Qualitative analysis of Group III(B) cations

In the last post of analytical chemistry we have separated III(B) cations from III(A) cations. On boiling with ammonium chloride NH₄Cl₃ and ammonia NH₃solution III(A) cations get precipitated as hydroxide, while III(B) cations remain in solution as sulphides. In the filtrate of III(A) we will perform tests for III(B) cations. First we have to re precipitate cations of III(B).

Step 1: Precipitation of III(B) cations as sulphide

To the filtrate, add 2-3ml of 2M ammonia NH₃ solution and heat, now pass hydrogen sulphide H₂S gas under pressure for 0.5 to 1 min. You will get the precipitate of III(B) cations which may contain CoS, NiS, MnS and ZnS. H₂S gas should be passed in hot solution for complete precipitation because ZnS and MnS form fine suspension if the solution is not hot enough.

           M²⁺ + S^2-⟶ MS

Wash the precipitate with the mixture of 1% ammonium chloride NH₄Cl solution and small amount of ammonium sulphide (NH₄)₂S solution and reject all the washings. You can get an idea of the cation by the colour of precipitate, if it is black it may contain NiS and CoS and it if white or dirty white it may contain ZnS and MnS. These cations have different solubility in HCl.ZnS and MnS are soluble in very dilute HCl while NiS and CoS require moderately dilute HCl. NiS and CoS are insoluble in cold dilute HCl.

Step 2 separation of Zn(II) and Mn(II) from Co(II) and Ni(II)

Transfer the precipitate to a small beaker. Add 5ml water and 5ml of dilute HCl, stir well and allow to stand for few minutes then filter. You will get ZnS and MnS in the form of filtrate and NiS and CoS will remain as residue.

MnS↓ + 2H⁺  ⟶ Mn²⁺ + H₂S↑

ZnS↓ + 2H⁺  ⟶ Zn²⁺ + H₂S↑

Step 3 Test for Nickel Ni(II) and Cobalt Co(II)

Dissolve the residue in aqua regia (3part conc. HCl: 1part conc. HNO₃). Solution may look white due to white sulphur, but on longer heating, sulphur is oxidised to sulphate and solution becomes clear.

CoS↓ + HNO₃ + 3HCl ⟶ Co²⁺ + S↓+ NOCl↑ + 2Cl^- + 2H₂O

NiS↓ + HNO₃ + 3HCl ⟶ Ni²⁺ + S↓+ NOCl↑ + 2Cl^- + 2H₂O

Divide the solution in two parts for the confirmatory tests of Ni(II) and Co(II).

Part 1 confirmatory test for Ni²⁺

Add 2ml ammonium chloride NH₄Cl 1M solution and 2M ammonia NH₃ solution until alkaline, and then excess of dimethylglyoxime reagent. Red precipitate of nickel dimethylglyoximeconfirms the presence of nickel.

You can also use spot test technique for the same test, place a drop of test solution and a drop of dimethylglyoxime reagent on a spot plate and add a drop of dilute ammonia solution. A red spot of nickel dimethylglyoxime is produced.

Ni²⁺  + 2C₄H₈O₂N₂⟶ Ni[C₄H₇O₂N₂]₂+ 2H⁺

Part 2 confirmatory test for Co²⁺

Add 1ml amyl alcohol, 2g solid ammonium thiocynate. Amyl alcohol layer becomes blue due to formation of tetrathiocobaltate(II) ions. This is known as Vogel reaction.

Co²⁺  + 4SCN^-⟶ [Co(SCN)₄]^2-

Step 4 Test for zinc Zn(II) and manganese Mn(II)

Filtrate may contain Mn²⁺ and Zn²⁺ and traces of Ni²⁺ and Co²⁺. Zinc and manganese both form hydroxide with NaOH but zinc hydroxide is soluble in excess of reagent. We can use this difference for the separation of Zn from Mn.

Boil the filtrate until H₂S is removed (evolution of H₂S gas can be identified with its characteristic odour. For confirmation you can  test it with lead acetate paper which turns black due to formation of lead sulphide H₂S + Pb²⁺ ⟶PbS↓ + 2H⁺). Cool the solution and add excess of 2M sodium hydroxide NaOH solution, followed by 1ml 3% hydrogen peroxide H₂O₂solution. Boil for 2-3 min then filter. You will get Mn²⁺ as precipitate and Zn²⁺ in the filtrate.

Mn²⁺ + 2OH^- ⟶ Mn(OH)₂↓

Hydrogen peroxide converts manganese (II) hydroxide into hydrated manganese dioxide.

Mn(OH)₂ ↓ + H₂O₂ ⟶ MnO(OH)₂↓ + H₂O

Zn²⁺ + 2OH^- ⇌ Zn(OH)₂↓

Zinc (II) hydroxide is soluble in excess of reagent by forming tetrahydroxozincate ion.

Zn(OH)₂ ↓ + 2OH^- ⇌ [Zn(OH)₄]^2-

Test for Zn²⁺in the filtrate

Divide the filtrate in to two parts.

Part 1: acidify with 2M acetic acid CH₃COOH and pass H₂S gas. White precipitate of zinc sulphide is formed.

Zn²⁺ + H_2­S + 2CH₃COO^- ⟶ZnS ↓ + 2CH₃COOH

Part 2: acidify with 1M sulphuric acid H₂SO₄, add 0.5ml of 0.1M cobalt acetate Co(CH₃COO)₂ solution and 0.5ml of ammonium tetratiocyanato-mercurate(II) regent and stir. Violet precipitate is obtained.

Zn²⁺ + [Hg(SCN)₄]^2-⟶ Zn[Hg(SCN)₄] ↓

Test for Mn²⁺in the precipitate

Precipitate largely contain MnO(OH)₂ and traces of Ni(OH)₂and Co(OH)₃. Dissolve the precipitate in 5ml of 8M nitric acid HNO₃, if it doesn’t dissolve completely then add few drops of 3% H₂O₂ solution. Boil to decompose excess of H₂O₂ and cool.

Add 0.05g sodium bismuthate NaBiO₃, stir and allow to settle. Purple solution of MnO₄^- is formed.

2Mn²⁺ + 5NaBiO₃ +14H⁺⟶ 2 (MnO₄)^- + 5Bi³⁺ + 5Na⁺+ 7H₂O

Now we have successfully analysed the III(B) group cations. In the filtrate of III(B) we will test for group IV cations. 

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