Group IIIB: (Zinc group) Nickel, Manganese, cobalt and Zinc Group reagent: Hydrogen sulphide (gas or saturated aqueous solution) in the presence of ammonia and ammonium chloride or ammonium sulphide solution. Separation of Group III B cations

Mn(OH)2, [Zn(NH3)4]2+, [Co(NH3)6]2+, [Ni(NH3)6]2+ Soluble

soluble complexes - Add NH4Cl/NH4OH - Add H2S water - Digest in a b.w.b. for 10 minutes (1) - Cool, centrifuge - Test for complete precipitation

Residue MnS  buff to pink

Centrifuge ZnS  white

Group IV Cation

CoS  black

NiS  black

Subsequent groups

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- Wash - Transfer the ppt. with 1M HCl - Stirr in a beaker 3 min on cold - Centrifuge

(2)

Residue CoS 

Centrifuge

NiS 

Zn2+ , ZnCl2

- Dissolve in aqua regia or NaOCl solution - Divide into equal parts

-Boil to expel H2S (3) (test with lead acetate paper) -Add xss dil NaOH -Add H2O2 solution, boil , centrifuge

Test for Co2+

Test for Ni2+ - Add NH4Cl/NH4OH solution till alkaline - Add xss DMG (5)

Red ppt.

Mn2+ MnCl2

a) Add Na2S2O3 + amyl alcohol + solid NH4SCN shake well

blue amyl alcohol layer b) + HAc + KNO2 (7) K3[Co (NO2)6]

yellow

Residue

Centrifugate

MnO(OH)2 or MnO2

[Zn(OH)4]2or ZnO22-

- Dissolve in conc. HNO3 - Add H2O2 & boil, Cool Add solid NaBiO3 or red lead - stirr and allow to settle

- Acidify with acetic acid (4) - divided into 2 portions

K4[Fe(CN)6]

purple solution of MnO42H2S

white ppt of Zn2[Fe(CN)6]

white ppt of ZnS Group IV Cation

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Notes : (1) Digesting the precipitates for 10 minutes in a hot water bath is important in order to "age" CoS and NiS. Both sulphides exist in two crystal forms (this phenomenon is called polymorphism), so they are converted to the less soluble αform by "aging" (leaving the precipitate in contact with the mother liquor for a period of time) in order not to dissolve in 1M HCl. ZnS and MnS are not affected by aging and will dissolve in 1M HCl. (2) 1M HCl is used to dissolve MnS nd ZnS while CoS and NiS by

aging

become

practically

insoluble

in

HCl

in

consequence of change in crystal form, these latter are soluble in conc. HNO3, aqua regia and sodium hypochlorite solution. (3) H2S must be removed before boiling with NaOH and H 2O2 for two reasons : (a) To prevent the formation of colloidal sulphur from the oxidation of H2S by H2O2. (b) To prevent the precipitation of ZnS. (4) The solution containing the soluble zincate ZnO22- has to be acidified with acetic acid before testing for zinc in order to decompose the complex and liberate the free Zn2+ Group IV Cation

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+ H2S 2-

ZnO2 + HAc

ZnS

White

Zn2+ + K4[Fe(CN)6]

Zn2[Fe(CN)6] white

(5) Ni2+ salts in solutions made slightly alkaline with ammonia give a red precipitate with dimethylglyoxime. The red precipitate does not form in acidic or in strongly alkaline solutions. Ferrous (red coloration), bismuth (yellow precipitate) and cobalt when present in large excess (brown coloration) interfere in ammoniacal solution. The influence of interfering elements can be eliminated by the addition of tartarate. Fe2+ must be oxidized to Fe3+, say by hydrogen peroxide and removed as Fe(OH)3  or as FeF63- (masking). (6) In testing for Co2+ by NH4SCN, Ni2+ forms green [Ni(SCN)]+ but the color remains in the aqueous solution and is not extractable in the organic layer. Fe3+ interferes giving red [Fe(SCN)]2+ extractable in the organic layer so it should be removed by adding sodium thiosulphate Na2S2O3 (masking agent) which reduces Fe3+ to Fe2+ or by complexation of Fe3+ with fluoride. Group IV Cation

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(7) A yellow precipitate of potassium cobaltinitrite is formed from a slightly acidic solution, so acetic acid is used for acidification. The precipitate does not form in alkaline or in strongly acidic solutions.

