Chem cations

Qualitative Analysis of Cations

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Classic Separation

Scheme for

Qualitative Analysis of

21 Common Cations

Zumdahl 7e, Figure 15.12.

Group I Cations

Ag+, Pb2+

Add dilute HCl

AgCl(s) white precipitate

PbCl2(s) white precipitate

Ag+ + Cl AgCl(s)

Pb2+ + 2Cl PbCl2(s)

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+



Precipitation of Bismuth Sulfide



From Left to Right: Cadmium Sulfide, Chromium(III)

Hydroxide, Aluminum Hydroxide, and Nickel(II) Hydroxide



Figure 15.13 The

Separation of the

Group I Ions in

the Classic

Scheme of

Qualitative

Analysis



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)



Flame Test

for Potassium



Flame Test

for Sodium

Preliminary Tests

1. Add H2O

2. Add NaOH

a. Dilute NaOH

b. Excess NaOH

3. Add NH3

a. Dilute NH3

b. Excess NH3

4. Add HCl

5. Add sulfide

a. 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


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)62+ + 2OH


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)


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+


Groups II and III Cations

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+


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


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)


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

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