valdosta state university experiment 10 group iii cation analysis part ii valdosta state university

Valdosta State University

Experiment 10Group III Cation Analysis

Part II



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.



Purpose

To determine which group III ions are present in an unknown solution.



Background


General Unknown

Group IIAcid Insoluble Metal Sulfides

Group IIIAlkaline Insoluble Metal Sulfides and Hydroxides

Group IVSoluble Metal

Ions

Group IInsoluble Metal Chlorides

and Ammonia

HCl

HCl / H2S

NH3 / H2S


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.


Background – Hydrogen Sulfide


CC

N

SH

HH

H

H

CC

O

OH

HH

H

+ 2 H2O + H+(aq) + NH4+(aq) + H2S(g)

H2S(aq) + 2 H2O(l) 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).


Background – Group III Separation Scheme


Group III Unknown

NiS, FeS, MnS, ZnS, Fe(OH)3, Al(OH)3

Group IV ions

Ni2+, Fe3+, Mn2+, Zn2+, Al3+

Waste

Al(OH)4-, Zn(OH)4

2-Fe(OH)3, Ni(OH)2, Mn(OH)2

MnO4-

purple

Fe(OH)3

Divide sample

Ni(NH3)62+

Ni(DMG)2

strawberry red ppt.

Fe(SCN)63-

blood red

Al(OH)3 Zn(NH3)42+

Al(OH)3aluminoncherry red ppt.

K2Zn3[Fe(CN)6]2white ppt.

NH3, H2S, heat

HCl, HNO3, heat

NaOH

NaBiO3

NH3

HCl / NH4SCN H2DMG

HNO3

NH3

aluminon, NH3

K4Fe(CN)6

HNO3




Group III unknown

NiS, FeS, MnS, Fe(OH)3, Al(OH)3, ZnS

Group IV ions

NH3, H2S, Heat

A – Preparation of Group III cations

• The group III ions are initially separated from the bulk solution by precipitation as either insoluble metal sulfides or hydroxides.

Ni2+(aq) + S2(aq) NiS(s) (black) Fe2+(aq) + S2(aq) FeS(s) (black) Zn2+(aq) + S2(aq) ZnS(s) (white) Mn2+(aq) + S2(aq) MnS(s) (pink) Al3+(aq) + 3 OH(aq) Al(OH)3(s) (white, gel)




Group III unknown


Group IV ions

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.

2 Fe3+(aq) + H2S (aq) 2 Fe2+(aq) + S(s) + 2 H+(aq)




Group III unknown


Group IV ions

NH3, H2S, Heat


• Alternately, the iron(III) can combine with the hydroxide ion and precipitate as iron(III) hydroxide.

Fe3+(aq) + 3 OH-(aq) Fe(OH)3 (rust color)




NiS, FeS, MnS, Fe(OH)3, Al(OH)3

Ni2+, Fe3+, Mn2+, Zn2+, Al3+Waste

HCl, HNO3, Heat

B1 – Separation of Group III cations

• Following the precipitation, the metal ions are combined with acid to form the free (and soluble) metal ions.

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)




Ni2+, Fe3+, Mn2+, Zn2+, Al3+

Al(OH)4-, Zn(OH)4


NaOH


• 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) Fe(OH)3(s) (rust-color)

Ni2+(aq) + 2 OH(aq) Ni(OH)2(s) (green)

Mn2+(aq) + 2 OH(aq) Mn(OH)2(s) (light brown)




Ni2+, Fe3+, Mn2+, Zn2+, Al3+

Al(OH)4-, Zn(OH)4


NaOH


• 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) Al(OH)3(s) (white, gelatinous)

Zn2+(aq) + 2 OH(aq) Zn(OH)2(s) (white)




Ni2+, Fe3+, Mn2+, Zn2+, Al3+

Al(OH)4-, Zn(OH)4


NaOH


Excess Acid

Al(OH)3(s) + 3H+(aq) Al3+ + 3 H2O(aq)

Zn(OH)2(s) + 2H+(aq) Zn2+ + 2 H2O(aq)




Ni2+, Fe3+, Mn2+, Zn2+, Al3+

Al(OH)4-, Zn(OH)4


NaOH


Excess Base

Al(OH)3(s) + OH(aq) Al(OH)4-(aq)

Zn(OH)2(s) + 2OH(aq) Zn(OH)42-(aq)




Fe(OH)3, Ni(OH)2, Mn(OH)2

Fe3+, Ni2+, Mn2+

C1 – Test for Mn2+, Fe3+, Ni2+

HNO3, KNO2• 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.

DivideSample




DivideSample

MnO4-

purple

NaBiO3

C2 – Test for 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.

14H+(aq) + 2Mn2+(aq) + 5BiO3-(s) 2 MnO4

-(aq) + 5Bi3+(aq) + 7H2O(l)

Fe3+, Ni2+, Mn2+




DivideSample

D1 – Separation of Fe3+ and Ni2+

Fe(OH)3 Ni(NH3)62+

NH3

• 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) Ni(NH3)6

2+(aq) (blue)

Fe3+, Ni2+, Mn2+




DivideSample

D2 – Test for Fe3+

Fe(OH)3 Ni(NH3)62+

NH3

Fe(SCN)63-

blood red

HCl / NH4SCN

• 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.

Fe3+(aq) + 6SCN-(aq) Fe(SCN)63-(aq) blood red

Fe3+, Ni2+, Mn2+




DivideSample

E – Test for Ni2+

Fe(OH)3 Ni(NH3)62+

NH3

Fe(SCN)63-

blood red

HCl / NH4SCN

Ni(DMG)2

strawberry red ppt.

H2DMG

• 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.

Ni(NH3)62+(aq) + 2 HC4H7N2O2(aq) 4NH3(aq) + 2NH4

+(aq) + Ni(C4H7N2O2)2(s) (red)

Fe3+, Ni2+, Mn2+




Al(OH)4-, Zn(OH)4

2-F1 – Separation of Al3+ and Zn2+

Al(OH)3 Zn(NH3)42+

NH3

HNO3

• 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) NH4+(aq) + Al(OH)3(s)

Zn2+(aq) + 4NH3(aq) Zn(NH3)42+(aq)




Al(OH)4-, Zn(OH)4

2-

F2 – Test for Al3+

Al(OH)3 Zn(NH3)42+

NH3

HNO3

Al(OH)3 aluminoncherry red ppt.

NH3, aluminon

• 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.

Al3+(aq) + NH3(aq) + H2O + aluminon(aq) NH4+(aq) + Al(OH)3aluminon(s) (red)




Al(OH)4-, Zn(OH)4

2-

F2 – Test for Al3+

Al(OH)3 Zn(NH3)42+

NH3

HNO3


NH3, aluminon

• To confirm that the red precipitate is the aluminum complex, ammonium carbonate is added.

• If the red color does not fade, aluminum is present.




Al(OH)4-, Zn(OH)4

2-

G – Test for Zn2+

Al(OH)3 Zn(NH3)42+

NH3

HNO3


NH3, aluminon

K2Zn3[Fe(CN)6]2

white ppt.

K4Fe(CN)6

• 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.

Zn2+(aq) + K+(aq) + Fe(CN)64 (aq) K2Zn3[Fe(CN)6]2 (s)




Zn2+ Al3+ 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.


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".


valdosta state university experiment 10 group iii cation analysis part ii valdosta state university

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