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Analysis of Aluminium Sulphate (Al₂(SO₄)₃)

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 a coloured / ammonium / acetate salt
SolubilitySoluble in waterCommon alum-type sulphate; formula is Al₂(SO₄)₃ not (Al)₂(SO₄)₃
Dry heatingWater of crystallisation may be lost; white residueNot a nitrate (no brown fumes)
Dilute H₂SO₄No gasNot carbonate
Conc. H₂SO₄No halide / NO₂ fumesNot Cl⁻ / Br⁻ / I⁻ / NO₃⁻
Flame testNo characteristic colourNot Ba / Sr / Ca / Cu

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 (Al³⁺)

ExperimentObservationInference
To the original solution add solid NH₄Cl and NH₄OH (Group III)White gelatinous ppt of Al(OH)₃Group III (Al³⁺) may be present
Confirmatory Tests
NaOH (excess)
Dissolve the ppt in dilute HCl. Add NaOH in excess and warm		White gelatinous ppt dissolves in excess NaOH (sodium aluminate)Al³⁺ is confirmed
Blue lake
To a portion add a drop of blue litmus and then NH₄OH along the sides		Blue floating lake in a colourless solutionAl³⁺ is confirmed

Ionic equations

  • Al³⁺ + 3NH₄OH → Al(OH)₃ ↓ + 3NH₄⁺
  • Al(OH)₃ + NaOH → Na[Al(OH)₄] (soluble)

Result

The given salt contains Al³⁺ as the cation (basic radical) and SO₄²⁻ as the anion (acidic radical). The salt is Aluminium Sulphate (Al₂(SO₄)₃).

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 NH₄Cl added before NH₄OH in Group III?

    Common-ion effect lowers [OH⁻] so Mg(OH)₂ does not ppt; only Al / Fe hydroxides do.

  2. Why is the ppt gelatinous?

    Al(OH)₃ is a hydrated gelatinous hydroxide.

  3. Why does it dissolve in excess NaOH?

    Al(OH)₃ is amphoteric and forms [Al(OH)₄]⁻.

  4. What is the blue lake test?

    Al(OH)₃ adsorbs blue litmus and floats as a blue lake.

  5. How is Al³⁺ distinguished from Zn²⁺?

    Zn is Group IV (white ZnS with H₂S / NH₄OH). Al ppt appears in Group III before that.

  6. Why BaCl₂ for the anion?

    Sulphate gives acid-insoluble BaSO₄.

  7. Is this the same as potash alum?

    Potash alum is K₂SO₄·Al₂(SO₄)₃·24H₂O. This writeup is the simple sulphate Al₂(SO₄)₃.

  8. Why is the salt white, not coloured?

    Al³⁺ has no d-d colour; unlike Cu²⁺ / Fe³⁺ / Ni²⁺.

Related salts

ऐलुमिनियम सल्फेट (हिन्दी में)

सूत्र Al₂(SO₄)₃ है, (Al)₂(SO₄)₃ नहीं। नमक सफेद और गंधहीन है।

क्षारीय मूलक वर्ग III: NH₄Cl + NH₄OH से सफेद जिलेटिनी अवक्षेप, ब्लू लेक परीक्षण।

अम्लीय मूलक सल्फेट: BaCl₂ से सफेद BaSO₄ जो सांद्र HCl में नहीं घुलता।