GROUP III CATIONS
A. INTRODUCTION
Ions in the Group: Al3+, Cr3+, Fe3+, Fe2+, Mn2+, Co2+, Ni2+, Zn2+
Precipitating Agent: S2- from thioacetamide in ammonia-ammonium chloride buffer. Cr3+ and Al3+
precipitate as hydroxides. The other ions in the group precipitate as sulfides.
Table III - 1
ION
Al3+
COLOR OF SOLUTION OF
ION
PRECIPITATED FORM
OF ION
COLOR OF THE
PRECIPITATE
Colorless
Al(OH)3
White
Dark Green and Blue
Cr(OH)3
Blue-Green
Fe3+
Red-Brown
Fe2S3
Black
Fe2+
Pale Green
FeS**
Black
Mn2+
Colorless at Low
MnS
Light Pink
*Cr3+
Concentrations
Co2+
Pale Red
CoS
Brown to Black
Ni2+
Pale Green
NiS
Brown to Black
Zn2+
Colorless
ZnS
White
*Cr may be present as CrO42- (yellow) or Cr2O72See note.
III-O and reaction III-O (C).
(yellow-orange).
**Any Fe3+ present in the solution will be reduced by H2S to Fe2+.
See reaction III-O (C).
B. QUALITATIVE ANALYSIS - GROUP III PROCEDURES
Procedure III-O Separation of Group III from Groups IV and V:
Place the decantate from the group II separation in a small beaker and evaporate to 2 ml if
necessary. (If you are using an unknown containing just group III cations, take about 2
ml of that solution). Add about 1 ml of NH4Cl solution; then add 15 MNH4 OH
dropwise with stirring until the solution is alkaline. (Several ml of NH 4OH may be
required depending upon the acidity of the original solution.) Add 1 ml (20 drops) of 1M
thioacetamide; carefully heat the mixture almost to boiling in a water bath and maintain
temperature for at least 3minutes. Centrifuge, then add 2 drops thioacetamide and heat to
test for complete precipitation. If precipitation is complete, transfer the solution and all
of the precipitate to a test tube. Centrifuge, label and save the decantate for group IV and
V analysis. Wash the precipitate once with 1% NH4NO3 solution and discard the wash
water.
Precipitate NiS, CoS, MnS, ZnS, FeS, Al (OH)3 , Cr (OH)3
Procedure III-1
Separation of Ni and Co from other Ions:
Add about 2 ml 3 m HCl to the precipitate and stir thoroughly. (Shaking the test tube is
NOT sufficient.) If you have a large amount of precipitate, it may be necessary to repeat
this extraction procedure. Save the solution for procedure III-4.
Precipitae NiS, CoS (Procedure III-2)
Decantate Al3+, Cr3+, Fe2+, Mn2+, Zn2+ (Procedure III-4)
Procedure III-2
Dissolving NiS and CoS:
To the precipitate from procedure III-1 add 10 drops concentrated HN03 and 10 drops
concentrated HCl. Heat and stir carefully until solid dissolves. Elemental sulfur, a solid,
will be formed. Centrifuge to remove the solid sulfur or carefully scoop it off the top of
the liquid. Discard the sulfur and divide the solution into two parts: (a) procedure III-3;
and (b) procedure III-3b.
Procedure III-3 (a) The Borax Bead Test for Co
A very sensitive test for Co2+ is known as the borax bead test. To prepare a borax bead,
dip a looped platinum wire into sodium borate and melt the borate in a burner flame.
Repeat until a clear bead fills the loop. Dip the bead into the solution in one of the test
tubes (a) from procedure III-2 and then heat in the flame. Repeat 4-5 times if necessary.
A blue color in the bead is a positive test for Co.
Procedure III-3 (b) Identification of Ni
Dilute the other part of the solution from procedure III-2 (b), to give a volume of 2 ml
and add concentrated NH4OH dropwise with thorough stirring until the solution is basic.
Add 2 ml of dimethylglyoxime solution. The appearance of a pink to red precipitate
confirms the presence of Ni.
Procedure III-4
Oxidation of Fe, Mn and Cr and Separation of Fe and Mn from Al, Cr and Zn
Place the decantate from Procedure III-1 in an evaporating dish and boil down to a
volume of 2 ml if necessary. Make the solution basic with 6 M NaOH, then add 0.25 g
Na2O2 cautiously to the solution. Warm (do not boil) with constant stirring until the
bubbling ceases. (Do NOT evaporate to dryness.) Add 1 ml distilled water, transfer all
of the solution to a test tube and centrifuge. Save the solution for procedure III-6.
