5&6 Analisis Kation
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Transcript of 5&6 Analisis Kation
Analisis Kation
19-2
Figure 19.16
The general procedure for separating ions in qualitative analysis
Add precipitating
ionC
entr
ifuge
Add precipitating
ion
Cen
trifu
ge
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
19-3
A qualitative analysis scheme for separating cations into five ion groups
Add 6M HCl
Cen
trifu
geAcidify to pH 0.5;
add H2S
Cen
trifu
ge
Add NH3/NH4
+ buffer(pH 8)
Cen
trifu
ge
Add (NH4)2HPO4
Cen
trifu
ge
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
19-4
Figure 19.17 A qualitative analysis scheme for separating cations into five ion groups
Add 6M HCl
Cen
trifu
geAcidify to pH 0.5;
add H2S
Cen
trifu
ge
Add NH3/NH4
+ buffer(pH 8)
Cen
trifu
ge
Add (NH4)2HPO4
Cen
trifu
ge
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
19-5
Using pH and complexation to Separate IonsFor Qualitative Analysis
19-6
Hot water
Pb2+AgCl(s); Hg2Cl2(s)
Pb C rO PbC rOaqueous aqueous so lid2
42
4
, ,
ppt
CrO aqueous42
,
19-7
NH3
HgNH2Cl(s)
Ag(NH3)6+
Hot water
ppt
CrO aqueous42
,
Pb2+AgCl(s); Hg2Cl2(s)
Ag NH AgNH
AgNH NH Ag NH
aqueous aqueous aqueous
aqueous aqueous aqueous
3 3
3 3 3 2
19-8
Figure 19.18Step 1
Add NH3(aq)
Cen
trifu
ge
Cen
trifu
ge
Step 2 Add HCl
Step 3 Add NaOH
Cen
trifu
geStep 4
Add HCl, Na2HPO4
Step 5 Dissolve in
HCl and add KSCN
A qualitative analysis scheme for Ag+,Al3+,Cu2+, and Fe3+
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Background
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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
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Background
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• 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
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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).
Thioacetamide
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Background – Group III Separation Scheme
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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
6 M NaOH
NaBiO3
Conc.NH3
HCl / NH4SCN H2DMG
HNO3
NH3
aluminon, NH3
K4Fe(CN)6
HNO3
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Background – Group III Separation Scheme
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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)
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Background – Group III Separation Scheme
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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
• 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
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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
• 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)
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Background – Group III Separation Scheme
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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)
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Background – Group III Separation Scheme
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Ni2+, Fe3+, Mn2+, Zn2+, Al3+
Al(OH)4-, Zn(OH)4
2-Fe(OH)3, Ni(OH)2, Mn(OH)2
6 M NaOH
B2 – Separation of Group III cations
• 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)
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Background – Group III Separation Scheme
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Ni2+, Fe3+, Mn2+, Zn2+, Al3+
Al(OH)4-, Zn(OH)4
2-Fe(OH)3, Ni(OH)2, Mn(OH)2
6 M NaOH
B2 – Separation of Group III cations
• 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)
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Background – Group III Separation Scheme
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Ni2+, Fe3+, Mn2+, Zn2+, Al3+
Al(OH)4-, Zn(OH)4
2-Fe(OH)3, Ni(OH)2, Mn(OH)2
6 M NaOH
B2 – Separation of Group III cations
Excess Acid
Al(OH)3(s) + 3H+(aq) Al3+ + 3 H2O(aq)
Zn(OH)2(s) + 2H+(aq) Zn2+ + 2 H2O(aq)
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Background – Group III Separation Scheme
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Ni2+, Fe3+, Mn2+, Zn2+, Al3+
Al(OH)4-, Zn(OH)4
2-Fe(OH)3, Ni(OH)2, Mn(OH)2
6 M NaOH
B2 – Separation of Group III cations
Excess Base
Al(OH)3(s) + OH(aq) Al(OH)4-(aq)
Zn(OH)2(s) + 2OH(aq) Zn(OH)42-(aq)
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Background – Group III Separation Scheme
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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
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Background – Group III Separation Scheme
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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+
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Background – Group III Separation Scheme
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DivideSample
D1 – Separation of Fe3+ and Ni2+
Fe(OH)3 Ni(NH3)62+
Conc. 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+
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Background – Group III Separation Scheme
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DivideSample
D2 – Test for Fe3+
Fe(OH)3 Ni(NH3)62+
Conc. 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+
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Background – Group III Separation Scheme
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DivideSample
E – Test for Ni2+
Fe(OH)3 Ni(NH3)62+
Conc. 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+
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Background – Group III Separation Scheme
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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) + 3 NH3(aq) + 3 H2O(l) 3 NH4+(aq) + Al(OH)3(s)
Zn2+(aq) + 4 NH3(aq) Zn(NH3)42+(aq)
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Background – Group III Separation Scheme
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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) + 3 NH3(aq) + 3 H2O + aluminon(aq) 3 NH4+(aq) + Al(OH)3aluminon(s) (red)
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Background – Group III Separation Scheme
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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
• To confirm that the red precipitate is the aluminum complex, ammonium carbonate is added.
