6142 M1 | [2] Qualitative Analysis
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Classification of Cations — Basis
Grouped according to their similarity to their reactions to the precipitants.
Group I — Name
Silver or Chloride Group
Group I — Group Precipitant
2M/3M HCl, NH₄Cl, or any soluble chloride salt.
Group I — Members
Ag⁺, Hg₂²⁺, Pb²⁺
Group I — Reaction Result
Precipitation of chloride salts (silver chloride, lead chloride, mercurous chloride).
Group II — Name
H₂S or Cu–Tin Group
Group II — Group Precipitant
Adjust H⁺ to 0.3M and saturate with H₂S.
Group II — Reaction Result
All members will be precipitated as sulfides.
Group II — Cu Subgroup
Cu²⁺, Cd²⁺, Pb²⁺, Bi³⁺
Group II — Cu Subgroup Solubility
Insoluble in Na₂S.
Group II — Tin Subgroup
Hg²⁺, As³⁺/As⁵⁺, Sn²⁺/Sn⁴⁺, Sn³⁺/Sn⁵⁺
Group II — Tin Subgroup Solubility
Soluble in Na₂S.
Group III — Name
(NH₄)₂S or Al–Fe Group
Group III — Group Precipitant
(NH₄)₂S or NH₄Cl, NH₃ and saturation of the solution with H₂S.
Group III — Formation of (NH₄)₂S
NH₄Cl + NH₃ + H₂S = (NH₄)₂S.
Group III — Reaction Result
All members will be precipitated as sulfides.
Group III — Al Subgroup
Al³⁺, Cr³⁺, Zn²⁺
Group III — Al Subgroup Solubility
Soluble in NaOH and Na₂O₂.
Group III — Fe Subgroup
Fe²⁺/Fe³⁺, Mn²⁺, Ni²⁺, Co²⁺
Group III — Fe Subgroup Solubility
Insoluble in NaOH and Na₂O₂.
Group III — Exception
Aluminum and chromium sulfides are highly unstable and will be oxidized to their hydroxide forms.
Group III — Final Products
Aluminum hydroxide and chromic hydroxide.
Group IV — Name
Alkaline Earth or (NH₄)₂CO₃ Group
Group IV — Group Precipitant
NH₃, NH₄Cl, (NH₄)₂CO₃ and 95% C₂H₅OH.
Group IV — Members
Ba²⁺, Sr²⁺, Ca²⁺, Mg²⁺.
Group IV — Purpose of Adding 95% Ethanol
Complete precipitation.
Group IV — Reaction Result
Members would be precipitated as carbonates.
Group V — Name
Alkali or Soluble Group.
Group V — Members
K⁺, Li⁺, Na⁺, NH₄⁺.
Group V — Group Precipitant
None.
Group V — Meaning of Soluble
No group precipitant.
Ag⁺ — Product
AgOH.
Ag⁺ — Visible Result
White precipitate.
Ag⁺ — With Excess NH₄OH
Ag(NH₃)₂⁺; soluble, colorless solution.
Cu²⁺ — Product
Cu(OH)NO₃.
Cu²⁺ — Visible Result
Green basic salt.
Cu²⁺ — With Excess NH₄OH
Cu(NH₃)₄²⁺ or Cu(NH₃)₄(OH)₂; soluble, blue solution.
Al³⁺ — Product and Visible Result
Al(OH)₃; white gelatinous precipitate.
Cr³⁺ — Product and Visible Result
Cr(OH)₃; grayish-green precipitate.
Zn²⁺ — Product and Visible Result
Zn(OH)₂; white precipitate.
Zn²⁺ — With Excess NH₄OH
Zn(NH₃)₄²⁺; soluble, colorless solution.
Mn²⁺ — Initial Product
Mn(OH)₂.
Mn²⁺ — Oxidation Product
MnO₂·H₂O.
Mn²⁺ — Visible Result
White to light-brown precipitate.
Fe³⁺ — Product and Visible Result
Fe(OH)₃; reddish-brown precipitate.
Fe²⁺ — Product and Visible Result
Fe(OH)₂; white to green to brown precipitate due to oxidation.
Co²⁺ — Product and Visible Result
Co(OH)NO₃; blue basic salt.
Co²⁺ — With Excess NH₄OH
Co(NH₃)₄²⁺; soluble, dirty-yellow solution.
Ni²⁺ — Product and Visible Result
Ni(OH)NO₃; green basic salt.
Ni²⁺ — With Excess NH₄OH
Ni(NH₃)₄²⁺; soluble, blue solution.
Ag⁺ — Product and Visible Result with NaOH
AgOH to Ag₂O (reduced form); white to brown precipitate.
Pb²⁺ — Product and Visible Result
Pb(OH)₂; white amorphous precipitate.
Pb²⁺ — With Excess NaOH
Pb(OH)₄²⁻ or Na₂PbO₂; colorless solution.
