1. Qualitative Analysis of Cations

2. Classic Separation
Scheme for
Qualitative Analysis of
21 Common Cations
Zumdahl 7e, Figure 15.12.
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3. Group I Cations
Ag+, Pb2+
Add dilute HCl
AgCl(s) white precipitate
PbCl2(s) white precipitate
Ag+ + Cl  AgCl(s)
Pb2+ + 2Cl  PbCl2(s)

4. Group II Cations
Cu(II), Cd(II), As(III), Sb(III), Bi(III), Sn(IV)
Add H2S (acidic)
CuS(s), CdS(s), As2S3(s), Sb2S3(s), Bi2S3(s), SnS2(s)
Cu2+(aq) + H2S(aq)  CuS(s) + 2H+

5. Precipitation of Bismuth Sulfide
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6. From Left to Right: Cadmium Sulfide, Chromium(III)
Hydroxide, Aluminum Hydroxide, and Nickel(II) Hydroxide
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7. Figure 15.13 The
Separation of the
Group I Ions in
the Classic
Scheme of
Qualitative
Analysis
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8. Aqueous Ammonia is Added to Silver Chloride (white). Silver Chloride,
Insoluble in Water, Dissolves to Form Ag(NH3)2+ (aq) and Cl-(aq)
AgCl(s) + 2NH3(aq) Ý Ag(NH3)2+(aq) + Cl (aq)
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9. Flame Test
for Potassium
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10. Flame Test
for Sodium
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11. Preliminary Tests
1. Add H2O 4. Add HCl
2. Add NaOH 5. Add sulfide
a. Dilute NaOH a. Acidic
b. Excess NaOH b. Basic
3. Add NH3 6. Add Na2CO3
a. Dilute NH3 7. Add H2SO4
b. Excess NH3 8. Flame tests

12. Preliminary Tests
• Test 1. Add water
– Insoluble oxychlorides may be observed as a faint
white cloudiness (turbidity)
• Sb3+ + H2O + Cl Ý SbOCl(s) + 2H+
• Bi3+ + H2O + Cl Ý BiOCl(s) + 2H+
– Increased acid concentration favors soluble form
All species are in aqueous solution unless indicated otherwise

13. Preliminary Tests
• Test 2. Reactions with NaOH
– Test 2a. Dilute NaOH.
• Most cations form insoluble hydroxides
– Mg2+ + 2OH  Mg(OH)2(s)
– Al3+ + 3OH  Al(OH)3(s)
– Test 2b. Excess NaOH
• Most metal hydroxides remain insoluble
– Mg(OH)2(s)
• Amphoteric hydroxides dissolve
– Al(OH)3(s) + OH  Al(OH)4
All species are in aqueous solution unless indicated otherwise

14. Reactions with NaOH
Amphoteric metal hydroxides

15. Preliminary Tests
• Test 3. Reactions with Ammonia
– Test 3a. Dilute NH3. Weak base raises pH.
• Mg2+ + 2NH3 + 2H2O  Mg(OH)2(s) + 2NH4+
• Ni2+ + 2NH3 + 2H2O  Ni(OH)2(s) + 2NH4+
– Test 3b. Excess NH3.
• Most metal hydroxides remain insoluble.
– Mg(OH)2(s)
• Soluble ammine complexes form in some cases.
– Ni(OH)2(s) + 6NH3  Ni(NH3)62+ + 2OH
All species are in aqueous solution unless indicated otherwise

16. Reactions with NH3
Soluble ammine complexes

17. Preliminary Tests
• Test 4. Add HCl
– Solubility Rule: All chlorides are soluble, except
those of Ag+, Pb2+ and Hg22+
– Reaction with HCl
• Ag+ + Cl  AgCl(s)
All species are in aqueous solution unless indicated otherwise

18. Preliminary Tests
• Test 5. Add sulfide
– Heating thioacetamide in water produces H2S
– Test 5a. Acid sulfide. The Group II cations precipitate
in acidic (0.3 M HCl) sulfide solution.
• Cu2+ + H2S  CuS(s) + 2H+
– Test 5b. Basic sulfide. The Group III cations
precipitate when the solution is made basic
• Zn2+ + H2S  ZnS(s) + 2H+
All species are in aqueous solution unless indicated otherwise

19. Groups II and III Cations

20. Preliminary Tests
• Test 6. Reactions with Na2CO3
– Most metal carbonates are insoluble
• Ni2+ + CO32  NiCO3(s)
• Ca2+ + CO32  CaCO3(s)
• Exceptions: K+, Na+, NH4+
– In sulfide separation scheme
• most cations are removed in Groups I, II, and III as insoluble
chlorides or sulfides
• the remaining insoluble carbonates precipitate as the Group
IV cations.
– Ca2+, Mg2+, Ba2+
All species are in aqueous solution unless indicated otherwise

21. Preliminary Tests
• Test 7. Add H2SO4
– Solubility rule
• Most sulfates are soluble, except those of Ba2+, Sr2+, and
Pb2+
– The insolubility of BaSO4 provides a basis to
distinguish it from the other Group IV insoluble
carbonates.
• Addition of acid causes carbonates to dissolve
– BaCO3(s) + 2H+  Ba2+ + H2O + CO2
• If H2SO4 is used as the acid, BaSO4 is formed.
– BaCO3(s) + 2H+ + SO42  BaSO4(s) + H2O + CO2
All species are in aqueous solution unless indicated otherwise

22. Preliminary Tests
• Test 8. Flame test
– Used primarily to help identify soluble cations
• Solubility rule:
– All Group IA salts (Li+, Na+, K+, Rb+, Cs+) and NH4+ salts are soluble.
• Na+, K+ yield positive flame tests.
• NH4+ is also a soluble cation, but yields no useful result in
the flame test. It is identified from its properties of volatility
and basicity
– Add base, heat, and test with moist litmus
– NH4+ + OH  NH3(g) + H2O(l)
All species are in aqueous solution unless indicated otherwise

23. Preliminary Tests and Sulfide
Separation Scheme
• Reactions in the
preliminary tests can
be used in conjunction
with the sulfide
scheme to identify all
21 cations.
• Especially useful are
– reactions with NaOH
– reactions with NH3