Analytical Chemistry: Uses of Ammonium Hydroxide and Sodium Hydroxide

ICSE Grade 10 Chemistry - Chapter 4

📚 Smart Summary

1. Ammonium Hydroxide (NH₄OH) - Introduction

Formula: NH₄OH (also written as NH₃ + H₂O). Weak base, aqueous solution of ammonia.

Preparation: NH₃ (ammonia gas) dissolved in water: NH₃ + H₂O ⇌ NH₄OH

Nature: Weak alkali, partial ionization. Pungent smell of ammonia.

Use as Reagent: Important analytical reagent to identify metal ions based on precipitate colors.

Physical Properties: Colorless solution, pungent odor, turns red litmus blue.

Chemical Properties: Reacts with acids to form ammonium salts. Example: NH₄OH + HCl → NH₄Cl + H₂O

2. Action of NH₄OH on Metal Salt Solutions

Calcium (Ca²⁺): No precipitate or white precipitate (slight). CaCl₂ + 2NH₄OH → Ca(OH)₂ + 2NH₄Cl

Lead (Pb²⁺): White precipitate of Pb(OH)₂, insoluble in excess. Pb(NO₃)₂ + 2NH₄OH → Pb(OH)₂↓ (white) + 2NH₄NO₃

Zinc (Zn²⁺): White precipitate, soluble in excess giving colorless solution. ZnSO₄ + 2NH₄OH → Zn(OH)₂↓ (white). Excess: Zn(OH)₂ + 4NH₄OH → [Zn(NH₃)₄](OH)₂ (soluble)

Iron(II) (Fe²⁺): Dirty green precipitate, insoluble in excess. FeSO₄ + 2NH₄OH → Fe(OH)₂↓ (dirty green) + (NH₄)₂SO₄

Iron(III) (Fe³⁺): Reddish-brown precipitate, insoluble in excess. FeCl₃ + 3NH₄OH → Fe(OH)₃↓ (reddish-brown) + 3NH₄Cl

Copper (Cu²⁺): Blue precipitate, soluble in excess giving deep blue solution. CuSO₄ + 2NH₄OH → Cu(OH)₂↓ (pale blue). Excess: Cu(OH)₂ + 4NH₄OH → [Cu(NH₃)₄](OH)₂ (deep blue)

3. Sodium Hydroxide (NaOH) - Introduction

Formula: NaOH. Common name: Caustic soda.

Nature: Strong alkali, complete ionization in water.

Physical Properties: White deliquescent solid, absorbs moisture from air.

Preparation: Electrolysis of brine (NaCl solution) - Chlor-alkali process.

Use as Reagent: Important analytical reagent for identification of metal ions.

Chemical Properties: Neutralizes acids, reacts with metals, reacts with ammonium salts liberating NH₃.

4. Action of NaOH on Metal Salt Solutions

Calcium (Ca²⁺): White precipitate, slightly soluble. CaCl₂ + 2NaOH → Ca(OH)₂↓ (white) + 2NaCl

Lead (Pb²⁺): White precipitate, soluble in excess. Pb(NO₃)₂ + 2NaOH → Pb(OH)₂↓ (white). Excess: Pb(OH)₂ + 2NaOH → Na₂PbO₂ + 2H₂O (sodium plumbite - soluble)

Zinc (Zn²⁺): White precipitate, soluble in excess. ZnSO₄ + 2NaOH → Zn(OH)₂↓ (white). Excess: Zn(OH)₂ + 2NaOH → Na₂ZnO₂ + 2H₂O (sodium zincate - soluble)

Aluminum (Al³⁺): White precipitate, soluble in excess. AlCl₃ + 3NaOH → Al(OH)₃↓ (white). Excess: Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O (sodium aluminate - soluble)

Iron(II) (Fe²⁺): Dirty green precipitate, insoluble in excess. Turns brown on standing (oxidation). FeSO₄ + 2NaOH → Fe(OH)₂↓ (dirty green) + Na₂SO₄

Iron(III) (Fe³⁺): Reddish-brown precipitate, insoluble in excess. FeCl₃ + 3NaOH → Fe(OH)₃↓ (reddish-brown) + 3NaCl

Copper (Cu²⁺): Blue precipitate, insoluble in excess. CuSO₄ + 2NaOH → Cu(OH)₂↓ (blue) + Na₂SO₄

5. Comparison and Identification Summary

Key Difference: NH₄OH forms complex ions (soluble) with Zn²⁺ and Cu²⁺. NaOH forms simple salts (zincate, plumbite soluble in excess).

Cu²⁺ Distinction: Blue ppt with both, but NH₄OH excess gives deep blue solution; NaOH excess keeps blue ppt.

Zn²⁺ Distinction: White ppt with both, soluble in excess of both, but nature of solution different (complex vs zincate).

Fe²⁺ vs Fe³⁺: Green ppt = Fe²⁺; Reddish-brown ppt = Fe³⁺. Both insoluble in excess reagent.

Amphoteric Behavior: Zn(OH)₂, Pb(OH)₂, Al(OH)₃ dissolve in excess NaOH (amphoteric hydroxides).

Practical Application: Used in qualitative analysis to identify unknown metal cations in solution.