Hg₂²⁺ — Product and Visible Result
HgOH to Hg₂O (reduced form); black precipitate.
Al³⁺ — Product and Visible Result
Al(OH)₃; white gelatinous precipitate.
Cr³⁺ — Product and Visible Result
Cr(OH)₃; grayish-green precipitate.
Cr³⁺ — With Excess NaOH
NaCrO₂; soluble, green solution.
Cr³⁺ — With Na₂O₂ or H₂O₂
Na₂CrO₄; yellow solution.
Zn²⁺ — Product and Visible Result
Zn(OH)₂; white precipitate.
Zn²⁺ — With Excess NaOH
Na₂ZnO₂; soluble, colorless solution.
Mn²⁺ — Product and Visible Result
Mn(OH)₂; white precipitate.
Mn²⁺ — With Na₂O₂ or H₂O₂
MnO₂·H₂O (oxidized form); brown precipitate.
Fe³⁺ — Product and Visible Result
Fe(OH)₃; reddish-brown precipitate.
Fe²⁺ — Product and Visible Result
Fe(OH)₂; white to green to brown precipitate.
Fe²⁺ — With Na₂O₂ or H₂O₂
Fe(OH)₃ (oxidized to higher valence); reddish-brown precipitate.
Co²⁺ — Product and Visible Result
Co(OH)₂; blue precipitate on warming, pink precipitate.
Co²⁺ — With Na₂O₂ or H₂O₂
Co(OH)₃ (oxidized to higher valence); brown or black precipitate.
Ni²⁺ — Product and Visible Result
Ni(OH)₂; apple-green precipitate.
Ni²⁺ — With H₂O₂
Ni(OH)₃ (oxidized to higher valence); green precipitate.
Ni²⁺ — With Na₂O₂
No oxidation.
Pb²⁺ — Product and Visible Result with H₂S
PbS; black precipitate.
Bi³⁺ — Product and Visible Result with H₂S
Bi₂S₃; dark-brown or brownish-black precipitate.
Cu²⁺ — Product and Visible Result with H₂S
CuS; black precipitate.
Cd²⁺ — Product and Visible Result with H₂S
CdS; yellow precipitate.
Hg²⁺ — Product and Visible Result with H₂S
HgS; white to yellow to brown to black precipitate.
Hg²⁺ — With Na₂S
Na₂HgS₂; soluble, colorless solution.
Sb³⁺ — Product and Visible Result with H₂S
Sb₂S₃ (only orange sulfide); orange or reddish-orange precipitate.
Sb³⁺ — With Na₂S
Na₃SbS₄; soluble, colorless solution.
As³⁺ — Product and Visible Result with H₂S
As₂S₂; yellow precipitate.
As³⁺ — With Na₂S
Na₃AsS₄; soluble, colorless solution.
Sn²⁺ — Product and Visible Result with H₂S
SnS; light-brown precipitate.
Sn²⁺ — With Na₂S
Na₂SnS₃; soluble, colorless solution.
Sn⁴⁺ — Product and Visible Result with H₂S
SnS₂; yellow precipitate.
Sn⁴⁺ — With Na₂S
Na₂SnS₃; soluble, colorless solution.
Cu Subgroup and Tin Subgroup — Distinguishing Reagent
Sodium sulfide (Na₂S).
Cu Subgroup and Tin Subgroup — Distinguishing Principle
Sodium sulfide forms soluble products with the tin subgroup, producing colorless solutions.
Stannous Sulfide and Stannic Sulfide — Same Sodium Salt Formula
Na₂SnS₃.
Stannous Sulfide — Reaction with Na₂S
SnS is oxidized to its higher valence and becomes stannic sulfide; the product formed is the same as that of stannic sulfide.
Al³⁺ — With NH₄Cl and NH₄OH
Al(OH)₃; white gelatinous precipitate.
Cr³⁺ — With NH₄Cl and NH₄OH
Cr(OH)₃; grayish-green precipitate.
Zn²⁺ — With NH₄Cl and NH₄OH
Soluble.
Zn²⁺ — With (NH₄)₂S
ZnS; white metallic precipitate.
ZnS — Distinguishing Characteristic
The only white metallic sulfide.
Mn²⁺ — With NH₄Cl and NH₄OH
Soluble.
Mn²⁺ — With (NH₄)₂S
MnS; flesh or salmon-colored precipitate.
MnS — Distinguishing Characteristic
The only flesh/salmon-colored sulfide.
Fe³⁺ — With NH₄Cl and NH₄OH
Fe(OH)₃; reddish-brown precipitate.
Fe³⁺ — With (NH₄)₂S
Fe₂S₃; black precipitate.
Fe²⁺ — With NH₄Cl and NH₄OH
Soluble.
Fe²⁺ — With (NH₄)₂S
FeS; black precipitate.