group iiib: (zinc group) nickel, manganese, cobalt and...

Group IV Cation 12

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 Centrifuge

Subsequent groups MnS ZnS CoS NiS buff to pink white black black

Group IV Cation Page 113

Red ppt. blue amyl alcohol layer

yellow

purple solution of MnO42-

Residue Centrifugate

- Wash - Transfer the ppt. with 1M HCl (2) - Stirr in a beaker 3 min on cold - Centrifuge

Residue Centrifuge

CoS NiS Zn2+ , Mn2+ ZnCl2 MnCl2

- 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 Ni2+

Test for Co2+

- Add NH4Cl/NH4OH solution till alkaline - Add xss DMG (5)

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

b) + HAc + KNO2 (7) K3[Co (NO2)6]

MnO(OH)2 or MnO2

[Zn(OH)4]2-

or 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

H2S

K4[Fe(CN)6]

white ppt

of ZnS

white ppt of Zn2[Fe(CN)6]


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 α-

form 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 H2O2

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+


(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.

Z n O 2 + H A c Z n 2 + 2 -

+ H 2 S

+ K 4 [ F e ( C N ) 6 ]

Z n S W h i t e

Z n 2 [ F e ( C N ) 6 ]

w h i t e


(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.


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(NH3)6]2+

is formed.

3- Ammonium sulphide solution :

A black precipitate of cobalt sulphide CoS is formed from

neutral or alkaline solution


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 K3[Co(CN)6].


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.


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]2-

Tetrathiocyanatocobaltate

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.


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-


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 3

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]2-


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.


H3C – C = N – OH

Ni2+ + 2 | H3C – C = N - OH

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 MnO4

-

N

OH

C CH3

CH3C

H3C C

O

O

N

NNi

OH

NCH3C

+ 2H+


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


A white precipitate of manganous hydroxide is initially

formed.

Mn2+ + 2OH- Mn(OH)2

The precipitate is insoluble in excess reagent. It rapidly

oxidizes on exposure to air becoming brown. 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



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.


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


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+ + 5C2O4

2- 2Mn2+ + 10 CO2 + 8H2O


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+


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.


Zn(OH)2 + 2H+ Zn2+ + 2H2O

Zn(OH)2 + 2OH- ZnO22- + 2H2O


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 + 2NH4+

Zn(OH)2 + 4NH3 [Zn(NH3)4]2+ + 2OH-


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.


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 K2Zn3 [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 iiib: (zinc group) nickel, manganese, cobalt and...

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