Group III Analysis

Your unknown may contain Ni2+ Co2+, Zn2+, Mn2+, Al3+, Cr3+, and Fe2+ or Fe3+. (No distinction will be made between the two oxidation states of iron.) These are the ions whose sulfides are soluble in acid but precipitate in ammonia solution, because the sulfide concentration is greatly increased. Al3+ and Cr3+ form hydroxides instead of sulfides.

EQUATIONS:

NH3+H2SNH4++ S2

Ni2++S2NiS(black or brown)

Co2++S2CoS(black or brown)

Mn2++S2MnS(light pink)

Zn2++S2 ZnS (white)

Fe2++S2FeS(black)

Al3++3OH-Al(OH)3(white)

Cr3++3OH Cr(OH)3(bluegreen)

PROCEDURE: Be sure to record all reagents added, observations, conclusions, and all equations.

A.PREPARATION OF GROUP III CATIONS.

If the unknown is a general unknown, use the supernatant liquid (or filtrate) from the Group II analysis and proceed to paragraph A.2. If the unknown is Group III only, start with paragraph A.1.

1.Treat 50 mg of unknown powder with 1 mL of12 M HCl. Immerse the test tube in boiling water and stir continuously for a few seconds. When the powder is entirely dissolved, dilute with 1 mL of water. Use 1 mL for analysis.

2.To the test solution in a test tube, add 15 M ammonia solution dropwise until it is basic to litmus. If only one drop of ammonia was required, add 1/2 mL of 1 M NH4Cl Add 3 additional drops of 15 M NH3,

3.Saturate the mixture with H2S by adding 6 drops of 1 M thioacetamide, heat in a hot water bath (approximately 95°C) for several minutes, and centrifuge. Save the precipitate for further Group III analysis. If this is a general unknown, save the solution for Groups IV and V; if this is Group III only, discard the solution.

B.SEPARATION AND IDENTIFICATION OF COBALTAND NICKEL.

THEORY AND EQUATIONS:

Except for nickel and cobalt, the insoluble sulfides and hydroxides dissolve in 1 M HCl:

MnS+2H+Mn2++ H2S

Al(OH)3+3H+ Al3++3H2O

NiS and CoS change into a different crystalline form, less soluble in HCl. We use this as means of separating nickel and cobalt from the other ions in this group.

Equations for reactions in the identification of nickel and cobalt:

CoS + 8H++ 2NO33Co2++ 2NO + 3S+4H2O

NiS + 8H++ 2NO33Ni2++ 2NO + 3S+ 4H2O

Co2++6NH3[Co(NH3)6]2+(pink)

Ni2++6NH3[Ni(NH3)6]2+ (blue)

Final test for nickel:

Final tests for cobalt:

[Co(NH3)6]2+ + 4NCS [Co(NCS)4]2 + 6NH3

Co2+ + 6NO2 [Co(NO2)6]4-

[Co(NO2)6]4 + NO2 +2H+[Co(NO2)6]3 + NO + H2O

3K+ + (Co(NO2)6]3 K3[Co(NO2)6]

B.PROCEDURE:Separation and Identification of Co and Ni.

1.Wash the precipitate from A.3 twice with 2 mL of water and discard the meanings. Add 10 drops of 1 M HCl to the precipitate and stir the mixture. Centrifuge and separate the mixture immediately; prolonged contact with the acid cause s some dissolution of CoS and NiS. Wash the precipitate with 4 drops of 1 M HCl and quickly centrifuge and separate again. Save the combined centrifugates (solutions) for Step C.

2.To the precipitate from Step B.1 (CoS and/or NiS) add 3 drops of 12 M HCl and 1 drop of concentrated HNO3 and heat the mixture in a hot water bath until any reaction stops. Separate any sulfur that forms and boil the solution to remove any excess HNO3 or oxides of nitrogen (brown gas). Add sufficient 6 M aqueous ammonia to make the solution slightly basic to litmus. Dilute the solution to 1 mL and divide it into three parts.

3.Test for Nickel: To one part of the solution from Step B.2, add one drop of dimethylglyoxime. The formation of a pink or red precipitate confirms the presence of nickel. (A yellow-brown solution may indicate cobalt, but it is not proof.)

4.Test for Cobalt: Acidify a second portion of the solution from B.2 with 1 M HCl and add several crystals of NH4CNS to it. Now add an equal volume of acetone and stir the mixture. A blue color proves the presence of cobalt. If the solution becomes red upon the addition of NH4CNS, iron (III) ions are present. Add 1 drop of 1 M NaF to the solution to complex the iron. Now. If the solution is bluishgreen, the presence of cobalt is confirmed.