Decantate: Al3+, CrO42+, Zn2+ (Procedure III-6)
Precipitate: Fe(OH)3, MnO(OH)2 (Procedure III-5)
Procedure III-5
Identification of Fe and Mn
Wash the precipitate from procedure III-4 with about 1 ml hot water and discard wash
water. Then add 2 ml concentrated HNO3 and 2 drops of concentrated HCl and heat with
stirring until the solid is completely dissolved. Any solid not dissolved may be discarded.
Divided the solution into 2 equal parts: (a) procedure III-5a; and (b) procedure III-5b.
Procedure III-5 (a) Identification of Mn
To part (a) of the solution, from procedure III-5 add 0.25 g PbO2 (lead peroxide) heat for
about one minute and then allow to cool. Centrifuge if necessary. A pink to purple
solution indicates the presence of Mn and MnO4-.
Procedure III-5(b) Identification of Fe
To the other half of the solution part (b) from procedure III-5, add 1 ml H2O, then add 1
ml of K4Fe(CN)6 and centrifuge. A deep blue precipitate confirms the presence of iron.
A sensitive test for Fe3+ is to add KSCN, deep red color solution. Fe(SCN) 2+ is a positive
test for iron. SEE THE NOTES - light blue precipitate might be due to impurities.
Procedure III-6
Separation of Al
To the solution from procedure III-4, carefully add concentrated HNO3 with stirring until
the solution is just acidic. Add 0.5 g NH4Cl, heat to boiling in a water bath and add 6 M
NH4OH until the solutioin is definitely basic. That is, add about 10 drops beyond the
amount necessary to first give a basic solution. Centrifuge. Save the solution for
procedure III-8.
Decantate: CrO42+, Zn2+
(Procedure III-8)
Precipitate: Al (OH)3
(Procedure III-7)
Procedure III-7
Identification of Al
The precipitate from procedure III-6, if pure Al(OH)3, will be white and gelatinous.
Several of the ions in group III will form colored hydroxides and may give a misleading
test. Small amounts of rust colored Fe(OH)3 may precipitate if iron was not completely
removed earlier. Wash precipitate with 2 ml of 1% NH 4Cl, centrifuge and discard
washing. To confIrm aluminum, dissolve the precipitate in 4M acetic acid and add 2-4
drops of “aluminon reagent” and add 6M NH4OH until the solution is basic. Centrifuge.
The formation of a reddish gelatinous precipitate confirms Al.
Procedure III-8
Identification of Cr and Zn
Add 4M acetic acid to the solutioin from procedure III-6 until acidic. Add 0.25 g solid
sodium acetate to buffer the solution and heat in a water bath to boiling. Add BaCl2
solution dropwise with stirring until precipitation is complete. The appearance of yellow
BaCrO4 confirms the presence of Cr. Test for complete precipitation. (If the solution is
not yellow, there is probably no chromium present so no precipitate will form.)
Procedure III-9
Identification of Zn
Centrifuge the solution from precedure III-8. To the clear, colorless decantate add
K4Fe(CN)6. Formation of a whte gelatinous precipate ZnFe(CN)6 confirms the presence
of Zn. (If Fe or Al were not completely removed previously, they may interfere with the
test for Zn.)
C. NOTES, PRECAUTIONS AND HINTS
III-0
The ions of group III form sulfides which are much more soluble than the sulfides of
group II ions. It is, therefore, necessary, to have a higher sulfide ion concentration in
order to precipitate these ions; so, a basic solution is used. A buffer of NH 4Cl-NH4OH is
used so that the pH will not be high enough to precipitate Mg(OH) 2 in this group. When
the solution is made basic with NH4OH, the hydroxides of Fe3+ (red-brown), Al3+ (white)
and Cr3+ (green) will precipitate and can be identified by their color if any of these metals
are present in the #3 oxidation state. In order to identify iron in the #2 oxidation state it is
necessary to test a small portion of the original unknown: add K3Fe(CN)6 dropwise. (Do
NOT add K4Fe(CN)6 since it will not give a test.) The appearance of the deep blue
precipitate of Prussian Blue confirms Fe2+. Chromium may be present in an unknown in
one of three different forms. As Cr3+ ion it gives a solution. If chromium is present in
the +6 oxidation state, it will form the CrO42- ion in a basic solution giving a yellow color
or in acid solutioin will exist as Cr2O72- giving a yellow-orange color. The important
point is this, if chromium is present in the +6 state, it will be reduced to the +3 state (see
equation III-0(c)) and the appropriate color change will occur. One last word about the
color of solution in group III. Due to the phenomenon of complementary color
interactions a solution may contain two colored ions yet be colorless. Due to color
mixing, two colors may combine so as to form a third totally different color. Finally,
during the course of the analysis as one ion is separated from the others, the color of the
solutions may change. You should use such color changes as aids in carrying out your
analysis.
