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Analysis of Zinc Sulphate (ZnSO₄)

Class 12 Chemistry Practical

How to do salt analysis · Viva · Quiz

Aim

To analyse the given inorganic salt for one acidic and one basic radical.

Preliminary Test

ExperimentObservationInference
Physical examinationWhite crystalline solid; odourlessNot Cu²⁺ / Fe / Ni
SolubilitySoluble in water (white vitriol)Soluble sulphate
Dry heatingWater of crystallisation lost; white residueNot a nitrate
Dilute / conc. H₂SO₄No characteristic gasNot CO₃²⁻ / Cl⁻ / Br⁻ / I⁻ / NO₃⁻
Flame testNo characteristic colourNot Ba / Ca / Sr / Cu
Action of NaOH on a drop of solutionWhite ppt soluble in excessZn²⁺ or Al³⁺ possible; groups decide

Test of Anion (SO₄²⁻)

ExperimentObservationInference
Confirmatory Tests
Barium chloride
Acidify the water / soda extract with dilute HCl and add BaCl₂		White ppt of BaSO₄, insoluble in conc. HCl and conc. HNO₃SO₄²⁻ is confirmed
Lead acetate
Acidify with acetic acid and add lead acetate		White ppt of PbSO₄SO₄²⁻ is confirmed

Ionic equations

  • Ba²⁺ + SO₄²⁻ → BaSO₄ ↓ (white, insoluble in acids)
  • Pb²⁺ + SO₄²⁻ → PbSO₄ ↓

Test of Cation (Zn²⁺)

ExperimentObservationInference
Pass H₂S through the solution made alkaline with NH₄OH (after Group III)White ppt of ZnSGroup IV (Zn²⁺) may be present
Confirmatory Tests
Sodium hydroxide
Add NaOH dropwise, then in excess, and warm		White ppt of Zn(OH)₂ dissolves in excess NaOHZn²⁺ is confirmed
Potassium ferrocyanide
Neutralise and add K₄[Fe(CN)₆]		Bluish-white ppt of zinc ferrocyanideZn²⁺ is confirmed

Ionic equations

  • Zn²⁺ + H₂S → ZnS ↓ (white) + 2H⁺ (alkaline medium)
  • Zn²⁺ + 2OH⁻ → Zn(OH)₂ ↓ ; Zn(OH)₂ + 2OH⁻ → [Zn(OH)₄]²⁻

Result

The given salt contains Zn²⁺ as the cation (basic radical) and SO₄²⁻ as the anion (acidic radical). The salt is Zinc Sulphate (ZnSO₄).

Precautions

  1. Use small quantities of salt and reagents; do not use excess.
  2. Keep the mouth of the test tube away from the face while heating.
  3. Use freshly prepared FeSO₄ for the brown ring test.
  4. Nessler’s reagent is toxic (mercury); handle with care and do not pipette by mouth.

Viva voce

  1. Why is H₂S passed in alkaline medium for Group IV?

    [S²⁻] is higher in alkaline solution, enough to ppt ZnS / MnS / NiS / CoS.

  2. Why not in acidic medium?

    In acid, [S²⁻] is too low for ZnS (Ksp) but enough for CuS (Group II).

  3. How is Zn distinguished from Al?

    Al ppts in Group III before H₂S; Zn does not.

  4. Why bluish-white ferrocyanide ppt?

    Zinc ferrocyanide is bluish-white; copper’s is chocolate-brown.

  5. Why BaCl₂ here?

    Anion is sulphate.

  6. Why white salt?

    Zn²⁺ is d¹⁰, so no d-d colour.

  7. Why heat with excess NaOH?

    Dissolution of Zn(OH)₂ is clearer on warming.

  8. Is ZnS soluble in HCl?

    Yes. CuS is not. Used when dissolving the Group IV ppt.

Related salts