Qualitative Analysis of IV Group Cations

Group IV  cations are calcium (II) Ca²⁺, strontium (II) Sr²⁺ and barium (II) Ba²⁺. Its group reagent is 1M solution of ammonium carbonate (NH₄)₂CO₃  in neutral or alkaline medium. The medium needs to be neutral or alkaline because it is readily decomposed even by weak acids like acetic acid.

CO₃^2-+ CH₃COOH ⇌ CO₂ ↑ + H₂O + CH₃COO^-

This reagent is easily hydrolyzed but in alkaline medium, the equilibrium shifts backwards and reagent remains un-hydrolysed.

CO₃^2-+ H₂O ⇌ HCO₃^- + OH^-

Cations of group IV precipitate in the form of carbonate. For the test of IV group cations, take the filtrate ofIII(B) group and follow these steps:

1.  Transfer the filtrate of III(B) to a porcelain dish and acidify it with dilute acetic acid. Evaporate it to a pasty mass, and allow to cool.
2. Add 3-4ml concentrated nitric acid HNO₃, and heat until the mixture is dry. Then heat more strongly until all ammonium salts are volatilised and allow it to cool.
3. Add 3ml 2M hydrochloric acid HCl and 10ml water, heat till warm while stirring to dissolve the salt.
4. Add 0.25g solid ammonium chloride NH₄Cl and add concentrated ammonia NH₃ solution to make it alkaline and add group reagent 1M ammonium carbonate (NH₄)₂CO₃in slight excess. Place it in water bath at 50-60°C for 5minutes and keep stirring. Boiling the reagent is necessary for the removal of ammonium hydrogen carbonate (NH₄)HCO₃and ammonium carbamate NH₄O(NH₂)CO₃. These salts are present as impurities in commercial (NH₄)₂CO₃and they can interfere in analysis, as they are soluble in water.
5. Filter and wash with little hot water. Precipitate may contain Barium carbonate BaCO₃, Strontium carbonate SrCO₃ and Calcium carbonate CaCO₃.

          Ba²⁺+ CO₃^2- ⟶ BaCO₃↓

          Ca²⁺+ CO₃^2- ⟶ CaCO₃↓

          Sr²⁺+ CO₃^2- ⟶ SrCO₃↓

Separation of Ba²⁺ from Sr²⁺ and Ca²⁺

Carbonates of all the three cations are insoluble in water but soluble in acetic acid and in dilute mineral acid. So, we can dissolve the precipitate of IV group cations in dilute acetic acid.

BaCO₃↓ + H⁺ ⟶ Ba²⁺ + H₂O + CO₂

CaCO₃↓ + H⁺ ⟶ Ca²⁺ + H₂O + CO₂

SrCO₃↓ + H⁺ ⟶ Sr²⁺ + H₂O + CO₂

This way we get thesolution of cations of IV group, let's call it solution (A). These cations form chromate salt on reacting with potassium chromate solution. These chromate salts have different solubilities; barium chromate is insoluble in dilute acetic acid while the other two are soluble. By using this difference in solubility, we can separate Ba²⁺ from others. 

Test a small amount of solution with potassium chromate K₂CrO₄ solution, a yellow precipitate indicates Ba²⁺. Solubility product of SrCrO₄ and CaCrO₄ is much higher than BaCrO₄, so they need much higher concentration of CrO₄^2- ion to precipitate them. That’s why they can’t get precipitated from dilute solution.

Ba²⁺ + CrO₄^2- ⟶ BaCrO₄ ↓

If Barium is absent then proceed without adding potassium chromate to the whole solution. If barium is present then heat the rest of the solution (A) till boiling and add slight excess of 0.1M potassium chromate K₂CrO₄ solution until the solution becomes yellow coloured and precipitation is complete. Filter and wash the precipitate with little hot water. The precipitate contains BaCrO₄. Keep the filtrate (B) and washings for the test of Sr²⁺ and Ca²⁺.

Confirmatory Test for Ba²⁺

Wash the precipitate with hot water. Dissolve it in concentrated hydrochloric HCl acid. HCl coverts barium chromate into soluble dichromate. Divide the solution in two parts.

BaCrO₄ ↓ + H⁺  ⟶ BaCr₂O₇+ H₂O

Part 1: Dilute it with water and add dilute sulphuric H₂SO₄acid. A white precipitate of barium sulphate is formed which is insoluble in dilute acids but soluble in concentrated sulphuric H₂SO₄ acid. This confirms Ba²⁺.

BaCr₂O₇ + H₂SO₄⟶ Ba SO₄ + H₂Cr₂O₇

Part 2: Evaporate the solution to dryness and apply flame test. Green or yellowish green flame confirms Ba²⁺.

Test for Sr²⁺ and Ca²⁺

If Ba²⁺ is absent: Boil the solution (a) for 1 min to expel excess CO₂  and test for Sr²⁺ and Ca²⁺.

If Ba²⁺ is present: Take the filtrate (b), which we saved after removal of Ba²⁺ as chromate, and neutralised it with 2M ammonia NH₃ solution and add excess of ammonium carbonate (NH₄)CO₃ solution or you can add a little solid sodium carbonate Na₂CO₃. A white precipitate indicates SrCO₃or CaCO₃ or both. Wash the precipitate with hot water and dissolve it in 4ml acetic acid and boil to remove excess CO₂.

Add 2ml saturated solution of ammonium sulphate (NH₄)₂SO₄ , followed by 0.2g sodium thiosulphate Na₂S₂O₃ and heat in a water bath for 5min and allow to stand for few minutes then filter. Sulphates of Sr²⁺ and Ca²⁺ are formed. Strontium sulphate is insoluble in ammonium sulphate solution, so the  white precipitate we get  here is strontium sulphate and calcium sulphate goes in the filtrate.

Ca²⁺+ SO₄^2- ⟶ CaSO₄

Sr²⁺+ SO₄^2-  ⟶ SrSO₄↓

Confirmatory Test for Sr²⁺

Transfer the white precipitate along with filter paper to a small crucible, heat until precipitate has charred. Moisten the ash with concentrated HCl and apply flame test. Crimson flame confirms Sr²⁺.

Confirmatory Test for Ca²⁺

Take the filtrate and add a little 0.1M ammonium oxalate (NH₄)₂C₂O₄solution and 2ml of 2M acetic acid CH₃COOH, warm the solution on a water bath. White precipitate of calcium oxalate is formed which is insoluble in water as well as in acetic acid. This white precipitate of calcium oxalate CaC₂O₄ confirms Ca²⁺.

Ca²⁺ + (COO)₂^2- ⟶ Ca(COO)₂ ↓

We have successfully separated the cations of group IV. In the next post of analytical chemistry we will analyse the group V cations in the filtrate of group IV.

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