III-1
NiS and CoS will dissolve in HCl solution but the rate of dissolving is much slower than
for the other ions in the group. It is therefore, necessary to limit the length of time during
which the sulfides are exposed to HCl. If there is no insoluble material remaining at the
end of procedure 3-1, this indicates that your sample contains neither Ni or Co.
III-2
The mixture of concentrated HNO3 and concentrated HCl is very corrosive and should be
handled with extream caution! Be careful not to mistake insoluble sulfur for the sulfides
you are trying to dissolve.
III-3(b)
It is necessary to completely neutralize the solutioin containing Co and Ni and then to
make the solution mildly basic since the dimethyglyomine reagent is decomposed in acid
solution and a misleading test result will be obtained. If Co is present in the solution, it
may tend to interfere with the Ni test. The Co will form a soluble brownish complex ion.
An excess of dimethylglyoxime is necessary to give the cherry-red precipitate to confirm
Ni.
III-4
The hydroxides of Al, Cr and Zn are amphoteric and will dissolve in the strongly based
solution. Caution! Na2O2 is highly reactive. Avoid contact with your skin and clothes.
Do not weigh Na2O2 on paper or any combustible material. A dry watch glass is
convenient for weighing. Add the Na2O2 to stand in contact with the air for prolonged
period before use. (That means put the top back on the can when you are finished getting
your Na2O2!)
III-5
A very sensitive test for Fe3+ may be made by forming the thiocynante-iron complex ion.
The procedure is: To about 10 drops of the solution from procedure III-5(b) (use solution
to which no K4Fe(CN)6 has been added), add 1 or 2 crystals of NH4SCN or KSCN. A
blood red solution is a positive test for iron. Iron is present in almost all chemicals in
very small amounts as an impurity picked up during manufacturing. For this reason the
production of a light pink solution with NH4SCN or KSCN is not a positive test.
III-6
Al(OH)3 will precipitate form solution only in a fairly narrow pH range. Al(OH) 3 will
dissolve in strongly acidic and in strongly basic solutions; hence the use of the bufer
solution. If Zn is present in the solution, white Zn(OH) 2 will precipitate upon initial
addition of NH4OH, but the soluble complex ion Zn (NH3)42+ will be formed upon
continued addition of NH4OH, but the soluble complex ion (Zn(NH3)42+ will be formed
upon continued addition of NH3.
D. CHEMICAL REACTIONS FOR THE GROUP III ANALYSIS
III-0(A) Production of sulfide ion from thioacetimide in basic soution:
CH3CSNH2 + H2O
H2S + 2OH-
CH3CONH2 + H2S
S2- + H2O
(B) Precipitation of the group III ions:
Al3+ + 3OH-
Al(OH)3
Cr3+ + 3OH-
Cr(OH)3
Fe3+ + S2-
Fe2S3
Mn2+ + S2-
MnS
Co2+ + S2-
CoS
Ni2+ + S2-
NiS
Zn2+ + S2-
ZnS
(C) Reduction of Fe(III) and Cr(VI):
Fe3+ + H2S + OH
Fe2+ + H2O + S
CrO42- + H2S + OHProcedure III-1
Cr3+ + H2O + S
FeS + HCl
Fe2+ + H2S
MnS + HCl
Mn2+ + H2S
Al(OH)3 + HCl
Al3+ + H2O
Cr(OH)3 + HCl
Cr3+ + H2O
ZnS + HCl
Zn2+ + H2S
Procedure III-2
Procedure III-3
Procedure III-4
Procedure III-5
CoS + NHO3
Co2+ + NO + S + H2O
NiS + NHO3
Ni2+ + NO + S + H2O
Co2+ + 6NH3
(Co(NH3)6)2+
(Pink)
Ni2+ + 6NH3
(Ni(NH3)6)2+
(Blue)
Co2+ + KNO2
K3Co(NO2)6 + NO
Fe2+ + Na2O2 + OH-
Fe(OH)3 + H2O
Mn2+ + Na2O2 + OH-
MnO(OH)2
Cr3+ + Na2O2 + OH-
CrO42-
Fe(OH)3 + H+
Fe(OH)3 + H2O
MnO(OH)2 + HCl
Mn2+ + Cl2 + H2O
Mn2+ + PbO2 + H+
MnO4- + Pb2+ + H2O
Fe3+ + K4Fe(CN)6
K3Fe(CN)6
Procedure III-7
Al3+ + OH-
Procedure III-8
CrO42- + Ba2+
Procedure III-9
Zn2+ + K4Fe(CN)6
Al(OH)3
BaCrO4
K2Fe(CN)6