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Group III Cation Page 95

Cation Group III (iron group)

Group IIIA: Fe (II) & (III) Al (III) Cr (III) & (VI) , Mn (II),(III)&(IV).

Group IIIB Ni (II), Mn (II), Co(II) & Zn(II)

This group comprises seven elements and is subdivided into

two subgroups:

l- Group IllA (The iron group): it includes iron (ll) and (Ill),

aluminium (Ill), chromium (Ill) and (Vl) and manganese (ll), (III)

and (IV).

2- Group IIIB (The zinc group): This includes nickel(lI), cobalt (ll),

manganese (ll) and zinc (ll).

The metals of this group are not precipitated by the group

reagents for groups l and ll, but are all precipitated in the

presence of ammonium chloride by hydrogen sulphide from their

solutions made alkaline with ammonia solution. Aluminium and


chromium are precipitated as the hydroxides owing to the

complete hydrolysis of the sulphides in aqueous solution. So, iron,

aluminum and chromium (often accompanied by a little

manganese) are precipitated as hydroxides by ammonia solution

in the presence of ammonium chloride and refer to as group III A,

while the other metals of the group remain in solution and may be

precipitated as sulphides by hydrogen sulphide and refer to as

group lllB.

Group IIIA: Iron, Aluminum, Chrome,

Manganese.

Group reagent: ammonium solution (ammonium hydroxide) in

presence of ammonium chloride


Separation of Cation Group III

Residue Centrifuge

Subsequent

groups

NaOH + H2O2 boil until evolution of ceases centrifugate

-Boil the centrifugate from GpII to get rid of H2S. - Boil with few drops HNO3 - Add solid NH4Cl and heat to boiling, cool add xss ammonia until the solution alkaline - Heat, cool and centrifugate

Fe(OH)3 Reddish brown

gelatinous

Cr(OH)3 Dirty green

gelatinous

Al(OH)3 White

gelatinous

Mn(OH)3 Pinkish

white

Centrifuge Residue

Fe(OH)3

CrO42-

Yellow chromate

AlO2-

colourless aluminate

Mn(OH)3

Residue + Conc HCl Divided into 2 portions

Deep blood red color with NH4SCH

Prussian blue ppt. with K4[Fe(CN)6]

Residue +conc HNO3 + red lead oxide, heat =violet solution of MnO4

2-

NH4CL boil, cool Al(OH)3

White gelatinous

1) HAc + Pb(Ac) = yellow ppt. of PbCrO4

2) Dil. HNO3 + amyl Alcohol + H2O2 = blue organic layer of chromium pentoxide


Notes

1. H2S must be completely removed to avoid:

The precipitation of group IV cations as sulphides upon

rendering the medium alkaline.

Its oxidation with HNO3 to colloidal S° that will be difficult to

separate.

its oxidation with HNO3 to sulphate that may precipitate

group V cations as sulphates.

2. Boiling with conc. HNO3; is to oxidize Fe2+ to Fe3+ to ensure

complete precipitation of iron as Fe(OH)3 which has much

lower solubility than Fe(OH)2. It is important to mention here

that the hydroxides of the lower oxidation states of many of

Group Ill elements are too soluble to be completely

precipitated by the weak base NH4OH in presence of NH4Cl

e.g.: Fe(OH)2 and Mn(OH)2. These two elements will be

present in the lower oxidation states due to the reducing

action of H2S used to precipitate the preceding group (Group

ll). In practice, atmospheric oxygen oxidizes Mn(OH)2 to

some extent so that manganese is encountered in both

Group Ill [where it is precipitated as Mn(OH)3 ] and Group lll

B [where it is precipitated as sulphide (MnS)].


3. NH4Cl is added to the precipitating agent of Group Ill to:

Decrease the ionization of ammonia solution by common

ion effect to prevent the precipitation of Group IV cations

and Mg as hydroxides with Group Ill cations (as they

require a higher (OH-) concentration).

