Qualitative analysis of V group cations

Group 5 cations are magnesium (II) Mg²⁺, potassium (I) K⁺ and sodium (I) Na⁺. These cations do not react with hydrochloric acid HCl, hydrogen sulphide H₂S, ammonium sulphide and ammonium carbonate (NH₄)₂CO₃. 

Magnesium does show similar reactions to IV^th group cations; it forms basic magnesium carbonate MgCO₃.Mg(OH)­₂.5H₂O with IVth group reagent ammonium carbonate (NH₄)₂CO₃. But this basic magnesium carbonate is soluble in presence of ammonium salts and therefore it doesn’t precipitate with IV group cations.

5Mg²⁺ + 6CO₃^2- + 7H₂O ⟶ 4 MgCO₃.Mg(OH)­₂.5H₂O ↓ + 2HCO₃^-

NH₄⁺ + CO₃² ⇌ NH₃ + HCO₃^-

Magnesium carbonate doesn’t precipitate in the presence of ammonium salts due to common ion effect. Solubility product of magnesium carbonate is quite high and cannot be achieved with lower concentration of carbonate ions. High concentration of ammonium ions shifts the equilibrium in forward direction which decreases the concentration of carbonate ions causing the basic magnesium carbonate to remain soluble.

Take the filtrate of IV group in a porcelain dish and evaporate it to a pasty mass. Add 3ml concentrated nitric acid HNO₃ to dissolve it, evaporate again to dryness and heat until white fumes of ammonium salts cease to evolve. If you get a white residue, it means group V is present.

Add 4ml water to the residue, stir and warm it up for 1minute and then filter. We will test for Mg(II) in the residue, and for K(I) and Na(I) in the filtrate. 

Confirmatory test for Mg²⁺

Dissolve the residue in a few drops of dil HCl and add 2-3ml water. Divide the solution in two parts.

Part 1

Add a little ammonium chloride NH₃Cl solution followed by ammonical oxine reagent (take 1ml 2% 8-hydroxyquinoline solution and add 2M acetic acid followed by 5ml 2M ammonia solution, warm to dissolve any precipitated oxine) and heat to boiling for 1-2minutes or till the odour of ammonia becomes noticeable. You will get a pale yellow precipitate of magnesium oxine Mg(C₉H₆ON)₂.4H₂O which confirms the Mg²⁺ ion.

Part 2

Take 1-2 drops of test solution in a spot plate and add 2-3 drops of magneson I reagent (4-(4-Nitrophenylazo)-resorcinol) and add 1 drop of 2M sodium hydroxide NaOH to make it alkaline. Blue colouration or blue precipitate is formed depending on the concentration of magnesium.

We will test for K(I) and Na(I) in the filtrate we got above. If the residue of V^th group dissolves completely, dilute it up to 6ml and filter if necessary. Divide this solution into three equal parts to test magnesium (II) Mg²⁺, potassium (I) K⁺ and sodium (I) Na⁺. In first part, directly apply magneson test for the confirmation of Mg(II) ion and in other two parts test for K(I) and Na(I).

Confirmatory test for Na⁺

Add a little uranyl magnesium acetate reagent, shake and allow to stand for few minutes. Yellow crystalline precipitate of sodium magnesium uranyl acetate is formed. If precipitation doesn’t occur, add 1/3^rdvolume of ethanol; it helps in precipitation.

Na⁺ + Mg²⁺ + 3UO₂²⁺+ 9CH₃COO^- ⟶NaMg(UO₂)₃(CH₃COO)₉ ↓

If you perform flame test persistent yellow flame confirms Na⁺.

Confirmatory test for K⁺

Add a little sodium hexanitritocobaltate(III) solution and a few drops of 2M acetic acid. Stir and then allow it to stand for 1-2 minutes. Yellow precipitate of potassium hexanitritocobaltate(III) is obtained. If precipitation doesn’t occur immediately, warm it a little; it will accelerate the precipitation.

3K⁺ + [Co(NO₂)₆]^3- ⟶  K₃[Co(NO₂)₆] ↓

The precipitate is insoluble in dilute acetic acid. If larger amount of sodium is present or you have added excess of reagent, then a mixed salt K₂Na[Co(NO₂)₆] is formed.

We have successfully separated the metal cations. Unlike cations, there is no well-defined system for analysis of anions. In the coming posts of analytical chemistry we will discuss the tests for anions.

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