UNIT-2E-Q.-A.-OF-GROUP-4-5-CATIONS

UNIT 2A

Qualitative Analysis of Cations

  • Instructor: Miroma R. Villacrusis

Group IV Cations

  • Cations involved: Ba²⁺, Sr²⁺, Ca²⁺

  • Known as "Alkaline earth group" or "Ammonium carbonate group"

  • Group reagent: NH4Cl + NH2OH + (NH4)2CO3

    • Cations precipitate as carbonate in alkaline medium.

  • Similar properties due to proximity in the periodic table make separation challenging.

Procedure for Separation from Group III-B

  1. Evaporate centrifugate until nearly dry.

  2. Add NH4Cl and (NH4)2CO3 solution.

  3. Heat to 60°C and wait for residue.

  4. Expected white precipitates:

  • BaCO3

  • SrCO3

  • CaCO3

Questions and Answers

Q1: Purpose of Evaporating Centrifugate

  • To expel H₂S, prevent oxidation of S²⁻ to SO₄²⁻, and stop Ba²⁺ and Sr²⁺ precipitation as SO₄²⁻.

  • CaSO4 has a higher Ksp, thus requires greater SO₄²⁻ concentration to precipitate.

Q2: Moderate Amount of NH4+ Addition

  • Excess NH4+ decreases carbonate ion concentration:

    • NH4+ + CO3²⁻ → NH3 + HCO3⁻

  • Solution must remain slightly basic; acidity can convert carbonate to soluble bicarbonate.

Q3: Use of NH4Cl

  • Reduces OH⁻ and CO3²⁻ ion concentrations by the common ion effect, preventing Mg precipitation as Mg(OH)2 or MgCO3.

  • Aids in coagulation of the precipitate.

Q4: Use of NH4OH

  1. Converts NH4HCO3 to (NH4)2CO3.

  2. Converts ammonium carbamate into carbonate.

  3. Ensures a mildly alkaline medium to prevent bicarbonate formation (soluble).

Q5: Heating to 60°C

  1. Converts HCO3⁻ to CO2.

  2. Aids in forming more crystalline precipitate.

  3. Excess heat loses NH3 and pushes equilibrium towards bicarbonate formation.

Carbonates and Sulfates of Group IV

  • Carbonates (BaCO3, SrCO3, CaCO3) are insoluble in acetic acid.

  • Sulfates are insoluble in weak acetic acid and strong acids.

Separation of Group IV

  • Precipitate added to acetic acid dissolves:

    • BaCO3 + 2 CH3COOH → Ba(CH3COO)2 + CO2 + H2O

    • SrCO3 + 2 CH3COOH → Sr(CH3COO)2 + CO2 + H2O

    • CaCO3 + 2 CH3COOH → Ca(CH3COO)2 + CO2 + H2O

  • Testing for Barium using K2CrO4:

    • Appearance of an orange-tinted solution indicates Ba presence, resulting in a yellow precipitate of BaCrO4.

Flame Test for Barium and Calcium

  • Barium yields an apple green color.

  • Calcium yields a brick red color.

Ammonium Ion Behavior Differences

  • Ammonium ion behaves as a weak base, differing from alkali metal ions.

Group V Cations

  • Cations involved: Mg²⁺, Na⁺, K⁺, NH4⁺

  • Commonly called the "Alkali metal group" or "Soluble group" because all salts are soluble.

  • Removal of traces of alkaline earth group using ammonium oxalate and sulfate for separation.

Testing Group V Cations

Testing for Na⁺ and K⁺

  • Removal of NH4⁺ is required as it interferes with K⁺ testing.

  • Use of potassium dihydrogen antimonate or magnesium uranyl acetate for Na⁺ detection.

  • K⁺ tested by sodium cobaltic nitrite.

Flame Test Results

  • Na⁺ shows golden yellow and K⁺ shows violet in flame tests.

Conclusion

  • Effective qualitative analysis requires knowledge of chemical behaviors, reactions, and group characteristics.