3. GROUP CATIONS Fe

3. GROUP CATIONS

Fe ³⁺ , Al ³⁺ , Cr ³⁺ , Ni ²⁺ , Co ²⁺ , Mn ²⁺ ,

Zn ²⁺

These cations precipitate as sulfides and hydroxides when reacted with hydrogen sulfide (H

S) or thioacetamide solution in basic medium buffered with NH

OH – NH

Cl. These cations do not precipitate with the reagents used for precipitation of the group I and II.cations.

Group III cations are divided into two:

1) Iron Group (Group 3A): Fe

, Al

, Cr

(precipitates in the form of hydroxides).

2) Zinc Group (Group 3B): Ni

, Co

, Mn

, Zn

(precipitates in the form of sulfides).

Systematic analysis of group III cations:

 20 drops of sample containing Fe

, Al

, Cr

, Ni

, Co

, Mn

, Zn

taken into a tube from the sample.

 Add 10 drops of 4 M ammonium chloride (NH

Cl) solution and 10 drops of 2 M ammonium hydroxide (NH

OH, %33) solution dropwise to the sample and check the basicity of the solution with a litmus paper.

 Then add 20 drops of NH

OH solution and Fe

, Al

and Cr

precipitate as their hydroxides.

 Heat the solution for 5 min at water bath and add 20 drops of thioacetamide solution (11%, w/v) and observe precipitates of sulfides of Ni

, Co

, Zn

and Mn

(COMMON PRECIPITATE)

 This precipitate contains Al(OH)₃, Cr(OH)₃, Fe(OH)₃, NiS, CoS, MnS, and ZnS and it is washed several times with 0.1 M NH₄Cl.

Add 20 drops of 6 M HCl onto the washed precipitate.

Centrifuge and separate the supernatant A and the precipitate A.

Precipitate A (CoS ve NiS):

 Precipitate A is taken into cruicible, heated on bunsen burber till the solute part evaporates if exist, and add 20 drops of aqua regia (15 drops of 6 M HCl + 5 drops of 6 M HNO

) to dissolve the precipitate.

 Add from NH

OH (%33) solution until it is basic (check with red litmus paper).

 Divide the solution into two parts. To the first part, add 10 drops of 0.1 M potassium ferricyanide (K

[Fe(CN)

]) solution and the dark green color is followed, which is the precipitate of cobalt ferricyanide (Co

[Fe(CN)

]). This shows the presence of Co

.

 To the second part, add 2 drops of 6 M NaOH and 2 drops of dimethyl glyoxime

(%1 in 96% alcohol), the rose color shows the presence of Ni

.

Supernatant A (Fe

, Al

, Cr

, Mn

, Zn

):

 Add 20 drops of 6 M NaOH and 20 drops of 3% H

O

(v/v) are added to the supernatant.

 Centrifuge and separate supernatant B and the precipitate B.

 Transfer precipitate B to a clean test tube, add 10 drops of HNO

(37.5%) and pinches of spatula sodium bismuthate (NaBiO

) from the side of the tube. The color of violet proves the presence of Mn

.

 Add 0.1 M K

[Fe(CN)

] to the precipitate B, the prussian blue precipitate proves the

presence of Fe

.

Supernatant B (Zn²⁺, Al³⁺, Cr³⁺):

 Add 20 drops of HNO₃ and NH₄OH to the solution until it is basic. Centrifuge and separate the supernatant C and the precipitate C

 Precipitate C is white aluminum hydroxide (Al(OH)₃).

 The yellow color of the supernatant C, which contains Cr³⁺ and Zn²⁺, indicates the presence of Cr³⁺.

 Add 20 drops of 2 M CH₃COOH to the supernatant C to make the medium acidic. Add pinches of spatula of sodium acetate (NaCH₃COO).

 Heat the solution in a water bath for 20 min and add 2 drops of 0.5 M barium chloride (BaCl₂) solution to the hot solution. The formed yellow precipitate (precipitate D) is barium chromate (BaCrO₄).

 Add 2 drops of thioacetamide solution (11% w/v) to the supernatant D and heat it up in the water bath with stirring for 5 minutes until the white precipitate is visible. The formation of dirty white zinc sulfide (ZnS) shows the presence of Zn²⁺.

SUPERNATANT A: (Fe³⁺, Al³⁺, Cr³⁺, Mn²⁺, Zn²⁺ )

dark green color Co ⁺²

• Transfer to a clean test tube

• 10 drops of HNO3

• pinches of spatula sodium bismuthate (NaBiO3) from the side of the tube

PRECIPITATE A: CoS, NiS

III. GROUP CATIONS

• 20 drops of sample

• 10 drops of 4 M NH₄Cl

• Dropwise 2 M NH4OH

• check the basicity of the solution with a litmus paper.

