Volumetric and Salt Analysis Techniques for Chemistry

Volumetric Analysis (Titration)

Experiment: KMnO₄ vs Mohr’s salt (Ferrous ammonium sulfate)

Indicator: Self-indicator (KMnO₄)

End point: Colourless → Permanent light pink

Ionic equations:

• Reduction: MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O
• Oxidation: Fe²⁺ → Fe³⁺ + e^–

Rapid Viva Questions

1. Why add dilute H₂SO₄?
   • Provides an acidic medium and prevents KMnO₄ from getting reduced to MnO₂ (brown precipitate).
2. Why not HCl?
   • HCl reduces KMnO₄, producing Cl₂ gas.
3. Why not HNO₃?
   • Strong oxidizer; interferes with the reaction.
4. Mohr’s salt formula?
   • FeSO₄ · (NH₄)₂SO₄ · 6H₂O
5. Why heat oxalic acid (if used)?
   • Heating speeds the reaction; Mohr’s salt usually reacts at room temperature.

Salt Analysis (Qualitative)

Order: Preliminary → Anion test → Cation test

Anion Analysis (Acid Radicals)

Order: Dilute acid → Concentrated acid → Independent tests

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Table (text version):

| GRP    | REAGENT         | OBSERVATION              | INFERENCE       | CONFIRMATORY TEST                  | 
|--------|-----------------|--------------------------|-----------------|---------------------------------| 
| DILUTE | Dil. H2SO4      | Brisk effervescence      | Carbonate (CO3) | Gas turns lime water milky      | 
|       | Dil. H2SO4      | Smell of burning sulphur | Sulphite (SO3)  | K2Cr2O7 paper turns Green        | 
|--------|-----------------|--------------------------|-----------------|---------------------------------| 
| CONC.  | Conc. H2SO4     | Pungent white fumes      | Chloride (Cl)   | AgNO3 + HNO3 -> White ppt       | 
|        | Conc. H2SO4     | Reddish-brown fumes      | Bromide (Br)    | AgNO3 + HNO3 -> Pale Yellow ppt | 
|        | Conc. H2SO4     | Violet fumes             | Iodide (I)      | AgNO3 + HNO3 -> Yellow ppt      | 
|        | Conc. H2SO4     | Brown fumes (add Cu)     | Nitrate (NO3)   | Brown Ring Test (at junction)   | 
|        | Conc. H2SO4     | Vinegar smell            | Acetate (CH3COO)| Ester Test -> Fruity smell      | 
|--------|-----------------|--------------------------|-----------------|---------------------------------| 
| INDEP. | BaCl2 (aq)      | White ppt                | Sulphate (SO4)  | Ppt insoluble in Conc. HCl      | 
|        | Amm. Molybdate  | Canary yellow ppt        | Phosphate (PO4) | (Boil with Conc HNO3 first)     | 

Cation Analysis (Basic Radicals)

(Add reagents to the original solution as indicated.)

| GRP | REAGENT (Add to O.S.)   | CATION      | PPT / COLOR      | CONFIRMATORY TEST                  | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| 0   | NaOH + Heat             | NH4+        | Ammonia smell    | Nessler's Reagent -> Brown ppt     | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| I   | Dil. HCl                | Pb2+        | White ppt        | Dissolve in hot water + KI         | 
|     |                         |             |                  | -> Golden Yellow ppt               | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| II  | Dil. HCl + H2S gas      | Cu2+        | Black ppt        | Dissolve in HNO3 + Excess NH4OH    | 
|     |                         |             |                  | -> Deep Blue Solution              | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| III | NH4Cl + Excess NH4OH   | Fe3+        | Red-brown ppt    | K-Ferrocyanide -> Prussian Blue    | 
|     |                         | Al3+        | Gelatinous White | Lake Test -> Blue floating mass    | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| IV  | NH4OH + H2S gas         | Zn2+        | Dirty White ppt  | NaOH + H2S -> White ppt            | 
|     |                         | Mn2+        | Buff / Flesh ppt | Melt (KOH+KNO3) -> Green mass      | 
|     |                         | Ni2+        | Black ppt        | DMG Test -> Rose Red ppt           | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| V   | (NH4)2CO3               | Ba2+        | White ppt        | Flame Test: Apple Green            | 
|     | (presence of NH4Cl)     | Sr2+        | White ppt        | Flame Test: Crimson Red            | 
|     |                         | Ca2+        | White ppt        | Flame Test: Brick Red              | 
|-----|-------------------------|-------------|------------------|------------------------------------| 
| VI  | Disodium H-Phosphate    | Mg2+        | White ppt        | Ash Test -> Pink Ash               | 
|--------------------------------------------------------------------------------|