To help the coagulation of the gelatinous precipitates of

Group llI

NH4OH NH4+ + OH-

NH4Cl NH4+ + Cl-

4. Al(OH)3 and Cr(OH)3 are amphoteric and dissolve in excess

alkali while Fe(OH)3 and Mn(OH)3 are not appreciably

amphoteric and do not dissolve. This allows the separation

of these elements into two divisions, since, when the mixed

precipitated hydroxides are treated with excess alkali

hydroxide; the former two will dissolve while the latter will

not. Although Cr(OH)3 is completely soluble in excess

sodium hydroxide as chromite CrO2-. H2O2 is added to

oxidize Cr (III) to the stable chromate CrO42- which is yellow

in colour because trivalent Cr is green and will mask the

white gelatinous Al(OH)3 later upon addition 0f NH4Cl to test


for Al3+ by re-precipitation of Al(OH)3 as chromite will also

re-precipitate as Cr(OH)3, while CrO42- is stable and does not

re-preeipitate upon addition of NH4Cl.

5. Conc. HNO3 must not be used to dissolve the precipitate as

it may oxidize SCN- ions to the red thiocyanogen (SCN)2

which will be indistinguishable from the red ferric thiocyanate

complex.

6. Conc. HCl must not be used to dissolve the precipitate

because Cl- is a reducing agent and will bleach the purple

color of MnO4- by reducing it to Mn2+.

7. Acetic acid, rather than a stronger acid (e.g. HCl), is used

for acidification before adding lead acetate to avoid

converting CrO42- to Cr2O7

2- which forms soluble PbCr2O7;

instead of the insoluble PbCrO4.

8. Chromium pentoxide is unstable in aqueous medium and

stable in organic solvents that’s why amyl alcohol is added.


I - Iron (L. Ferrum), Fe

Iron forms two important series of salts : the ferrous salts which

may be regarded as derived from ferrous oxide FeO in which the

metal is divalent, and the ferric salts, which may be regarded as

derived from ferric oxide Fe2O3 in which the metal is trivalent.

Reactions of ferrous and ferric ions

1- Sodium hydroxide solution

Fe2+

Fe2+ + 2 OH- → Fe(OH)2

ferrous hydroxide . white

- Upon exposure to air Ferrous hydroxide is rapidly oxidized

yielding reddish- brown ferric hydroxide.

4 Fe (OH)2 + 2 H2O + O2 → 4 Fe(OH)3


Fe3+

Ferric ion gives reddish-brown precipitate of ferric hydroxide,

insoluble in excess of the reagent (distinction from aluminium and

chromium)

Fe3+ + 2 OH- → Fe(OH)3

2. Ammonia solution:

Fe2+

Precipitation of ferrous hydroxide occurs. In the presence of

excess of ammonium chloride solution, the common ion effect of

the ammonium ions lowers the concentration of the hydroxyl ions

to such an extent that the solubility product of Fe(OH)2 is not

attained and precipitation does not occur. Similar remarks apply

to nickel, cobalt, zinc, manganese and also magnesium.

Fe3+

Reddish-brown gelatinous precipitate of fem hydroxide

Fe(OH)3 insoluble in excess of the reagent (no soluble ammine

complex) but soluble in acids.

The solubility product of ferric hydroxide is so small that

complete precipitation takes place even in the presence of


ammonium salts (distinction from ferrous, nickel, cobalt,

manganese, Zinc and magnesium).

- Ferric hydroxide does not dissolve in strong alkali (as Al3+, Cr3+,

Zn2+) or in excess NH4OH (as Co2+, Ni2+, Zn2+)

- Ferric hydroxide is re-precipitated by:

o NH4OH/NH4Cl buffered solution.

o Alkali carbonates and hydroxides

CO32- + H2O HCO3

- + OH-

Fe3+

Fe(OH)3

3. Hydrogen sulphide:

Fe2+

No precipitation takes place in acid solution since the sulphide ion

concentration [S2-] is insufficient to exceed the solubility product

of ferrous sulphide.

Fe3+

In the presence of acid, hydrogen sulphide reduces ferric ions to

ferrous with separation of sulphur.

2 Fe3+ + H2S → 2 Fe2+ + 2 H+ + S

Group III Cation 4

4. potassium cyanide solution (Poison)

Fe2+

Yellowish-brown precipitate of ferrous cyanide is formed. This

latter is soluble in excess reagent forming a yellow solution of

ferrocyanide

Fe2+ + 2 CN- → Fe(CN)2

Fe(CN)2 + 4 CN- → [Fe(CN)6 ]4-

Fe3+

Ferric ion in the presence of a low concentration of CN- ion is

precipitated as Fe(OH)3 by the OH- ion resulting from hydrolysis

of cyanide. .