• 20 drops of NH₄OH eklenir.

• Heat the solution for 5 min at water bath and add 20 drops of thioacetamide solution

COMMON PRECIPITATE

Al(OH)₃, Cr(OH)₃, Fe(OH)₃, NiS, CoS, MnS, ZnS

• Wash several times with 0.1 M NH₄Cl

• Add 20 drops of 6 M HCl

• Centrifuge and separate the supernatant A and the precipitate A.

• aken into cruicible, heated on bunsen burber till the solute part evaporates

• 20 drops of aqua regia (15 drops of 6 M HCl + 5 drops of 6 M HNO3) to dissolve the precipitate.

• Add 2 M NH₄OH check the basicity with red litmus paper.

• 2 drops of 6 M NaOH

• 2 drops of dimethyl glyoxime

• Add 0.1 M (K4[Fe(CN)6])

Violet color, Mn²⁺

white aluminum hydroxide (Al(OH)₃)

2 drops of thioacetamide solution heat it up in the water bath with stirring for 5 minutes until the white

precipitate is visible.

• 20 drops of 6 M NaOH

• 20 drops of 3%

• H₂O₂ Centrifuge and separate supernatant B and the precipitate B.

PRECIPITATE B Fe(OH)₃, MnO₂

• 20 drops of HNO₃

• add NH₄OH until it is basic.

• Centrifuge and separate the supernatant C and the precipitate C

SUPERNATANT B: Zn²⁺, Al³⁺, Cr³⁺

• 10 drops of 0,1 M (K4[Fe(CN)6])

rose color Ni ⁺²

prussian blue precipitate Fe³⁺

PRECIPITATE C

SUPERNATANT D

Divide the sample into two

Divide the sample into two

SUPERNATANT A: (Fe³⁺, Al³⁺, Cr³⁺, Mn²⁺, Zn²⁺ )

• Transfer to a clean test tube

• 10 drops of HNO₃

• pinches of spatula sodium bismuthate (NaBiO₃) from the side of the tube

III. GROUP CATIONS

• 20 drops of sample

• 10 drops of 4 M NH₄Cl

• Dropwise 2 M NH₄OH

• check the basicity of the solution with a litmus paper.

• 20 drops of NH₄OH eklenir.

• Heat the solution for 5 min at water bath and add 20 drops of thioacetamide solution

COMMON PRECIPITATE

Al(OH)₃, Cr(OH)₃, Fe(OH)₃, NiS, CoS, MnS, ZnS

• Wash several times with 0.1 M NH₄Cl

• Add 20 drops of 6 M HCl

• Centrifuge and separate the supernatant A and the precipitate A.

• aken into cruicible, heated on bunsen burber till the solute part evaporates

• 20 drops of aqua regia (15 drops of 6 M HCl + 5 drops of 6 M HNO3) to dissolve the precipitate.

• Add 2 M NH₄OH check the basicity with red litmus paper.

• Add 0.1 M (K₄[Fe(CN)₆])

SUPERNATANT C: Cr³⁺, Zn²⁺

Violet color, Mn²⁺

white aluminum hydroxide (Al(OH)₃)

2 drops of thioacetamide solution heat it up in the water bath with stirring for 5 minutes until the white

precipitate is visible.

• 20 drops 2 M CH₃COOH

• pinches of spatula of sodium acetate (NaCH₃COO)

• Heat the solution in a water bath for 20 min

• add 2 drops of 0.5 M barium chloride (BaCl₂) solution to the hot solution.

• 20 drops of 6 M NaOH

• 20 drops of 3%

• H2O2 Centrifuge and separate supernatant B and the precipitate B.

PRECIPITATE B Fe(OH)₃, MnO₂

• 20 drops of HNO3

• add NH4OH until it is basic.

• Centrifuge and separate the supernatant C and the precipitate C

SUPERNATANT B: Zn²⁺, Al³⁺, Cr³⁺

prussian blue precipitate Fe³⁺

PRECIPITATE C

PRECIPITATE D

Yellow precipitate barium chromate

BaCrO₄

SUPERNATANT D

dirty white precipitate

ZnS Divide the sample into two

When excess NH

is added to the starting solution, red brown color indicates the

presence of the Fe

, and/or colorless and gelatinous precipitate indicates the

presence of Al

.