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Analysis of Lead Nitrate (Pb(NO₃)₂)

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 acetate / ammonium
SolubilitySoluble in waterAll nitrates are soluble
Dry heatingDecrepitates (crackling); brown fumes of NO₂; yellow residue of PbOLead nitrate (typical dry heating)
Flame testNo apple-green / brick-red / crimsonNot Ba / Ca / Sr
Dilute H₂SO₄No CO₂Not carbonate
Conc. H₂SO₄Brown fumes of NO₂, stronger with copper turningsNO₃⁻ may be present

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

  • 2Pb(NO₃)₂ → 2PbO + 4NO₂ ↑ + O₂ ↑ (dry heating)
  • 2NO₃⁻ + 4H₂SO₄ + 6Fe²⁺ → 2NO + 6Fe³⁺ + 4SO₄²⁻ + 4H₂O
  • FeSO₄ + NO → [Fe(H₂O)₅NO]SO₄ (brown ring)

Test of Cation (Pb²⁺)

ExperimentObservationInference
To the original solution add dilute HClWhite ppt of PbCl₂, soluble in hot water, reappears on coolingGroup I (Pb²⁺) may be present
Confirmatory Tests
Potassium iodide
To the hot solution of the ppt add KI		Yellow ppt of PbI₂ (golden spangles on cooling)Pb²⁺ is confirmed
Potassium chromate
To the acidified solution add K₂CrO₄		Yellow ppt of PbCrO₄, insoluble in acetic acidPb²⁺ is confirmed

Ionic equations

  • Pb²⁺ + 2Cl⁻ → PbCl₂ ↓
  • Pb²⁺ + 2I⁻ → PbI₂ ↓ (yellow)
  • Pb²⁺ + CrO₄²⁻ → PbCrO₄ ↓ (yellow)

Result

The given salt contains Pb²⁺ as the cation (basic radical) and NO₃⁻ as the anion (acidic radical). The salt is Lead Nitrate (Pb(NO₃)₂).

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 does lead nitrate crackle?

    Trapped water / crystals burst on heating (decrepitation), together with decomposition.

  2. Why brown fumes?

    NO₂ from thermal decomposition of nitrate.

  3. Why is the residue yellow?

    PbO (litharge) is yellow.

  4. Why freshly prepared FeSO₄?

    Fe²⁺ oxidises on standing and the ring fails.

  5. Why pour H₂SO₄ down the side?

    To form two layers so the ring appears at the junction.

  6. Why Group I before nitrate confirmatory?

    You may do anion first on soda extract; cation on original solution with HCl.

  7. Does nitrite also give a brown ring?

    Nitrite gives a ring even with dilute H₂SO₄; nitrate needs conc. H₂SO₄.

  8. Why is Pb(NO₃)₂ soluble unlike PbCl₂?

    Nitrates of all metals are soluble.

Related salts

लेड नाइट्रेट (हिन्दी में)

शुष्क ऊष्मा: चटचटाहट, भूरे NO₂ धुएँ, पीला PbO अवशेष। यह पहचान बहुत महत्वपूर्ण है।

अम्लीय मूलक नाइट्रेट: ब्राउन रिंग परीक्षण। क्षारीय मूलक लेड: वर्ग I, KI से पीला अवक्षेप।

शुष्क ऊष्मा पर भूरे धुएँ और पीला अवशेष नाइट्रेट की पहचान है।