Experiment 10
Group III Cation Analysis
Part II
Valdosta State University
Important Announcements
There will be no laboratory lecture for CHEM 1212L on
Wed Nov 7, Wed Nov 14, or Wed Nov 21 (Thanksgiving).
The next laboratory lecture for Experiment #12 will meet on
Wed, Nov 28.
The final exam for the lab will be at that same time, Wed
Nov 28.
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Purpose
To determine which group III ions are present in an
unknown solution.
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Background
General Unknown
HCl
Group I
Insoluble Metal Chlorides
and Ammonia
HCl / H2S
Group II
Acid Insoluble Metal Sulfides
NH3 / H2S
Group III
Alkaline Insoluble Metal
Sulfides and Hydroxides
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Group IV
Soluble Metal
Ions
Background
• For this experiment, the group III ions are Fe3+, Ni2+, Mn2+,
Al3+ and Zn2+.
• These ions initially precipitate as either metal sulfides (in
an alkaline environment) or metal hydroxides.
• This requires the chemist to generate a small quantity of
sulfide ion to precipitate the metals.
• A convenient source of S2- is thioacetamide, which
decomposes when heated to give hydrogen sulfide
(H2S) which yields S2- in chemical reactions.
• A reagent that is made and consumed in the same
flask is said to be produced in situ.
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Background – Hydrogen Sulfide
S
H
H
C
C
H
O
N
H
H
+ 2 H2O +
H
H+(aq)
H
C
C
H
O
+ NH4+(aq) + H2S(g)
H
H2S(aq) + 2 H2O(l) D 2 H3O+(aq) + S2-(aq)
• The addition of base to the second reaction consumes the hydronium ion
and drives the reaction to the right, increasing the concentration of S2-(aq).
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Background – Group III Separation Scheme
Group III Unknown
NH3, H2S,
heat
NiS, FeS, MnS, ZnS,
Fe(OH)3, Al(OH)3
Group IV ions
HCl, HNO3, heat
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
Waste
NaOH
Fe(OH)3, Ni(OH)2,
Mn(OH)2
Al(OH)4-, Zn(OH)42-
HNO3
Divide
sample
HNO3
NaBiO3
NH3
MnO4purple
NH3
Ni(NH3)62
Fe(OH)3
+
HCl /
NH4SCN
H2DMG
Fe(SCN)63blood red
Ni(DMG)2
strawberry red
ppt.
Al(OH)3
aluminon,
NH3
Al(OH)3aluminon
cherry red ppt.
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Zn(NH3)42+
K4Fe(CN)6
K2Zn3[Fe(CN)6]2
white ppt.
Background – Group III Separation Scheme
A – Preparation of Group III cations
Group III unknown
NH3, H2S, Heat
NiS, FeS, MnS, Fe(OH)3,
Al(OH)3, ZnS
Group IV ions
• The group III ions are initially separated
from the bulk solution by precipitation as
either insoluble metal sulfides or
hydroxides.
Ni2+(aq) + S2(aq) D NiS(s) (black)
Fe2+(aq) + S2(aq) D FeS(s) (black)
Zn2+(aq) + S2(aq) D ZnS(s) (white)
Mn2+(aq) + S2(aq) D MnS(s) (pink)
Al3+(aq) + 3 OH(aq) D Al(OH)3(s) (white, gel)
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Background – Group III Separation Scheme
A – Preparation of Group III cations
Group III unknown
NH3, H2S, Heat
NiS, FeS, MnS, Fe(OH)3,
Al(OH)3, ZnS
Group IV ions
• Since iron has two common oxidation
states, its chemistry in this step is more
complex.
• If iron(III) is present it is reduced to iron(II)
and elemental sulfur in produced.
2 Fe3+(aq) + H2S (aq)  2 Fe2+(aq) + S(s) + 2 H+(aq)
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Background – Group III Separation Scheme
A – Preparation of Group III cations
Group III unknown
• Alternately, the iron(III) can combine with the
hydroxide ion and precipitate as iron(III)
hydroxide.
NH3, H2S, Heat
NiS, FeS, MnS, Fe(OH)3,
Al(OH)3, ZnS
Group IV ions
Fe3+(aq) + 3 OH-(aq) D Fe(OH)3 (rust color)
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Background – Group III Separation Scheme
B1 – Separation of Group III cations
NiS, FeS, MnS, Fe(OH)3,
Al(OH)3
HCl, HNO3, Heat
Waste
• Following the precipitation, the metal
ions are combined with acid to form the
free (and soluble) metal ions.
