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Qualitative Analysis of Cations
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15–2
Classic Separation Scheme for Qualitative Analysis of 21 Common Cations
Zumdahl 7e, Figure 15.12.
Group I CationsAg+, Pb2+
Add dilute HCl
AgCl(s) white precipitatePbCl2(s) white precipitate
Ag+ + Cl AgCl(s)
Pb2+ + 2Cl PbCl2(s)
Group II CationsCu(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+
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15–5
Precipitation of Bismuth Sulfide
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15–6
From Left to Right: Cadmium Sulfide, Chromium(III) Hydroxide, Aluminum Hydroxide, and Nickel(II) Hydroxide
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15–7
Figure 15.13 The Separation of the Group I Ions in the Classic Scheme of Qualitative Analysis
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15–8
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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15–9
Flame Test for Potassium
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15–10
Flame Test for Sodium
Preliminary Tests
1. Add H2O
2. Add NaOHa. Dilute NaOH
b. Excess NaOH
3. Add NH3
a. Dilute NH3
b. Excess NH3
4. Add HCl
5. Add sulfidea. Acidic
b. Basic
6. Add Na2CO3
7. Add H2SO4
8. Flame tests
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
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
Reactions with NaOH
Amphoteric metal hydroxides
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)6
2+ + 2OH
All species are in aqueous solution unless indicated otherwise
Reactions with NH3
Soluble ammine complexes
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
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
Groups II and III Cations
Preliminary Tests
• Test 6. Reactions with Na2CO3
– Most metal carbonates are insoluble• Ni2+ + CO3
2 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
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
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
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