I- Cobalt, Co Cobalt exists in two oxidation states: cobalt (II) or cobaltous ion Co2+ and cobalt (III) or cobaltic ion. Cobaltous salts are pink when hydrated and blue when anhydrous or undissociated.

Reactions of cobalt (II) ions, Co2+

1- Sodium hydroxide solution : Alkali hydroxides precipitate on cold a blue basic salt Co(OH)+ which on warming or sometimes upon addition of excess reagent is converted into pink cobaltous hydroxide Co(OH)2 and this is gradually oxidized by atmospheric oxygen into the brownish black cobaltic hydroxide. Group IV Cation

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4Co(OH)2  + O2 + 2H2O  4Co(OH)3 

In this respect cobalt differs from nickel whose hydroxide Ni(OH)2 is not oxidized by atmospheric oxygen.

2- Ammonia solution : In the absence of ammonium salts (e.g. NH4Cl) small amounts of ammonia precipitate the blue basic salt Co(OH)+. The excess of the reagent dissolves the precipitate as the yellowish-brown hexammine-cobaltate (II) ions [Co(NH3)6]2+ which is slowly oxidized into the red brown hexammine-cobaltate (III) ions [Co(NH3)6]3+ upon exposure to air. On the other hand, in the presence of ammonium salt, ammonium hydroxide does not give a precipitate with cobaltous salts because [OH-] is reduced to such an extent that the solubility product of the basic salt or the hydroxide is not reached in which case the complex [Co(NH 3)6]2+ is formed.

3- Ammonium sulphide solution : A black precipitate of cobalt sulphide CoS is formed from neutral or alkaline solution Group IV Cation

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Co2+ + S2-  CoS 

The precipitate is insoluble in acetic acid or dilute hydrochloric acid but is readily soluble in hot concentrated nitric acid and in aqua regia with the separation of sulphur.

3CoS  + 2NO3- + 8H+  3Co2+ + 3S  + 2NO  + 4H2O CoS  + NO3- + 3Cl- + 4H+  Co2+ + S + NOCl + 2Cl- + 2H2O

The precipitate also dissolves in sodium hypochlorite solution.

CoS  + 2OCl- + 4H+ + 4Cl-  [CoCl4]2- + SO2  + 2H2O

4- Potassium cyanide solution : It gives a reddish-brown precipitate of cobaltous cyanide Co(CN)2 soluble in excess of the reagent to form a brown solution of potassium cobaltocyanide K4[Co(CN)6] which upon prolonged boiling in air or by heating with hydrogen peroxide turns yellow due to oxidation to potassium cobalticyanide K 3[Co(CN)6]. Group IV Cation

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5- Confirmatory tests of cobalt :

a) Potassium nitrite solution : Potassium nitrite added to slightly acidic solution of Co2+ (acetic acid is used for acidification) produces a yellow precipitate of potassium cobaltinitrite.

Co2+ + 7NO2- + 2H+ + 3K+  K3[Co(NO2)6]  + NO  + H2O

The precipitate does not form in alkaline or in strongly acidic solutions as strong acids decompose the reagent.

2NO2- + 2H+  2HNO2  NO2  + NO  + H2O unstable

While strong alkalis will precipitate cobalt as hydroxide. Ni2+ ion does not interfere with this test because it does not form a precipitate with potassium nitrite.

Group IV Cation

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b) Ammonium thiocyanate test: (Vogel reaction) On adding a concentrated NH4SCN solution or a few crystals of the solid to a neutral or acid solution of cobalt (II), a blue color appears owing to the formation of the complex [Co(SCN)4]2-. If amyl alcohol is added and the solution is shaken, the blue color passes into the alcohol layer (distinction from nickel).

Co2+ + 4SCN-  [Co(SCN)4]2Tetrathiocyanatocobaltate

The free acid H2[Co(SCN)4] is much more soluble in ether and in amyl alcohol than its salts, hence it is best to strongly acidify the solution to render the test more sensitive. Interference : Ni2+ forms green [Ni(SCN)]+ but the color remains in the aqueous layer and is not extractable in the organic solvent. Fe3+ interferes with the test giving red [Fe(SCN)] 2+ and should be removed either by reduction to Fe2+ with sodium thiosulphate or by complexation of ferric with fluoride or citrate.

Group IV Cation

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II- Nickel, Ni

The common oxidation state of nickel is +2 though + 4 is also known e.g. NiO2. Nickelous salts are green when hydrated and yellow to yellowish brown when anhydrous.