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
3NiS(s) +8H+(aq) + 2NO3(aq)  3Ni2+(aq) + 2NO(g) + 3S(s) + 4H2O(l)
FeS(s) + 2 H+ (aq)  Fe2+(aq) + H2S(aq)
3Fe2+(aq) + 4H+(aq) + NO3(aq)  3Fe3+(aq) + NO(g) + 2H2O(l)
MnS(s) + 2 H+(aq)  Mn2+(aq) + H2S(aq)
ZnS(s) + 2 H+(aq)  Zn2+(aq) + H2S(aq)
Al(OH)3(s) + 3 H+(aq)  Al3+(aq) + H2O(l)
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Background – Group III Separation Scheme
B2 – Separation of Group III cations
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
NaOH
Fe(OH)3, Ni(OH)2, Mn(OH)2
Al(OH)4-, Zn(OH)42-
• Aluminum and zinc ions are amphoteric.
• This means that at high acid or base
concentrations, these metals form soluble
complexes, but precipitate at moderate pH.
• Iron, manganese and nickel form insoluble
hydroxides at high pH.
Fe3+(aq) + 3 OH(aq) D Fe(OH)3(s) (rust-color)
Ni2+(aq) + 2 OH(aq) D Ni(OH)2(s) (green)
Mn2+(aq) + 2 OH(aq) D Mn(OH)2(s) (light brown)
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Background – Group III Separation Scheme
B2 – Separation of Group III cations
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
NaOH
Fe(OH)3, Ni(OH)2, Mn(OH)2
Al(OH)4-, Zn(OH)42-
• Aluminum and zinc ions are amphoteric.
• This means that at high acid or base
concentrations, these metals form soluble
complexes, but precipitate at moderate pH.
• Iron, manganese and nickel form insoluble
hydroxides at high pH.
Al3+(aq) + 3 OH(aq) D Al(OH)3(s) (white, gelatinous)
Zn2+(aq) + 2 OH(aq) D Zn(OH)2(s) (white)
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Background – Group III Separation Scheme
B2 – Separation of Group III cations
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
NaOH
Fe(OH)3, Ni(OH)2, Mn(OH)2
Al(OH)4-, Zn(OH)42-
Excess Acid
Al(OH)3(s) + 3H+(aq) D Al3+ + 3 H2O(aq)
Zn(OH)2(s) + 2H+(aq) D Zn2+ + 2 H2O(aq)
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Background – Group III Separation Scheme
B2 – Separation of Group III cations
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
NaOH
Fe(OH)3, Ni(OH)2, Mn(OH)2
Al(OH)4-, Zn(OH)42-
Excess Base
Al(OH)3(s) + OH(aq) D Al(OH)4-(aq)
Zn(OH)2(s) + 2OH(aq) D Zn(OH)42-(aq)
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Background – Group III Separation Scheme
Fe(OH)3, Ni(OH)2, Mn(OH)2
HNO3, KNO2
Fe3+, Ni2+, Mn2+
Divide
Sample
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C1 – Test for Mn2+, Fe3+, Ni2+
• The precipitate is redissolved by adding
acid to the precipitate.
• The addition of nitric acid neutralizes the
sodium hydroxide and regenerates the
free cations.
• There is no easy method which will allow
Mn2+, Fe3+ and Ni2+ to be separated;
therefore, the sample is divided.
Background – Group III Separation Scheme
C2 – Test for Mn2+
Fe3+,
Ni2+,
Mn2+
Divide
Sample
NaBiO3
• If sodium bismuthate is added to a
solution containing manganese(II), a
redox reaction occurs resulting in the
formation of the purple permanganate
ion.
MnO4purple
14H+(aq) + 2Mn2+(aq) + 5BiO3-(s)  2 MnO4-(aq) + 5Bi3+(aq) + 7H2O(l)
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Background – Group III Separation Scheme
D1 – Separation of Fe3+ and Ni2+
Fe3+,
Ni2+,
Mn2+
Divide
Sample
NH3
Fe(OH)3
Ni(NH3)62+
• The nickel and iron ions can be
separated by the addition of ammonia.
• The increased pH causes the formation
of the insoluble iron(III) hydroxide.
• The nickel ion combines with ammonia
to form a soluble complex ion,
hexaamminenickel(II).
Fe3+(aq) + 3NH3(aq) + 3H2O(l)  3NH4+(aq) + Fe(OH)3(s) (brown)
Ni2+(aq) + 6NH3(aq) D Ni(NH3)62+(aq) (blue)
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Background – Group III Separation Scheme
D2 – Test for Fe3+
Fe3+,
Ni2+,
Mn2+
• The presence of the iron(III) ion is
confirmed by the addition of ammonium
thiocyanate.
• If iron(III) is present, a blood red solution
forms.
