Qualitative analysis of Group III(A) cations

In the previous post of salt analysis we have learnt why we have to remove interfering radicals before 3^rd group analysis. After removing the interfering radicals, we will proceed to analyse the 3^rd group cations in the filtrate. To separate the radicals of 3^rdgroup we need to get them precipitated. The group reagent of 3^rd group is ammonium sulphide solution or hydrogen sulphide gas in the presence of ammonia and ammonium chloride. When we add group reagent to the filtrate we will get precipitate of 3^rd gr cations.

Cations of this group are cobalt(II) Co²⁺, Nickel(II) Ni²⁺, iron(II) Fe²⁺, iron(III) Fe³⁺, chromium(III) Cr³⁺, aluminium(III) Al³⁺, zinc(II) Zn²⁺, manganese(II) Mn²⁺, manganese(VII) Mn⁷⁺,  

Iron, aluminium and chromium and sometimes Manganese are precipitated as hydroxide by ammonia solution in presence of ammonium chloride while others are precipitated with ammonium sulphide in the form of sulphides. That’s why 3^rd group is further divided into A and B group.

III(A) is known as Iron group and it consists of iron(III) Fe³⁺, chromium(III) Cr³⁺, aluminium(III) Al³⁺.

III(B) is known as Zinc group and it consists of cobalt(II) Co²⁺, Nickel(II) Ni²⁺, zinc(II) Zn²⁺, manganese(II) Mn²⁺, manganese(VII) Mn⁷⁺.

Now let's continue from the group II filtrate and discuss the steps we have to follow to separate III(A) cations

Qualitative analysis of Group III(A) cations 

Step 1: Precipitation of III(A) cations as Hydroxide - To the filtrate, add few drops of nitric acid HNO₃ to convert ferrous Fe²⁺ into ferric Fe³⁺. Then add solid ammonium chloride NH₄Cl₃ and ammonia NH₃ solution and boil. On boiling you will get precipitate of III(A) cations which may contains Fe(OH)₃, Cr(OH)₃, Al(OH)₃ and a little MnO(OH)₂. Solubility product of iron(III) hydroxide is very small so it precipitates completely.

Fe³⁺ +3NH₃ + 3H₂O ⟶ Fe(OH)₃↓ + 3NH₄⁺

Ammonium chloride should be added in excess otherwise III(B) cations may get precipitated here. But avoid too much excess of ammonium chloride and ammonia otherwise Cr³⁺ may not precipitate and Al³⁺ may form colloidal solution. .

Cr³⁺ + 3NH₃ + 3H₂O ⟶ Cr(OH)₃↓ + 3NH₄⁺

Cr(OH)₃↓ + 6 NH₃ ⟶ [Cr(NH₃)₆]³⁺ + 3OH^-

Al(OH)₃↓ + OH^- ⟶ AlO₂^- + 2H₂O

Step 2: Separation of Fe(III) and Mn(II) from Al(III) and Cr(III) – Wash the precipitate with hot 

water and transfer to a test tube. Add a little water and add 1-2g of sodium peroxide or sodium hydroxide solution and bromine water/ hydrogen peroxide H₂O₂. Boil and filter in hot. On boiling, hydroxides of aluminium and chromium dissolve by forming meta- aluminate and chromate ion, and iron and manganese remain in the solution.

Al(OH)₃↓ + OH^- ⟶ AlO₂^- + 2H₂O

2Cr(OH)₃↓ + 2O₂^2- ⟶ 2CrO₄^2- + 2 OH^- + 2H₂O

Filter the residue to test for iron and manganese, and keep the filtrate for the test of aluminium and chromium.

Step 3: Test for iron and manganese – if residue is black, it means iron and manganese both are present and if it is reddish brown then only iron is present. Divide the residue into two parts.

Part 1 confirmatory test for Fe³⁺- dissolve the precipitate in dil HCl.

Fe(OH)₃ + H⁺ ⟶ Fe³⁺ + 3H₂O

Add few drops of potassium hexacyanoferrate (II) K₄[Fe(CN)₆] solution, intense blue precipitate of iron(III) hexacyanoferrate will appear.

4Fe³⁺ + 3 [Fe(CN)₆]^4- ⟶ Fe₄[Fe(CN)₆]₃ ↓

Excess of reagent dissolves it completely and intense blue solution is obtained and on adding sodium hydroxide solution to this blue solution, a red precipitate of iron(III) hydroxide is obtained.

Fe₄[Fe(CN)₆]₃ + 12OH^- ⟶ 4Fe(OH)₃ ↓ + 3[Fe(CN)₆]^4-

Part 2 confirmatory test for Mn²⁺- dissolve the precipitate in 1ml concentrated nitric acid HNO₃, if necessary add 1-2 drops of H₂SO₃. Add 0.05-0.1g sodium bismuthate NaBiO₃ and shake. A violet solution of permanganate will appear.

2Mn²⁺ + 5NaBiO₃ + 14H⁺ ⟶ 2MnO^-₄ + 5Bi³⁺ + 5Na⁺ + 7H₂O

Step 4: Test for aluminium and chromium – The filtrate from step 2 may contain CrO^-₄ and [Al(OH)₄]^-. If it is yellow, it means chromium might be present but if it is colourless then chromium is absent and you don’t need to test for it.

Divide the filtrate into 3 parts, we will test for chromium in 2 parts and in the third part we will test for aluminium.

Part 1 Test for Cr (III): acidify with dilute acetic acid CH₃COOH and add 0.25M lead acetate Pb(CH₃COO)₂ solution, a yellow precipitate of lead chromate is formed.

2CrO₄^2- + Pb²⁺ ⟶ PbCrO₄↓

This yellow precipitate is soluble in sodium hydroxide solution. it conforms the presence of Cr(III).

PbCrO₄↓ + 4OH^- ⟶ PbO₂^2- + CrO₄^2- + 2H₂O

Part 2 Test for Cr (III): acidify it with dilute nitric acid HNO₃, cool thoroughly and add 1ml amyl alcohol (2-methyl-butane-4-ol) and 3-4 drops of 3% hydrogen peroxide solution H₂O₂. Shake well and allow the two layers to separate. Upper blue layer contains perchromic acid (chromium pentoxide).

2 CrO₄^2- + 2H⁺ + 3 H₂O₂ ⟶ 2CrO₅ + 4 H₂O

Part 3 Test for Al (III): To test for aluminium acidify the filtrate with dilute HCl (test with litmus paper) then add 2M ammonia solution NH₃ until just alkaline. Heat till boiling. White gelatinous precipitate of aluminium hydroxide is obtained.

Al³⁺ + 3NH₃ + 3H₂O ⟶ Al(OH)₃↓ +3NH₄⁺

Filter this precipitate and dissolve a small portion of it into 1ml hot dil HCl. Cool and add 1ml 6M ammonium acetate solution and 0.5ml of 1M aluminon reagent (0.1g tri-ammonium arurine- tricarboxylate O(COONH₄)C₆H₃=C[C₆H₃(OH) COONH₄]₂dissolved in 100ml water), stir it and add ammonium carbonate solution. a red coloured precipitate confirms the presence of Al(III). In this post we discussed analysis of the III(A) group cations. In the next post of salt analysis we will continue with the test test for III(B) cations in the filtrate of III(A).

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