Reactions of nickel (II) ions, Ni2+

1- Sodium hydroxide solution: Gives a green precipitate of nickelous hydroxide Ni(OH) 2 insoluble in excess of the reagent but soluble in ammonia solution. Ni2+ + 2OH-  Ni(OH)2 

Nickelous hydroxide is not oxidized by exposure to air but can be oxidized to black nickelic hydroxide Ni(OH)3  by sodium hypochlorite solution.

2Ni(OH)2  + OCl- + H2O  2Ni(OH)3  + Cl-

Group IV Cation

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2- Ammonia solution: Forms a green precipitate of nickel (II) hydroxide which dissolves in excess reagent.

Ni2+ + 2NH3 + 2H2O  Ni(OH)2  + 2NH4+ Ni(OH)2 + 6NH3  [Ni(NH3)6]2+ + 2OH-

The solution of hexammine - nickelate (II) ions is deep blue. The solution is not oxidized upon boiling with free exposure to air or upon the addition of hydrogen peroxide (difference from cobalt). If ammonium salts as NH4Cl are present no precipitation occurs but the complex is formed immediately because the concentration of hydroxyl ions is so reduced that the solubility product of the hydroxide is not attained.

3- Ammonium sulphide solution: A black precipitate of nickel sulphide is formed from neutral or slightly alkaline solutions.

Ni2+ + S2-  NiS 

Group IV Cation

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The precipitate is practically insoluble in cold dilute hydrochloric acid (distinction from the sulphides of manganese and zinc) and in acetic acid, but dissolves in hot concentrated nitric acid and in aqua regia with the separation of sulphur.

3NiS  + 2NO3- + 8H+  3Ni2+ + 3S  + 2NO  + 4H2O NiS+ NO3- + 3Cl- + 4H+  Ni2+ + S  + NOCl  + 2Cl- + 2H2O

Sodium hypochlorite solution also dissolves nickel sulphide NiS  + 2OCl- + 4H+ + 4Cl-  [NiCl4]2- + SO2  + 2H2O

4- Potassium cyanide solution: A green precipitate of nickel (II) cyanide is formed

Ni2+ + 2CN-  Ni(CN)2 

The precipitate is readily soluble in excess reagent forming the complex salt potassium nickelocyanide (yellow solution).

Ni(CN)2  + 2CN-  [Ni(CN)4]2Group IV Cation

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If this solution is heated with sodium hypobromite solution (prepared in situ by adding bromine water to sodium hydroxide solution), the complex decomposes and a black precipitate is formed (difference from cobalt). The precipitate is formulated as Ni(OH)3 or NiO2. This can be used as a test for nickel in presence of cobalt.

2[Ni(CN)4]2- + 9BrO- + 4OH- + H2O  2Ni(OH)3 + 8CNO- + 9Br-

5- Potassium nitrite solution: No precipitate is produced in presence of acetic acid (difference from cobalt).

6- Confirmatory test of nickel:

Dimethylglyoxime test: (C4H8O2N2) A red precipitate of nickel dimethylglyoxime is obtained from solutions just made alkaline with ammonia or buffered with sodium acetate.

Group IV Cation

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Ni

2+

H3C – C = N – OH + 2 | H3C – C = N - OH

H3C

C

H3C

C



OH

O

N

N

C

Ni N

N

O

OH

C

CH3 CH3

+ 2H+

Interference Ferrous (red coloration), bismuth (yellow precipitate) and large

amounts

of

cobalt

(brown

coloration)

interfere

in

ammoniacal solution. The influence of interfering elements can be eliminated by the addition of tartarate.

Ferrous can be

oxidized to ferric, say by hydrogen peroxide and removed as Fe(OH)3 or as complex with fluoride (This is called masking).

III- Manganese, Mn The important oxidation states of manganese encountered in qualitative analysis are +2, +3, +4, +6 and +7. Representative examples are: Mn(OH)2 , Mn(OH)3 or MnO(OH)2 , MnO2 , MnO42- and MnO4Group IV Cation

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Reactions of manganese (II) ions (manganous), Mn2+

1- Sodium hydroxide solution: A white precipitate of manganous hydroxide is initially formed.

Mn2+ + 2OH- 

Mn(OH)2 

The precipitate is insoluble in excess reagent. oxidizes on exposure to air becoming brown.

It rapidly

The brown

compound is either manganic hydroxide Mn(OH) 3, or hydrated manganese dioxide MnO2.xH2O

4Mn(OH)2 + 2H2O + O2  4Mn(OH)3

The addition of a little hydrogen peroxide converts manganous hydroxide into hydrated manganese dioxide.

