BP106T Unit 5 – Electrolytes, Dental Products & Radiopharmaceuticals
Unit 5 covers advanced analytical chemistry techniques: Complexometric titrations (EDTA chelation structure, 1:1 metal-EDTA stoichiometry, metallochromic indicators: Eriochrome Black T, Xylenol Orange; Masking and Demasking agents; official assays of Calcium Gluconate and Zinc Sulphate), Precipitation titrations (Mohr's, Volhard's, and Fajan's adsorption indicator methods; assay of Sodium Chloride), and Gravimetric analysis (precipitation, digestion, filtration, washing, ignition, and weighing of Barium as Barium Sulphate).
📋 Prescribed Syllabus Topics
📖 Detailed Lecture Notes & Solved Examples
11. Complexometric Titrations: EDTA Chelation & Metallochromic Indicators
Dedicated Webpage➔Complexometric Titration is a volumetric method based on the formation of an un-ionized, water-soluble, stoichiometric coordination complex (chelate) between a metal cation and a complexing ligand. Disodium Ethylenediaminetetraacetate (Disodium EDTA, $Na_2H_2Y\cdot 2H_2O$) is the standard hexadentate chelating titrant. It coordinates through two amino nitrogen atoms and four carboxylate oxygen atoms, enveloping metal cations ($Ca^{2+}, Mg^{2+}, Zn^{2+}, Al^{3+}$) in an octahedral cage with strict 1 : 1 metal-to-ligand stoichiometry regardless of metal cation valence.EDTA ek hexadentate chelating ligand hai jo Calcium aur Zinc jaise metal ions ke sath 1 : 1 ratio me pakka complex banata hai. Eriochrome Black T (EBT) indicator pH 10 par pehle metal ke sath judkar Wine-Red color deta hai, aur jab saara metal EDTA se jud jata hai toh end point par solution deep Blue ho jata hai.
▶Metallochromic Indicator Mechanism (Eriochrome Black T / Mordant Black II)
How metal-indicator color transitions indicate the exact equivalence point.
| Pharmaceutical Drug Assay | Buffer System | Metallochromic Indicator | Sharp End-Point Color Transition |
|---|---|---|---|
| Calcium Gluconate IP | Strong NaOH (pH 12 - 13) | Calcon / Patton-Reed's Indicator | Pink / Red -> Pure Bright Blue |
| Zinc Sulphate IP | Ammonia-Ammonium Chloride (pH 10) | Eriochrome Black T (Mordant Black II) | Wine-Red -> Deep Blue |
| Aluminium Hydroxide Gel | Acetate Buffer (pH 4.5, Back Titration) | Dithizone / Xylenol Orange | Yellow -> Red (Back titration with $ZnSO_4$) |
| Bismuth Subcarbonate | Dilute Nitric Acid (pH 1 - 2) | Xylenol Orange | Red-Violet -> Lemon Yellow |
- •1 : 1 Stoichiometry: 1 mole of EDTA always reacts with exactly 1 mole of metal cation ($Ca^{2+}, Zn^{2+}, Fe^{3+}, Al^{3+}$).
- •Stability Constant ($K_f$): The Metal-EDTA complex must be significantly more stable than the Metal-Indicator complex ($K_{\text{Metal-EDTA}} > K_{\text{Metal-Indicator}}$).
- •Masking Agents: Reagents added to prevent interfering metal ions from reacting with EDTA without physically separating them (e.g., Potassium Cyanide $KCN$ masks $Cu^{2+}, Zn^{2+}, Ni^{2+}$, while allowing $Ca^{2+}$ and $Mg^{2+}$ to titrate).
- •Demasking Agents: Reagents that selectively break down a masked complex, releasing the metal for subsequent titration (e.g., Formaldehyde-Acetic acid demasks $Zn^{2+}$ from its cyanide complex).
22. Precipitation Titrations: Mohr's, Volhard's & Fajan's Methods
Dedicated Webpage➔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).Precipitation titration me Silver Nitrate (AgNO3) se precipitate banta hai. Mohr's method me neutral pH par Potassium chromate se Brick-Red (eent jaisa laal) Ag2CrO4 banta hai. Volhard's method me acidic medium me excess AgNO3 daal kar thiocyanate se back-titration karte hain aur AgCl ko coat karne ke liye Nitrobenzene milate hain. Fajan's method me Fluorescein dye precipitate par chipak kar pink color deti hai.
▶Comparison of the Three Classic Argentometric Titration Methods
Reactions, conditions, and official pharmacopoeial applications.
| 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 |
- •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.
33. Gravimetric Analysis: Principles & Assay of Barium as Barium Sulphate
Dedicated Webpage➔Gravimetric Analysis is a classical quantitative analytical technique based on the separation of an analyte from solution as a sparingly soluble, chemically stable precipitate, followed by filtration, washing, thermal drying or high-temperature furnace ignition, and highly accurate weighing on an analytical balance. Gravimetric steps strictly follow: (1) Sample Dissolution $\rightarrow$ (2) Precipitation with excess precipitating reagent $\rightarrow$ (3) Digestion / Ostwald Ripening (heating on a water bath below boiling for 1-2 hours to dissolve small unstable micro-crystals and recrystallize onto larger, purer crystal lattices, reducing co-precipitation) $\rightarrow$ (4) Filtration using Ashless Filter Paper (Whatman No. 42) $\rightarrow$ (5) Washing to remove adsorbed impurities $\rightarrow$ (6) Drying and Ignition in a tared silica crucible at 800-900°C $\rightarrow$ (7) Cooling in a desiccator and weighing to constant mass.Gravimetric analysis me liquid sample me precipitating agent daal kar insoluble BaSO4 banaya jata hai. Digestion (water bath par garam karna) se chote crystals ghulkar bade aur shuddh crystals ban jate hain. Phir Ashless filter paper se chhan kar 800-900°C par jalaya jata hai aur Gravimetric factor (0.5884) se exact wajan nikala jata hai.
▶Gravimetric Assay of Barium Sulphate ($BaSO_4$)
Standard calculation using the Gravimetric Factor.
- •Digestion (Ostwald Ripening): Minimizes co-precipitation (inclusion/occlusion) and produces large, easily filterable crystalline precipitates.
- •Ashless Filter Paper: Leaves less than 0.0001 g of ash residue upon combustion in a furnace.
- •Coprecipitation: Contamination of precipitate by substances that are normally soluble under experimental conditions (Occlusion, Surface adsorption, Isomorphous inclusion).
- •Postprecipitation: Deposition of a second, normally soluble substance onto the primary precipitate surface after a prolonged delay (e.g., $MgC_2O_4$ depositing onto $CaC_2O_4$).
- In a 10-mark question on Argentometric Titrations, contrast Mohr's, Volhard's, and Fajan's methods in terms of principle, pH conditions, indicators, and end-point color transitions.
- Explain the sequential steps of Gravimetric Analysis and define Digestion, Coprecipitation, and Postprecipitation.