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Analysis of Aluminium Nitrate (Al(NO₃)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 copper / iron / ammonium
SolubilitySoluble in waterNitrates are soluble
Dry heatingMay give brown fumes of NO₂ on strong heatingNitrate indicated
Conc. H₂SO₄ + Cu turningsBrown fumes; solution may turn blue from CuNO₃⁻ may be present
Dilute H₂SO₄No CO₂Not carbonate
Flame testNo characteristic colourNot Ba / Ca / Sr / Cu

Test of Anion (NO₃⁻)

ExperimentObservationInference
Heat the salt with conc. H₂SO₄ and copper turningsBrown fumes of NO₂; solution often turns blue with CuNO₃⁻ may be present
Confirmatory Test
Brown ring
To 1 mL of the solution add freshly prepared FeSO₄, then pour conc. H₂SO₄ slowly down the side		Brown ring at the junction of the two layers (nitroso-ferrous sulphate)NO₃⁻ is confirmed

Ionic equations

  • 2NO₃⁻ + 4H₂SO₄ + 6Fe²⁺ → 2NO + 6Fe³⁺ + 4SO₄²⁻ + 4H₂O
  • FeSO₄ + NO → [Fe(H₂O)₅NO]SO₄ (brown ring)

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 + OH⁻ → [Al(OH)₄]⁻

Result

The given salt contains Al³⁺ as the cation (basic radical) and NO₃⁻ as the anion (acidic radical). The salt is Aluminium Nitrate (Al(NO₃)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 brown ring?

    Nitroso-ferrous sulphate at the junction with conc. H₂SO₄.

  2. Why Group III for Al?

    Al(OH)₃ ppts with NH₄Cl + NH₄OH.

  3. Why amphoteric?

    Al(OH)₃ dissolves in excess NaOH; Fe(OH)₃ does not.

  4. Why NH₄Cl in Group III?

    To keep [OH⁻] low.

  5. Why not BaCl₂?

    Anion is nitrate, not sulphate.

  6. Why white salt?

    Al³⁺ is colourless.

  7. Why Cu turnings with conc. H₂SO₄?

    They reduce nitrate to NO₂, a preliminary nitrate test.

  8. Does Al³⁺ give a flame colour?

    No useful flame test in school labs.

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