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Analysis of Magnesium Sulphate (MgSO₄)

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 (Epsom salt); odourlessNot copper / iron
SolubilitySoluble in waterMgSO₄ is soluble
Dry heatingWater of crystallisation lostHydrated sulphate
Flame testNo characteristic colourNot Ba / Ca / Sr
Dilute / conc. H₂SO₄No characteristic gasNot CO₃²⁻ / halide / nitrate
Groups I-VNo ppt with the group reagentsProceed to Group VI

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 (Mg²⁺)

ExperimentObservationInference
Confirmatory Tests
Disodium hydrogen phosphate
Groups I-V reagents give no ppt. Add Na₂HPO₄ and scratch the walls		White crystalline ppt of MgNH₄PO₄Group VI (Mg²⁺) is confirmed
Magneson
To the alkaline solution add a drop of Magneson I		Blue lake / blue pptMg²⁺ is confirmed

Ionic equations

  • Mg²⁺ + NH₄⁺ + PO₄³⁻ → MgNH₄PO₄ ↓

Result

The given salt contains Mg²⁺ as the cation (basic radical) and SO₄²⁻ as the anion (acidic radical). The salt is Magnesium Sulphate (MgSO₄).

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 scratch the walls?

    MgNH₄PO₄ is slow to crystallise; scratching starts precipitation.

  2. Why NH₄⁺ in the ppt?

    The ppt is magnesium ammonium phosphate.

  3. Why Magneson?

    Adsorption indicator / lake test for Mg(OH)₂.

  4. Why is Mg not Group V?

    In the presence of NH₄Cl, MgCO₃ does not ppt.

  5. Why white?

    Mg²⁺ has no d-electrons of a chromophore.

  6. Epsom salt?

    MgSO₄·7H₂O.

  7. Why BaCl₂ still works?

    Sulphate is independent of the cation group.

  8. Could this be ZnSO₄?

    Zn would have given white ZnS in Group IV.

Related salts