Divide
Sample
NH3
Fe(OH)3
Ni(NH3)62+
HCl / NH4SCN
Fe(SCN)63blood red
Fe3+(aq) + 6SCN-(aq) D Fe(SCN)63-(aq) blood red
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Background – Group III Separation Scheme
E – Test for Ni2+
Fe3+,
Ni2+,
Mn2+
• The presence of the nickel ion is
confirmed by the addition of
dimethylglyoxime.
• Dimethylglyoxime combines with the
nickel ion to form a complex which forms
a strawberry red precipiate.
Divide
Sample
NH3
Ni(NH3)62+
Fe(OH)3
HCl / NH4SCN
H2DMG
Fe(SCN)63blood red
Ni(DMG)2
strawberry red ppt.
Ni(NH3)62+(aq) + 2 HC4H7N2O2(aq)  4NH3(aq) + 2NH4+(aq) + Ni(C4H7N2O2)2(s) (red)
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Background – Group III Separation Scheme
Al(OH)4-,
Zn(OH)42-
HNO3
NH3
Al(OH)3
Zn(NH3)42+
F1 – Separation of Al3+ and Zn2+
• Careful control of pH allows for the
separation of aluminum and zinc ions.
• The solution is made very slightly basic.
• At these conditions, the aluminum ion
precipitates as aluminum hydroxide.
• The zinc ion remains in solution.
Al3+(aq) + 3NH3(aq) + 3H2O(l) D NH4+(aq) + Al(OH)3(s)
Zn2+(aq) + 4NH3(aq) D Zn(NH3)42+(aq)
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Background – Group III Separation Scheme
Al(OH)4-, Zn(OH)42-
NH3
HNO3
Al(OH)3
Zn(NH3)42+
NH3,
aluminon
F2 – Test for Al3+
• A successful test for aluminum requires
that the previous reactions and their pH
control were properly performed.
• If not, false positive tests result.
• The test for aluminum requires the free
aluminum ion to react with ammonia in the
presence of a reagent called aluminon and
form a red precipitate.
• Be careful, if there is iron or zinc left in the
sample, a red precipitate will form resulting
in a false positive.
Al(OH)3 aluminon
cherry red ppt.
Al3+(aq) + NH3(aq) + H2O + aluminon(aq) D NH4+(aq) + Al(OH)3aluminon(s) (red)
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Background – Group III Separation Scheme
F2 – Test for Al3+
Al(OH)4-, Zn(OH)42-
• To confirm that the red precipitate is the
aluminum complex, ammonium carbonate
is added.
• If the red color does not fade, aluminum is
present.
NH3
HNO3
Al(OH)3
Zn(NH3)42+
NH3,
aluminon
Al(OH)3 aluminon
cherry red ppt.
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Background – Group III Separation Scheme
G – Test for Zn2+
Al(OH)4-, Zn(OH)42-
NH3
HNO3
Zn(NH3)42+
Al(OH)3
NH3,
aluminon
K4Fe(CN)6
Al(OH)3 aluminon
cherry red ppt.
• To test for the zinc ion, a solution of
potassium hexacyanoferrate(II) is
added to the test solution.
• If zinc is present a white precipitate
forms.
• The exact color of the precipitate can
vary depending on the presence of
other ions.
• If iron is present the color can change
to yellow, green or blue.
K2Zn3[Fe(CN)6]2
white ppt.
Zn2+(aq) + K+(aq) + Fe(CN)64 (aq) D K2Zn3[Fe(CN)6]2 (s)
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Background – Group III Separation Scheme
Zn2+
Al3+
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Ni2+
Fe3+
Mn2+
Safety
• Concentrated HNO3 causes severe burns to the skin which can take weeks
to completely heal and also can cause serious eye damage. This chemical
also destroys books and clothing. If you get any on you, wash the affected
area(s) with copious quantities of water for ten minutes.
• Sodium hydroxide causes severe eye damage! If you spill some in your
eyes, wash the eyes for at least 15 minutes. Get immediate medical
attention.
• Concentrated ammonia (NH3) is corrosive. If you spill some on your skin,
wash the affected area for five minutes. If the acids get in the eyes, rinse the
eyes out at the eyewash station for fifteen minutes and get prompt medical
attention. In addition, the vapors of concentrated ammonia are very irritating.
• Other 6 M acids can cause burns. If you spill some on your skin, wash the
affected area with water for five minutes.
• Thioacetamide should always be handled in solution. Carefully wash any of
the spilled reagent from your skin. Avoid breathing the toxic fumes of H2S
which are evolved from this reagent.
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Waste Disposal
Because the solutions used in this experiment contain ions
that can hazardous to the environment and to human
health, all solutions and precipitates used in this
experiment must be placed in the container marked
"Recovered Metals and Metal Ions".
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