Quick Viva Tips

• NH₄⁺ + NaOH → NH₃ (gas)
• Group III hydroxides precipitate due to the common-ion effect
• Distinguish Prussian Blue vs Lake Test
• Know Ni confirmatory DMG test
• Flame test colors for Ba, Sr, Ca

Organic Functional Group Analysis

Functional Group — Test — Reagent — Observation

Carboxylic Acid | NaHCO₃ test | Sodium bicarbonate solution (NaHCO₃) | Brisk effervescence (CO₂)
Alcohol (-OH) | Ceric Ammonium Nitrate test | Ceric ammonium nitrate in dilute nitric acid | Red color
Aldehyde (-CHO) | Tollen’s test | Ammoniacal silver nitrate (Tollen’s reagent) | Silver mirror
Ketone (>C=O) | Sodium Nitroprusside test | Sodium nitroprusside + NaOH | Red / Wine red
Phenol (-OH) | Neutral FeCl₃ test | Neutral ferric chloride solution | Violet color
Amine (-NH₂) | Azo dye test | Nitrous acid (NaNO₂ + HCl, cold) + β-naphthol | Orange / Red azo dye
Amine (-NH₂) | Litmus test (alt.) | Moist red litmus paper | Turns blue

Viva Tip: Alcohols also give red with Ceric Ammonium Nitrate; phenols may give false positives with strong oxidizers.

Master Viva Voce

Salt Analysis Viva

• NH4Cl suppresses NH4OH ionization (common-ion effect).
• Brown ring: [Fe(H₂O)₅NO]²⁺.
• Nessler’s reagent: K₂HgI₄ (detects NH₄⁺).
• Blue litmus → red with ammonium salts (acidic).

General & Project Viva

• Standard solution → exact molarity known.
• Molality > Molarity (volume changes with temperature; mass does not).
• Project: Aim + one application + main reaction.

Extra Veteran Tips

• Memorize gas evolution (NH₃, CO₂).
• Clear distinction: Prussian Blue vs Lake Test.
• Quick glance tables = 2–3 min full recall.

Quick Viva “Must Knows”

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1. Titration (KMnO₄ vs Mohr’s Salt):
   • Indicator: KMnO₄ (self-indicator).
   • Acid used: Dilute H₂SO₄ (prevents MnO₂ brown ppt).
   • Endpoint: Colourless to permanent light pink.
   • Equation: MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O.
2. Brown ring test (Nitrate):
   • Formula: [Fe(H₂O)₅NO]²⁺.
   • Name: Nitroso ferrous sulphate.
3. Group III logic:
   • Why NH4Cl before NH4OH? To suppress ionization of NH4OH (common ion effect) so only Group III precipitates.
4. Flame test colors:
   • Barium (Ba): Apple green.
   • Strontium (Sr): Crimson red.
   • Calcium (Ca): Brick red.
   • Copper (Cu): Bluish green.
5. Differences:
   • Qualitative = WHAT is present (salt analysis).
   • Quantitative = HOW MUCH is present (titration).
   • Double salt (Mohr’s salt) vs complex salt (K₄[Fe(CN)₆]).
   • Double salts dissociate completely; complexes do not.

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