5.Test for Cobalt: Acidify a third portion of the solution from B.2 with 6 M acetic acid, and add several large crystals of KNO2. Warm the mixture. A yellow precipitate confirms the presence of cobalt.

C.SEPARATION AND IDENTIFICATION OF MANGANESE AND IRON.

THEORY AND EQUATIONS: Nitric acid is used to oxidize iron to +3, so that the more insoluble iron (III) hydroxide can form when NaOH is added. A small amount of NaOH will precipitate all of the ions that remain in the solution from Step B.1:

3Fe2++ 4H++ NO33Fe3+ + NO + 2H2O

Fe3++3OHFe(OH)3(reddishbrown)

Mn2++2OHMn(OH)2 (white)

A13++3OHAl(OH)3(white gelatinous)

Cr3++3OHCr(OH)3(dark greenblue)

Zn2++2OHZn(OH)2 (white)

Excess NaOH dissolves the amphoteric hydroxides of Al, Cr, and Zn by forming the hydroxy complex ions:

Al(OH)3+OH[Al(OH)4](colorless)

Cr(OH)3+OH(Cr(OH)4](green)

Zn(OH)2+ 2OH[Zn(OH)4]2(colorless)

In the NaOH solution, hydrogen peroxide oxidizes both Mn and Cr. Manganese is oxidized to MnO2 and/or Mn(OH)3 (brown precipitate), which are more insoluble than Mn(OH)2. Chromite ion, Cr(OH)4 (green solution), is oxidized to chromate ion, CrO42 (yellow solution). (It is essential that ALL the chromium be completely oxidized to chromate, as Cr(III) will interfere with the aluminum test.)

Mn(OH)2+H2O2MnO2+ H2O

Mn(OH)2+H2O22Mn(OH)3

2[Cr(OH)4] + 3H2O2 + 2OH2CrO42+ 8H2O

Ferric hydroxide is dissolved in nitric acid and the presence of iron is confirmed by adding thiocyanate ion, resulting in a deep red solution of the complex ion [Fe(NCS)]2+.

Fe(OH)3+ 3H+Fe3++ 3H2O

Fe3++SCN[Fe(NCS)]2+

Nitrite ion is added to reduce MnO2 and Mn(OH)3 to Mn2+. Sodium bismuthate oxidizes Mn2+ to red (pink to violet) MnO4 confirming the presence of manganese.

MnO2 + 2H+ + NO2  Mn2+ + NO3 + H2O

2Mn2+ + 5BiO3 + 14H+  2MnO4 + 5Bi3+ + 7H2O

C.PROCEDURE for Separation and identification of Mn and Fe

1.Transfer the solution from Step B.1 (Mn2+, Fe2+, Al3+, Cr3+, and Zn2+) to a casserole, add 1 mL of 6 M HNO3 and evaporate to a moist residue. Take up the residue in 1 mL of water and transfer the solution to a test tube. Add 6 M NaOH dropwise until precipitation begins, then add 10 more drops excess. Now add 6 drops of 3% hydrogen peroxide and heat it in a hot water bath for 5 minutes. Separate the mixture and wash the residue with 10 drops of water containing 1 drop of 6 M NaOH. Save the combined solutions for Part D.

2.The residue from Part C.1 may contain Fe(OH)3, Mn(OH)3, or MnO2. Treat the residue with 15 drops of 6 M HNO3 and 2 drops of 1 M NaNO2. Stir and heat in a hot water bath. Separate any residue that remains. Heat the solution to boiling. Cool, and divide it into two parts.

3.Test for Iron: Dilute one part of the solution to 1 mL and add 2 or 3 crystals of NH4CNS. If iron is present, the solution will turn red. WARNING: Trace amounts of iron may be present as impurities in reagents and may give a slight coloration for this test. Disregard a slight coloration; if iron is present in the unknown, a deeper red should result.)

4.Test for Manganese: To the second part of the solution from Step C.2 add a small quantity of solid NaBiO3 and a few drops of 6 M HNO3. The formation of a pink, red, or purple color confirms the Presence of manganese. (The color may fade due to the presence of reducing agents. If any red color appears even for an instant, you have manganese.)

