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
Group III Cation
Page 95
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
Group III Cation
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Separation of Cation Group III
-Boil the centrifugate from GpII to get rid of H 2S.
- Boil with few drops HNO3
- Add solid NH4Cl and heat to boiling, cool add xss
ammonia until the solution alkaline
- Heat, cool and centrifugate
Residue
Centrifuge
Fe(OH)3
Cr(OH)3
Al(OH)3
Mn(OH)3
Reddish brown
gelatinous
Dirty green
gelatinous
White
gelatinous
Pinkish
white
Subsequent
groups
NaOH + H2O2 boil until evolution of
ceases centrifugate
Residue
Fe(OH)3
Residue + Conc
HCl
Divided into 2
portions
Deep blood
red color with
NH4SCH
Group III Cation
Centrifuge
Mn(OH)3
Residue +conc
HNO3 + red lead
oxide, heat =violet
solution of MnO42-
Prussian blue ppt.
with K4[Fe(CN)6]
AlO2colourless
aluminate
NH4CL boil, cool
Al(OH)3
White gelatinous
CrO42Yellow
chromate
1)
HAc + Pb(Ac) =
yellow ppt. of PbCrO4
2)
Dil. HNO3 +
amyl Alcohol + H2O2
= blue organic layer of
chromium pentoxide
Page 97
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)].
Group III Cation
Page 98
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-

NH4+
+ Cl-
NH4Cl
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
Group III Cation
Page 99
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 Cr2O72- 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.
Group III Cation
Page 100
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 
Group III Cation
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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
Group III Cation
Page 102
ammonium
salts
(distinction
from
ferrous,
nickel,
cobalt,
manganese, Zinc and magnesium).
- Ferric hydroxide does not dissolve in strong alkali (as Al 3+, 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
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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 ]4Fe3+
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 + OH3+
Fe
-
+ 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
Group III Cation
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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
Group III Cation
Page 105
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.
Group III Cation
Page 106
2 Fe2+ + H2O2 + 2H+ →
2 Fe3+
3 Fe2+ + NO3- + 4H+ →
3 Fe3+ + NO + 2 H2O
+ 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.
Group III Cation
Page 107
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).
Group III Cation
Page 108
[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 + 3H+
 Al(OH)3  + H2O
 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
Group III Cation
Page 109
2 CrO42-
+ 2 H+  2 Cr2O72- + 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-
Group III Cation
Page 110
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 S2Group III Cation
+
6 H2O  2 Cr(OH)3  + 3 H2S 
Page 111
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 CrO42- + 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 
Group III Cation
Page 112
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+
 Cr2O72- + 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
Group III Cation
Page 113
Group III Cation
Page 114