NOTES ON ANALYSIS OF GROUP 3 CATIONS
STEP
Reagents
A. Precipitation of Group 3 Cations
1
6M HCl
2
Distilled Water
3
5% T.A.
4
Heating
5
Conc. NH4OH
6
Heating
NOTES
To adjust pH suitable for precipitation of
Group 3 cations
To precipitate sulfides of group 3 cations
Production of sulfide ion from thioacetamide in basic soution:
(A) CH3CSNH2 + H2O → CH3CONH2 + H2S
(B) H2S + 2 OH- →
S2- + H2O
To precipitate the hydroxides
Precipitation of the group III ions:
(C) Al3+ + 3OH- → Al(OH)3
(D) Cr3+ + 3OH- → Cr(OH)3
(E) Fe3+ + S2- → Fe2S3
(F) Mn2+ + S2- → MnS
(G) Co2+ + S2- → CoS
(H) Ni2+ + S2- → NiS
(I) Zn2+ + S2- → ZnS
Residue contains CoS, NiS, FeS, MnS, ZnS,
Al(OH)3, Cr(OH)3
B. Separation & Identification of Cobalt & Nickel
7
Wash with D.W.
Residue B-2
1M HCl
Dissolves FeS, MnS, ZnS,
Cr(OH)3
(Supernatant B-2 and B-3)
Residue: CoS and NiS
Residue B-3
Conc. HCl
Mixture dissolves CoS and NiS and
(J) FeS + HCl → Fe2+ + H2S
Al(OH)3, (K) MnS + HCl → Mn2++ H2S
(L) Al(OH)3 + HCl → Al3+ + H2O
(M) Cr(OH)3 + HCl → Cr3+ + H2O
(N) ZnS + HCl → Zn2+ + H2S
(O) CoS + HNO3
→ Co2+ + NO + S + H2O
STEP
Supernatant
B-5
Reagents
NOTES
Conc. HNO3
forms NiCl64- and CoCl64-
Heating
Residue B-5 contains dark chunks of sulfur
Boiling
Removes excess nitric acid (HNO3)
Conc. NH4OH
Precipitates Fe(OH)3
(P) NiS + HNO3
→
Ni2++ NO + S + H2O
Distilled Water
Test for
Nickel
DMG
Forms the Ni(C4H6N2O2)2 , a red nickel
complex with two molecules of
dimethylglyoxime
(DMG)
Test for
Cobalt
1M HCl
Acidify the solution.
NH4CNS Crystals
Forms complex with cobalt as Co(CNS)42(blue green in the absence of iron)
Acetone
Stabilizes and intensifies the color
Eliminates interference of iron Fe3+ by
forming FeF63� thereby liberating the
1M NaF
blue-green color of Co(SCN)42- appear.
C. Separation of Manganese & Iron
Supernatant
B-2 & B3
3M HNO3
Oxidize Fe2+ to Fe3+ to insure complete
precipitation of iron as Fe(OH)3; may also
be used to destroy organic matter that may
form complex with aluminum.
Co2+ + 4SCN- → Co(CNS)42-
STEP
Reagents
Distilled Water
3M NaOH
NOTES
Precipitates the hydroxides of iron and
manganese while Al(OH)4 � , Zn(OH)42� ,
Cr(OH)4� remains in solution
3% H2O2
Residue C-6
+ 3OH- →
+ 3OH- →
+ 3OH- →
+ 2OH- →
Fe(OH)3
Al(OH)3
Cr(OH)3
Zn(OH)2
(U) Fe2+ + H2O2 + OH- → Fe(OH)3 + H2O
Oxidizes Cr3+ to CrO42-, Mn2+ to Mn4+
(V) Mn2+ + H2O2 + OH- → MnO(OH)2
(MnO2)
(W) Cr3+ + H2O2 + OH- →
CrO42-
Heating
Distilled Water
3M NaOH
Residue C-9
(Q) Fe3+
(R) Al3+
(S) Cr3+
(T) Zn2+
3M HNO3
1M NaNO2
Keeps the amphoteric hydroxides Al(OH)4 �, (X) Al(OH)3 + OH- → [Al(OH)4]
(Y) Cr(OH)3 + OH- → [Cr(OH)4]Zn(OH)42� in solution
(Z)
Zn(OH)2 + OH- → [Zn(OH)4]2(AA) Fe(OH)3 + H+
Fe3+ + 2H2O
→
Dissolves hydroxides of iron and manganese
(AB) MnO(OH)2 + H+
→ Mn2+ + Cl2 + H2O
Oxidizing agent
Heating
Supernatant
C-11
Test for Iron
Test for
Manganese
Heating
Distilled Water
NH4CNS
Combines with Fe3+ as Fe(SCN)2+
(AC) Fe3+ + SCN- →
Fe(SCN)2+ (blood-red color)
NaBiO3
Oxidizes Mn2+ or Mn4+ to MnO4�
(AD) Mn2+ + NaBiO3 + H+ →
3M HNO3
Provides acidic medium for oxidation of
Mn2+ to MnO4�
MnO4- + Pb2+ + H2O
(pink to purple color)
STEP
Reagents
D. Separation & Identification of Aluminum
Supernatant
C-6 & C-9
Residue D-3
NOTES
Al(OH)4 � + 4H+ → Al3+ + 4H2O
3M HOAC
Dissolves Al(OH)4 � liberating Al3+
3M NH4OH
Makes solution just basic enough to
precipitate Al3+ as Al(OH)3, the white
gelatinous precipitate
Al3+ + 3OH- → Al(OH)3
3M HOAC
Dissolves the white gelatinous precipitate
Al(OH)3 + 3H+ → Al3+ + 3H2O
Zn(OH)4 2� + 4H+ → Zn2+ + 4H2O
3
+ Al3+
Ammonium salt of ATA
HO
HO
+
O
O
+
Aluminon Rgt.
Ammonium salt of aurin tricarboxylic acid
(ATA) forms an insoluble red chelate with
Al3+
OH
NH 4
O
OH
O
O
O
O
Red color ppt of aluminum
with aluminon reagent
-
Al
O
3+
O
O
HO
HO
-
O
O
O
O
H
O
O
H
HO
O
O
Destroys the red precipitates formed by
aluminon reagent with iron and chromium
NH 4
O
O
1M (NH4)2CO3
-
-
STEP
Reagents
NOTES
E. Identification & Chromium & Zinc
Supernatant
D-3
3M HOAC
Prevents conversion of CrO42- to Cr2O72Test for
Chromium
Precipitates CrO42- as lead chromate PbCrO4
0.1 M Pb(OAc)2
(yellow ppt)
Test for Zinc
5% T.A.
CrO42- +
Pb2+ → PbCrO4 (yellow ppt)
Precipitates Zn2+ as Zinc Sulfide (ZnS)
Zn2+ + S2- →
Dissolves the ZnS but not sulfur confirms
presence of Zn.
ZnS + H+
ZnS
Heating
Precipitate
3M HCl
→ Zn2+ + H2S