H2O + CN- → HCN + OH-

Fe3+ + 3 OH- → Fe(OH)3 𝒙𝒔𝒔 𝑪𝑵−

[Fe(CN)6]3-

5- Potassium ferrocyanide solution:

Fe2+

In the complete absence of air, a white precipitate of potassium

ferrous fenocyanide is produced


Fe2+ + [Fe(CN)6]4- → K2Fe[Fe(CN)6]

ferrous ferrocyanide White

Under ordinary atmospheric conditions partial oxidation of Fe2+ to

Fe3+ occurs with the formation of potassium ferric ferrocyanide

Fe3+ + [Fe(CN)6]4- → KFe[Fe(CN)6]

ferric ferrocyanide Prussian blue Fe3+

An intense blue precipitate of ferric ferrocyanide (Prussian blue) is

formed.

4 Fe3+ + 3 [Fe(CN)6]4- → Fe4[Fe(CN)6]3

6. Potassium ferricyanide solution:

Fe2+

A dark blue precipitate is produced.

Fe2+ + [Fe(CN)6]3- → KFe[Fe(CN)6]

ferrous ferricyanide turnbull's blue


This was formally termed Turnbull's blue. It is now considered to

be identical with Prussian blue (polassium ferric ferrocyanide)

Fe3+

A brown coloration is produced due lo ferric ferricyanide.

Fe3+ + [Fe(CN)6]3- → Fe[Fe(CN)6]

ferric ferricyanide

brown solution

7. Ammonium thiocyanide solution:

Fe2+

No coloration is obtained with pure ferrous salts (distinction from

ferric salts).

Fe3+

Ferric ion gives a blood red color

Fe3+ + SCN- → [Fe(SCN)]2+

8. Oxidation of iron (II) to iron (III)

Oxidation occurs slowly upon exposure to air. Rapid oxidation is

effected by concentrated nitric acid, hydrogen peroxide and other

reagents.


2 Fe2+ + H2O2 + 2H+ → 2 Fe3+ + 2 H2O

3 Fe2+ + NO3- + 4H+ → 3 Fe3+ + NO + 2 H2O

9. Reduction of iron (III) to iron (II):

This may be accomplished by nascent hydrogen (e.g. from zinc

and dilute hydrochloric or sulphuric acid), stannous chloride,

hydrogen sulphide and other reducing agents.

2 Fe3+ + Sn2+ → 2Fe2+ + Sn4+

II. Aluminium (L. Alumen (Alum)), Al:

Aluminium forms only one series of salts derived from the oxide

Al2O3

Reactions of the aluminium ion Al3+

1- Ammonia solution :

A white gelatinous precipitate of aluminium hydroxide. The

presence of an ammonium salt is important in preventing the

formation of colloidal Al(OH)3.


Al3+ + NH4OH → Al(OH)3

2. Sodium hydroxide solution:

A white precipitate of aluminium hydroxide is formed Al(OH)3 .

It is an amphoteric hydroxide. It is soluble in excess of the reagent

with the formation of aluminate AlO2- or tetrahydroxoaluminate

[Al(OH)4]- .

Al3+ + 3 OH- → Al(OH)3

Al(OH)3 + xss NaOH → AlO2- or [Al(OH)4]

- .

soluble aluminate

The reaction is a reversible one, and any reagent which will

reduce the hydroxyl ion concentration sufficiently should cause

the reaction to proceed from right to left with the consequent

precipitation of aluminium hydroxide. This may be effected with:

a) A solution of ammonium chloride (The hydroxyl ion

concentration is reduced owing to the formation of the weak

base ammonia which can be removed as ammonia gas by

heating).


[Al(OH)4]- + NH4

+ Al(OH)3 + NH3 + H2O

b) By the addition of acid, a large excess of acid will cause the

precipitated hydroxide to redissolve.

[Al(OH)4]- + H+ Al(OH)3 + H2O

Al(OH)3 + 3H+ Al3+ + H2O

3. Ammonium sulphide solution;

A white precipitate of aluminium hydroxide is formed.