Mn(OH)2 + H2O2  MnO2 + 2H2O Group IV Cation

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2- Ammonia solution: Partial precipitation of white manganous hydroxide soluble in solutions of ammonium salts.

Mn2+ + 2NH3 + 2H2O  Mn(OH)2  + 2NH4+

On exposure to air brown manganic hydroxide or hydrated manganese dioxide is precipitated from the ammoniacal solution.

3- Ammonium sulphide solution: A pink precipitate of manganous sulphide is formed.

Mn2+ + S2-  MnS 

The precipitate is readily soluble in dilute acids as dilute hydrochloric acid (distinction from cobalt and nickel) and in acetic acid (distinction from cobalt, nickel and zinc).

MnS + 2H+  Mn2+ + H2S  Group IV Cation

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4- Confirmatory tests of manganese:

a) Reaction with lead dioxide and concentrated nitric acid: On boiling a dilute solution of manganese (II) ions with lead dioxide PbO2 or red lead Pb3O4 (a mixture of PbO2 – PbO oxide, which yields the dioxide in the presence of nitric acid) and a little concentrated nitric acid, then diluting and allowing the suspended solid containing unattacked lead dioxide to settle, the supernatant liquid acquires a purple or violet – red color due to permanganate.

2Mn2+ + 5PbO2 + 4H+  2MnO4- + 5Pb2+ + 2H2O

The solution must be free from hydrochloric acid and chlorides as well as other reducing agents as oxalates which will reduce the resulting permanganate to colorless manganese (II) ions.

2MnO4- + 16H+ + 10Cl-

 2Mn2+ + 5Cl2  + 8H2O

2MnO4- + 16H+ + 5C2O42-  2Mn2+ + 10 CO2  + 8H2O

Group IV Cation

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b) Reaction with sodium bismuthate (NaBiO3): When sodium bismuthate solid is added to a cold solution of manganous salt in dilute nitric acid or in dilute sulphuric acid the purple color of permanganate is produced

2Mn2+ + 5NaBiO3 + 14H+  2MnO4- + 5Bi3+ + 5Na+ + 7H2O

IV- Zinc, Zn Zinc is encountered in the +2 oxidation state, its compounds are usually colorless.

Reactions of zinc (II) ions, Zn2+

1- Sodium hydroxide solution: A white gelatinous precipitate of zinc hydroxide is formed Zn2+ + 2OH-

 Zn(OH)2 

The precipitate is soluble in dilute acids and in excess sodium hydroxide with the formation of zincate [ZnO22- or Zn(OH)42-] and is therefore amphoteric.

Group IV Cation

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Zn(OH)2  + 2H+

Zn2+ + 2H2O

Zn(OH)2  + 2OH-

ZnO22- + 2H2O

2- Ammonia solution: White precipitate of zinc hydroxide readily soluble in excess of the reagent and in solutions of ammonium salts owing to the production of tetramminezincate.

The non-precipitation of zinc

hydroxide by ammonia solution in the presence of ammonium chloride is due to the lowering of the hydroxyl ion concentration (by common ion effect) to such a value that the solubility product of Zn(OH)2 is not attained. Zn + 2NH3 + 2H2O Zn(OH)2  + 4NH3

Zn(OH)2  + 2NH4+ [Zn(NH3)4]2+ + 2OH-

3- Ammonium sulphide solution: White precipitate of zinc sulphide is formed from neutral or alkaline solutions.

It is insoluble in excess of the reagent, in

acetic acid and in solutions of caustic alkalis but dissolves in dilute mineral acids.

Group IV Cation

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Zn2+ + S2-  ZnS 

Zinc sulphide is also precipitated from solutions of alkali zincates. Na2ZnO2 + H2S

ZnS + 2NaOH

4- Confirmatory test of zinc:

Potassium ferrocyanide test:

A white precipitate of zinc ferrocyanide Zn2 [Fe(CN)6] which is converted by excess of the reagent into K 2Zn3 [Fe(CN)6]2 . The precipitate is insoluble in dilute acids but dissolves in solutions of caustic alkalis as sodium hydroxide.

2Zn2+ + K4 [Fe(CN)6]  Zn2 [Fe(CN)6] 3Zn2+ + 2K+ + 2[Fe(CN)6]4-  K2Zn3 [Fe(CN)6]2 

Group IV Cation

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