• If the red color does not fade, aluminum is present.
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Background – Group III Separation Scheme
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Al(OH)4-, Zn(OH)4
2-
G – Test for Zn2+
Al(OH)3 Zn(NH3)42+
NH3
HNO3
Al(OH)3 aluminoncherry red ppt.
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.
3 Zn2+(aq) + 2 K+(aq) + 2 Fe(CN)64 (aq) K2Zn3[Fe(CN)6]2 (s)
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Background – Group III Separation Scheme
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Zn2+ Al3+ Ni2+ Fe3+ Mn2+
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Background
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Group IV Unknown
BaCrO4
yellow ppt.
Flame Test Na+
orange-yellow flame
Ca2+, Mg2+
HC2H3O2, K2CrO4
Ba2+ Flame Testapple - green
BaSO4
white ppt.
6M HCl
6M H2SO4
CaC2O4
white ppt.Mg2+
MgNH4PO46H2Owhite ppt.
(NH4)2C2O4
NH3(aq), NaH2PO4, heat
Flame Testred-orange
6M HCl
Flame Test K+
lavender flame
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Background – Group IV Separation Scheme
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A – Flame test for Na+ and K+
• Insoluble salts of sodium and potassium are not known.
• One method of determining the presence of these ions is the flame test.
Group IV Unknown
Flame Test Na+
orange-yellow flameFlame Test K+
lavender flame
Na K
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Background – Group IV Separation Scheme
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B – Test for Ba2+
• The formation of a yellow precipitate on the addition of potassium chromate indicates the presence of the barium ion.
HC2H3O2,K2CrO4
BaCrO4
yellow ppt.
Ba2+
6M HCl
Flame Testapple - green
BaSO4
white ppt.
6M H2SO4
Ba2+(aq) + K2CrO4(aq) BaCrO4(s) + 2K+(aq)
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Background – Group IV Separation Scheme
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B – Test for Ba2+
• A flame test (apple green) is used to confirm the presence of the ion.
HC2H3O2,K2CrO4
BaCrO4
yellow ppt.
Ba2+
6M HCl
Flame Testapple - green
BaSO4
white ppt.
6M H2SO4
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Background – Group IV Separation Scheme
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B – Test for Ba2+
• The final test for barium is the formation of a white precipitate on the addition of a sulfate to the solution.
HC2H3O2,K2CrO4
BaCrO4
yellow ppt.
Ba2+
6M HCl
Flame Testapple - green
BaSO4
white ppt.
6M H2SO4
Ba2+(aq) + H2SO4(aq) BaSO4(s) + 2H+(aq)
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Background – Group IV Separation Scheme
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C – Test for Ca2+
• The calcium ion is separated from magnesium by precipitating calcium oxalate.
Ca2+, Mg2+
CaC2O4
(NH4)2C2O4
Mg2+
Flame testred-orange
6M HCl
Ca2+(aq) + (NH4)2C2O4(aq) CaC2O4(s) + 2NH4
+(aq)
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Background – Group IV Separation Scheme
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C – Test for Ca2+
• A flame test (red-orange flame) is used to confirm the presence of the calcium ion.
Ca2+, Mg2+
CaC2O4
(NH4)2C2O4
Mg2+
Flame testred-orange
6M HCl
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Background – Group IV Separation Scheme
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C – Test for Mg2+
• The magnesium ion is precipitated as a white solid (ammonium phosphate) in an alkaline solution.
Ca2+, Mg2+
CaC2O4
K2C2O4
Mg2+
Flame testred-orange
6M HCl
Mg2+(aq) + NH3(aq) + HPO42-(aq) MgNH4PO4(s)
MgNH4PO4–6H2Owhite ppt.
NH3(aq)Na2HPO4(aq)
Sample Problem 19.12 Separating Ions by Selective Precipitation
SOLUTION:
PROBLEM: A solution consists of 0.20M MgCl2 and 0.10M CuCl2. Calculate the [OH-] that would separate the metal ions as their hydroxides. Ksp of Mg(OH)2= is 6.3x10-10; Ksp of Cu(OH)2 is 2.2x10-20.
PLAN: Both precipitates are of the same ion ratio, 1:2, so we can compare their Ksp
values to determine which has the greater solubility.
It is obvious that Cu(OH)2 will precipitate first so we calculate the [OH-] needed for a saturated solution of Mg(OH)2. This should ensure that we do not precipitate Mg(OH)2. Then we can check how much Cu2+ remains in solution.
Mg(OH)2(s) Mg2+(aq) + 2OH-(aq) Ksp = 6.3x10-10
Cu(OH)2(s) Cu2+(aq) + 2OH-(aq) Ksp = 2.2x10-20
[OH-] needed for a saturated Mg(OH)2 solution =
Ksp
[Mg2]
6.3x10 10
0.20= 5.6x10-5M