3 Al3+ + 3 S2- + H2O 2 Al(OH)3 + 3H2S

III. Chromium [from Gr. Chroma (color)], Cr

The chromium atom exhibits three oxidation slates: +2 in

chromous compounds (not of interest in qualitative analysis); +3

in chromic compounds; and +6 in chromates and dichromates

Chromate ions are yellow while dichromates have an orange

color. Chromates are readily transformed to dichromates upon

addition of acid


2 CrO42- + 2 H+ 2 Cr2O7

2- + H2O

The reaction is reversible. In neutral (or alkaline) solutions the

chromate ion is stable, if acidified, dichromate ions will be

predominant, Chromate and dichromate ions are strong oxidizing

agents. Their reactions will be discussed with the anions.

Reactions of Chromium (III) ions, Cr3+

1. Ammonia solution:

Grey-green to grey-blue gelatinous precipitate of chromium

(III) hydroxide, Cr(OH)3, slightly soluble in excess of the

precipitant on cold forming a violet or pink solution containing

the complex ion [Cr(NH3)6]3+. Upon boiling the solution,

chromium hydroxide is precipitated. Hence for complete

precipitation of chromium as the hydroxide it is essential to boil

the solution and to avoid excess ammonia solution.

Cr3+ + 3 NH3 Cr(OH)3 + 3 NH4+

Cr(OH)3 + 6 NH3 [Cr(NH3)6]3+ + 3OH-


2- Sodium hydroxide solution:

A precipitate of chromic hydroxide is formed

Cr3+ + 3 OH- → Cr(OH)3

The precipitate is readily soluble in acids and also in excess of

the reagent forming a green solution containing chromite ions

CrO2- or tetrahydroxochromate ions [Cr(OH)4]

-.

Cr(OH)3 + NaOH → Na[CrO2] + 2 H2O

Cr(OH)3 + OH- [Cr(OH)4]- .

Upon boiling, chromium (Ill) hydroxide precipitates again.

3. Sodium carbonate isolation:

A precipitate of chromium (III) hydroxide is formed

2 Cr3+ + 3 CO32- + 3 H2O 2 Cr(OH)3 + 3 CO2

4. Ammonium sulphide solution:

A precipitate of chromium (Ill) hydroxide is formed:

2 Cr3+ + 3 S2- + 6 H2O 2 Cr(OH)3 + 3 H2S


5. Oxidation of chromium (III) to chromate:

A yellow solution of chromate can be produced by adding

excess of sodium hydroxide solution to a solution of chromium (ill)

salt, followed by a few ml of (20 - volume) hydrogen peroxide. The

excess of hydrogen peroxide can be removed by boiling the

mixture for 2 – 3 minutes.

2 Cr3+ + 3 H2O2 + 10 OH- → 2 CrO4

2- + 8 H2O

2 H2O2 (boil)→ O2 + 2 H2O

6- Confirmatory tests of chromium, after oxidation to

chromate:

Chromate ions can be identified with one of the following tests:

a) Barium chloride test :

By acidifying with acetic acid and adding barium chloride solution,

a yellow precipitate of barium chromate is formed.

Ba2+ + CrO42- BaCrO4


b) Chromium pentoxide or perchromic acid test:

By acidifying the chromate solution with dilute sulphuric acid or

dilute nitric acid, then adding few ml of amyl alcohol and finally

hydrogen peroxide, a blue color is formed which can be extracted

in the organic layer by shaking. In this reaction orange dichromate

is formed and is converted to the unstable chromium pentoxide or

perchromic acid which dissolves in and colors the alcohol layer

blue.

2CrO42- + 2H+ Cr2O7

2- + H2O

H2Cr2O7 + 4H2O2 → 2CrO5 + 5H2O

Chromium pentoxide has the following

structure

Since there is no change in the oxidation state of chromium (+6),

this is considered as an oxygenation reaction and not an oxidation

reaction.

In aqueous solution the blue color fades rapidly due to the

decomposition of chromium pentoxide to chromium (III) and

oxygen that’s why amyl alcohol is added.

4CrO5 + 12H+ → 4Cr3+ + 7O2 . + 6H2O

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