thiocyanate (NH4SCN) using Ferric Ammonium Sulphate [Ferric Alum] indicator.
•• Reactions:
•1. Ag+ (excess) + Cl- -> AgCl v + Ag+ (unreacted)
•2. Back Titration: Ag+ (unreacted) + SCN- -> AgSCN (White precipitate) v
•3. End Point: Fe(3+) (from alum) + SCN- -> [Fe(SCN)]2+ (Blood-Red Complex)
•• Precaution for Chloride Assay: AgCl is more soluble than AgSCN (Ksp AgCl = 1.8x10^-10 > Ksp AgSCN = 1.0x10^-12). Unreacted SCN- can displace Cl- from AgCl:
•AgCl + SCN- -> AgSCN + Cl- (Causes fading end point and high titer error).
•Correction: Add 2-3 mL of Nitrobenzene to coat and coagulate the AgCl precipitate, or filter off AgCl before back-titrating!
•C. FAJAN'S METHOD (Adsorption Indicator Method):
•• Analyte: Halide titrated directly with standard AgNO3 using an organic dye adsorption indicator (Fluorescein, Eosin, Dichlorofluorescein).
•• Mechanism:
•- Before End Point: Excess Cl- adsorbs onto AgCl precipitate particles -> Primary adsorbed layer is Cl-, attracting secondary counter-ions (Na+) [Colloid has negative charge; Fluorescein anion is repelled -> Greenish-yellow in solution].
•- At End Point: Excess Ag+ replaces Cl- in primary layer -> Colloid acquires positive charge -> Adsorbs modified Fluoresceinate dye anion onto precipitate surface -> Electronic resonance shift produces a brilliant SALMON-PINK / ROSE-RED color directly on precipitate surface!
Comparison & Reference Table:
| Method Name | Titration Type | Medium / pH | Indicator Used | End-Point Visual Observation |
|---|---|---|---|---|
| Mohr's Method | Direct Titration | Neutral / Mildly basic (pH 6.5 - 9.0) | Potassium Chromate ($K_2CrO_4$) | Formation of Brick-Red $Ag_2CrO_4$ precipitate |
| Volhard's Method | Indirect Back-Titration | Strongly Acidic (Dilute $HNO_3$) | Ferric Ammonium Sulphate (Alum) | Blood-Red soluble $[Fe(SCN)]^{2+}$ complex |
| Fajan's Method | Direct Adsorption | Neutral / Weakly Acidic (pH 4 - 7) | Fluorescein / Dichlorofluorescein | Salmon-Pink color on surface of $AgCl$ particles |
📋Key Takeaways & Exam Highlights
- Mohr's Method: Direct, neutral pH (6.5-9.0), $K_2CrO_4$ indicator $\rightarrow$ Brick-Red $Ag_2CrO_4$ precipitate.
- Volhard's Method: Indirect back-titration, acidic medium ($HNO_3$), Ferric alum indicator $\rightarrow$ Blood-Red $[Fe(SCN)]^{2+}$ complex; Nitrobenzene added to coat $AgCl$.
- Fajan's Method: Direct, adsorption indicator (Fluorescein/Eosin) $\rightarrow$ Salmon-Pink surface adsorption.
- Official Assay of Sodium Chloride IP: Standard Volhard's method with nitrobenzene.
🇮🇳 हिन्दी व्याख्या (2. Precipitation Titrations: Mohr's, Volhard's & Fajan's Methods)
मोहर विधि (Mohr's) में न्यूट्रल pH पर क्रोमेट इंडिकेटर से ईंट जैसा लाल (Brick-red) अवक्षेप बनता है। वोल्हार्ड विधि (Volhard's) में एसिडिक माध्यम में नाइट्रोबेंजीन और फेरिक एलम से खून जैसा लाल (Blood-red) रंग बनता है। फजान विधि (Fajan's) में फ्लोरोसीन डाई सतह पर चिपककर गुलाबी रंग देती है।