Separation of Group III Cations
The group III cations form
insoluble sulfide or hydroxide
salts in basic solution. They
are distinguished from group
II cations because these salts
dissolve in acid.
Al3+
Cr3+
Fe2+,
Fe3+
Ni2+
Co2+
Zn2+
Chemistry 123 – Dr. Woodward
Al3+, Cr3+, Fe2+, Fe3+, Ni2+, Co2+, Zn2+
H2O2 + HCl
Al3+, Cr3+, Fe3+, Ni2+, Co2+, Zn2+
Removes excess
H2O2, be careful
not to overdo it.
Evaporate to a paste
NH4Cl acts as a
buffer to keep the
solution from
becoming too basic
Dissolve with H2O/HCl
NH4Cl + NH3
Al(OH)3, Fe(OH)3, Cr(OH)3
Al3+(aq)
+
3OH−(aq)
H2O2 oxidizes any
Fe2+ to Fe3+. The
solution is acidic to
prevent oxidation of
Cr3+ and Co2+.
Ni(NH3)62+, Co(NH3)62+, Zn(NH3)62+
Ni2+(aq)
↔ Al(OH)3(s)
white ppt
+ 6NH3(aq) ↔ Ni(NH3)62+(aq)
blue
Cr3+(aq) + 3OH−(aq) ↔ Cr(OH)3(s)
graygray-green ppt
Co2+(aq) + 6NH3(aq) ↔ Co(NH3)62+(aq)
pink
Fe3+(aq) + 3OH−(aq) ↔ Fe(OH)3(s)
rust ppt
Zn2+(aq) + 4NH
Zn(NH
)42+(aq)
3(aq) ↔123
Chemistry
– Dr. 3Woodward
colorless
Why is the H2O2 pretreatment needed?
Later on we will separate iron from chromium and
aluminum based on the insolubility of its hydroxide salt.
Consider the differences in solubility of Fe(OH)3 and
Fe(OH)2:
Fe(OH)2
Cr(OH)3
Al(OH)3
Fe(OH)3
Ksp
Ksp
Ksp
Ksp
=
=
=
=
7.9
6.7
4.6
6.3
×
×
×
×
10−15
10−31
10−33
10−38
This separation will not work if the iron is Fe2+. The
hydrogen peroxide oxidizes any Fe2+ to Fe3+:
2Fe2+(aq) + H2O2(aq) + 2H+(aq) → 2Fe3+(aq) + 2H2O(aq)
Chemistry 123 – Dr. Woodward
1
Aluminum Subgroup
Don’t leave solution
in test tube overnight
or Al3+ will leach
from the glass
Al(OH)3, Fe(OH)3, Cr(OH)3
NaOH + H2O2
Al(OH)4−
Fe(OH)3
CrO42−
colorless sol’n
rust ppt
yellow sol’n
Cr(OH)3(s) + 3H2O2 + 4OH−
↔ 2CrO42−(aq) + 8H2O
Al(OH)3(s) + OH− ↔ Al(OH)4−(aq)
1. Acidify
2. NH3 + NH4Cl
Ppt should be white.
If it is not clean it up
or you may get a false
test for Al3+
Al(OH)3 white ppt
CrO42− yellow sol’n
Chemistry 123 – Dr. Woodward
Nickel Subgroup
Ni(NH3)62+, Co(NH3)62+, Zn(NH3)62+
Make sure to
heat as directed.
Don’t cut short
heating time or
separation of Zn
may not work.
NH3, CH3CSNH2 + heat
NiS, CoS, ZnS
The separations in
this subgroup are
not very clean. Pay
attention to colors to
get clues. Follow
directions closely.
Ni(NH3)62+ + HS− ↔ NH4+ + 5NH3 + NiS(s)
NiS(s) black ppt
Co(NH3)62+ + HS− ↔ NH4+ + 5NH3 + CoS(s)
CoS(s) black ppt
Zn(NH3)42+ + HS− ↔ NH4+ + 3NH3 + ZnS(s)
ZnS(s) white ppt
Cold 1 M HCl
Zn2+
NH3, CH3CSNH2 + heat
ZnS
white ppt
Make sure the ppt
is white. If not
you have Co
and/or Ni
contamination
NiS, CoS
Chemistry 123 – Dr. Woodward
Nickel and Cobalt
NiS (black ppt), CoS (black ppt)
Dissolve in aqua regia (HCl + HNO3)
NiCl42− (blue soln), CoCl42− (blue soln)
NH3, HDMG
NH4SCN,
acetone
Ni(DMG)2
Co(NCS)42−
brilliant red ppt
Blue soln
Co contamination
will form a brown
ion. If observed
add more HDMG
Chemistry 123 – Dr. Woodward
2
Qualitative Analysis of a Simple Salt
Experiment 26
Ag+, Pb2+, Bi3+, Cu2+
Al3+, Cr3+, Ni2+, Co2+
Zn2+, Sb3+/Sb5+
Sn2+/Sn4+, Fe2+/Fe3+
Cl−, Br−, I−,
SO42−, SO32−, S2−
PO43−, NO3−, CO32−
Examples: Bi(OH)3, ZnCO3, Ni(NO3)2, CoS,
CoS, AgCl,
AgCl, Cu3(PO4)3
Your assignment is to identify both the
cation and the anion
Chemistry 123 – Dr. Woodward
Initial Characterization
• Note the color and physical appearance (lime green
powder, shiny black crystals, wet transparent solid,
etc.)
• Test the solubility in water
• Test the solubility in HCl, HNO3, NaOH & NH3
• Note the color of the solution when you test solubility
– Some ions will be different colors than their salts (i.e. NiS is
black, Ni2+ ions in aqueous solution are green, Ni(NH3)62+ is blue)
– The appearance of a color in solution can be a clue that the
salt is slightly soluble (even though not all of the solid
dissolves)
Chemistry 123 – Dr. Woodward
Solubility and your Anion
Anion
Solubility
Exceptions
Chloride, Cl−
Soluble
AgCl, PbCl2
Br−
Soluble
AgBr, PbBr2
Soluble
AgI, PbI2, BiI3
Soluble
PbSO4, Ag2SO4
Bromide,
Iodide, I−
Sulfate, SO42−
Insoluble
None
Carbonate, CO32−
Sulfite, SO3
Insoluble
None
Phosphate, PO4
Insoluble
None
2−
3−
Chemistry 123 – Dr. Woodward
3
Colors of Ions in Solution
Color
Ions
Color
Ions
Green
Pink
Co2+
Blue
Cu2+, Fe2+, Cr3+,
Ni2+
Cu2+, Co2+, Ni2+
Orange
Cr2O72−, NO2, I2
Purple
Cr3+, Co2+
Yellow
Colorless
Ag+, Pb2+, Bi3+
Sb3+, Sb5+
Colorless
CrO42−, Fe3+, I2,
Sxn−
2+
Sn , Sn4+, Al3+,
Zn2+
Chemistry 123 – Dr. Woodward
4