1. Experiment 10 Group III Cation Analysis Part II
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2. 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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3. Purpose
To determine which group III ions are present in an unknown solution.
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4. Background
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5. 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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6. Background – Hydrogen Sulfide
H2S(aq) 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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7. Background – Group III Separation Scheme
HNO3
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8. Background – Group III Separation Scheme
Group III unknown
A – Preparation of Group III cations
NH3, H2S, Heat
The group III ions are initially separated from the bulk solution by precipitation as either insoluble metal sulfides or hydroxides.
NiS, FeS, MnS, Fe(OH)3, Al(OH)3, ZnS
Group IV ions
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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9. Background – Group III Separation Scheme
Group III unknown
A – Preparation of Group III cations
NH3, H2S, Heat
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.
NiS, FeS, MnS, Fe(OH)3, Al(OH)3, ZnS
Group IV ions
2 Fe3+(aq) + H2S (aq)  2 Fe2+(aq) + S(s) + 2 H+(aq)
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10. Background – Group III Separation Scheme
Group III unknown
A – Preparation of Group III cations
NH3, H2S, Heat
Alternately, the iron(III) can combine with the hydroxide ion and precipitate as iron(III) hydroxide.
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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11. Background – Group III Separation Scheme
B1 – Separation of Group III cations
NiS, FeS, MnS, Fe(OH)3, Al(OH)3
Following the precipitation, the metal ions are combined with acid to form the free (and soluble) metal ions.
HCl, HNO3, Heat
Waste
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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12. Background – Group III Separation Scheme
B2 – Separation of Group III cations
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
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.
NaOH
Fe(OH)3, Ni(OH)2, Mn(OH)2
Al(OH)4-, Zn(OH)42-
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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13. Background – Group III Separation Scheme
B2 – Separation of Group III cations
Ni2+, Fe3+, Mn2+, Zn2+, Al3+
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.
NaOH
Fe(OH)3, Ni(OH)2, Mn(OH)2
Al(OH)4-, Zn(OH)42-
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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14. 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 Al H2O(aq)
Zn(OH)2(s) + 2H+(aq) D Zn H2O(aq)
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15. 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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16. Background – Group III Separation Scheme
C1 – Test for Mn2+, Fe3+, Ni2+
Fe(OH)3, Ni(OH)2, Mn(OH)2
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.
HNO3, KNO2
Fe3+, Ni2+, Mn2+
Divide Sample
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17. Background – Group III Separation Scheme
C2 – Test for Mn2+
Fe3+, Ni2+, Mn2+
If sodium bismuthate is added to a solution containing manganese(II), a redox reaction occurs resulting in the formation of the purple permanganate ion.
Divide Sample
NaBiO3
MnO4- purple
14H+(aq) + 2Mn2+(aq) + 5BiO3-(s)  2 MnO4-(aq) + 5Bi3+(aq) + 7H2O(l)
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18. Background – Group III Separation Scheme
D1 – Separation of Fe3+ and Ni2+
Fe3+, Ni2+, Mn2+
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).
Divide Sample
NH3
Fe(OH)3
Ni(NH3)62+
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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19. 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)63-
blood red
Fe3+(aq) + 6SCN-(aq) D Fe(SCN)63-(aq) blood red
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20. 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
Fe(OH)3
Ni(NH3)62+
HCl / NH4SCN
H2DMG
Fe(SCN)63-
blood 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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21. Background – Group III Separation Scheme
F1 – Separation of Al3+ and Zn2+
Al(OH)4-, Zn(OH)42-
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.
HNO3
NH3
Al(OH)3
Zn(NH3)42+
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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22. Background – Group III Separation Scheme
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)4-, Zn(OH)42-
NH3
HNO3
Al(OH)3
Zn(NH3)42+
NH3, aluminon
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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23. 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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24. Background – Group III Separation Scheme
G – Test for Zn2+
Al(OH)4-, Zn(OH)42-
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.
NH3
HNO3
Al(OH)3
Zn(NH3)42+
NH3, aluminon
K4Fe(CN)6
Al(OH)3 aluminon cherry red ppt.
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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25. Background – Group III Separation Scheme
Zn2+
Al3+
Ni2+
Fe3+
Mn2+
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26. 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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27. 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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