D.SEPARATION AND IDENTIFICATION OF ALUMINUM, CHROMIUM, AND ZINC

THEORY AND EQUATIONS:

Acidification will convert the hydroxy complexes of aluminum and zinc to the hydrated ions, and will convert chromate ion to dichromate ion:

[Al(OH)4]+4H+Al3++ 4H2O

[Zn(OH)4]2+4H+Zn2++ 4H2O

2CrO42+2H+Cr2O72+ H2O

Addition of NH3 converts aluminum to insoluble aluminum hydroxide, separating it from soluble chromate ions and tetramminezinc(II) ions. (If chromium was not completely oxidized to chromate, Cr(OH)3 will precipitate and may give a false test for aluminum.)

Al3++3NH3+3H2OAl(OH)3+ 3NH4+

Cr2O72+2OH 2CrO4+ H2O

Zn2++4NH3Zn(NH3)4]

Aluminum is confirmed by precipitating aluminum hydroxide in the presence of a red dye. A yellow solution (chromate ion) indicates the presence of chromium, which is confirmed by precipitating lead chromate:

Pb(CH3COO)2+ CrO42-PbCrO4+ 2CH3COO-

The presence of zinc is confirmed in two ways: (1) by adding thioacetamide and precipitating white ZnS from the ammonia solution, and (2) by precipitating zinc hexacyanoferrate(II), which is also white:

2K++ Zn2++ [Fe(CN)6]4K2Zn[Fe(CN)6]

D.PROCEDURE for Separation and Identification of Al, Cr, Zn

1.To the solution from Part C.1 (CrO42, [Al(OH)4] and/or [Zn(OH)4]2), add 6 M acetic acid until the solution is acid to litmus, then add two or three drops of the acid in excess. Now add 6 M aqueous NH3 until distinctly basic to litmus. If a white gelatinous precipitate forms, often difficult to see, it is probably aluminum hydroxide. Separate the precipitate (for Step D.2), and save the solution, half for Step D.3, and half for Step D.4.

2.Confirmation of Aluminum: Dissolve the precipitate in 6 M acetic acid and add 2 drops of Aluminum Reagent (aluminon dye) and enough 1M (NH4)2CO3 to make the solution basic. The formation of a reddishcolored (dyed) precipitate confirms the presence of aluminum.

3.Test for Chromium: To one portion of the solution from Step D.1, add 1 M acetic acid until acid to litmus. Then add 2 drops of 0.1 M lead acetate solution. A yellow precipitate of PbCrO4 confirms chromium. A white precipitate is lead sulfate; if necessary, add 1 M ammonium acetate to dissolve the white precipitate, and then look for the yellow solid.

4.Test for Zinc: To the second portion of the solution from Step D.1, add 5 drops of thioacetamide solution and heat the mixture in a hot water bath. A white precipitate, soluble in 6 M HCl, indicates the presence of zinc. Heat the solution to expel H2S and then add 6 M aqueous NH3 until neutral. Add 10 drops of 1 M HCl and 5 drops of 0.1 M K4[Fe(CN)6]. Formation of a white precipitate, K2Zn(Fe(CN)6], proves that zinc is present.

Ni2+, Co2+, Mn2+, Fe3+, Zn2+, Al3+, Cr3+, Groups IV and V

[H2S, NH3]{Step A.1-3}

NiS, CoS, MnS, FeS, ZnS, Al(OH)3, Cr(OH)3 Groups IV and V

1M HCl {Step B.1}

NiS, CoSMn2+, Fe2+, Zn2+, Al3+, Cr3+

{Step B.2}[HNO3, Heat] {Step C.1}

[HCl, HNO3] [NaOH, H2O2]

Ni2+, Co2+ Mn(OH)3, MnO2, Fe(OH)3

{Step C.2}

[NH3] [HNO3, NO2-]

Ni(NH3)62+, Co(NH3)62+ Mn2+, Fe3+

{Step B.3} {Step B.4} {Step C.3}

[DMG][HCl, SCN-] {Step C.4} [SCN-]

[NaBiO3]

Ni(DMG)2Co(CNS)4-2 FeSCN2+

RedBlue-Green MnO4- Red

Pink toZn(OH)4-, Al(OH)4-, CrO4-2

{Step B.5} Violet

[HOAc, KNO2] [HOAc] {Step D.1}

K3[Co(NO2)6]Zn2+, Al3+, Cr2O72-

Yellow

[NH3]

Al(OH)3Zn(NH3)42+, CrO4-2

{Step D.2}

[HNO3] {Step D.3} {Step D.4}

[Aluminon, NH4)2CO3] [Pb2+] [H2S]

Al(OH)3PbCrO4 ZnS Plus

Dyed redYellow White followup

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