Cation and Anion Analysis

General Chemistry: Chapter 19Prentice-Hall © 2002 Slide 2 of 34

Qualitative Cation Analysis• An analysis that aims at identifying the cations

present in a mixture but not their quantities.• Think of cations in solubility groups according to the

conditions that causes precipitation

chloride group hydrogen sulfide group

ammonium sulfide group carbonate group.

–Selectively precipitate the first group of cations then move on to the next.

• Acid–base chemistry, precipitation reactions, oxidation–reduction, and complex ion formation all apply to an area of analytical chemistry called classical qualitative inorganic analysis.

• “Qualitative” signifies that the interest is in determining what is present.– Quantitative analyses are those that determine how much of a

particular substance or species is present.

• Although classical qualitative analysis is not used as widely today as are instrumental methods, it is still a good vehicle for applying all the basic concepts of equilibria in aqueous solutions.

Qualitative Inorganic Analysis

Indicators of chemical reactions• Sometimes, it is difficult to

determine whether a chemical reaction has happened or not. There are several indicators for this:– Production of a solid, liquid, or

gas– Generation/consumption of heat– Color change– Production of light

Qualitative Analysis

• Separation and identification of cations, such as Ag+, Ba2+, Cr3+, Fe3+, Cu2+, etc. can be carried out based on their different solubility and their ability to form complex ions with specific reagents, such as HCl, H2SO4, NaOH, NH3, and others.

• Separation and identification of anions, such as Cl-, Br-, I-, SO4

2-, CO32-, PO4

3-, etc., can be accomplished using reagents such as AgNO3, Ba(NO3)2 under neutral or acidic conditions.

The General Procedure for Separating Ions in Qualitative Analysis

Prentice-Hall © 2002 General Chemistry: Chapter 19 Slide 8 of 34

Qualitative Cation Analysis

In acid, H2S produces very little S2–, so only the most-insoluble sulfides precipitate.

In base, there is more S2–, and the less-insoluble sulfides also precipitate.

Some hydroxides also precipitate here.

Prentice-Hall © 2002 General Chemistry: Chapter 19 Slide 9 of 34

Qualitative Cation Analysis

• If aqueous HCl is added to an unknown solution of cations, and a precipitate forms, then the unknown contains one or more of these cations: Pb2+, Hg2

2+, or Ag+.

• These are the only ions to form insoluble chlorides.• Any precipitate is separated from the mixture and further

tests are performed to determine which of the three Group 1 cations are present.

• The supernatant liquid is also saved for further analysis (it contains the rest of the cations).

• If there is no precipitate, then Group 1 ions must be absent from the mixture.

Cation Group 1

• Precipitated PbCl2 is slightly soluble in hot water.

• The precipitate is washed with hot water, then aqueous K2CrO4 is added to the washings.

• If Pb2+ is present, a precipitate of yellow lead chromate forms, which is less soluble than PbCl2.

• (If all of the precipitate dissolves in the hot water, what does that mean?)

• If Pb2+ is absent, then the washings just become tinged yellow but no precipitate is in evidence.

Cation Group 1 (cont’d)Analyzing for Pb2+

• Next, any undissolved precipitate is treated with aqueous ammonia.

• If AgCl is present, it will dissolve, forming Ag(NH3)2+ (the

dissolution may not be visually apparent).

• If Hg22+ is present, the precipitate will turn dark gray/

black, due to a disproportionation reaction that forms Hg metal and HgNH2Cl.

• The supernatant liquid (which contains the Ag+, if present) is then treated with aqueous nitric acid.

• If a precipitate reforms, then Ag+ was present in the solution. if no precipitate forms, then Ag+ was not present in the solution.

Cation Group 1 (cont’d)Analyzing for Ag+ and Hg2

2+

Cation Group 1 (continued)Analyzing For Ag+

• Next, the undissolved precipitate is treated with aqueous ammonia.

• If AgCl is present, it will dissolve in this solution.• If there is any remaining precipitate, it is separated

from the supernatant liquid and saved for further analysis.

• The supernatant liquid (which contains the Ag+, if present) is then treated with aqueous nitric acid.

• If a precipitate reforms, then Ag+ was present in the solution, if no precipitate forms, then Ag+ was not present in the solution.

Cation Group 1 (continued)Analyzing For Hg2

2+

• When precipitate was treated with aqueous ammonia in the previous step, any Hg2

2+ underwent an oxidation-reduction reaction to form a dark gray mixture of elemental mercury and HgNH2Cl that precipitates from the solution.

• If this dark gray precipitate was observed, then mercury was present in the original unknown sample.

• If this dark gray precipitate was not observed, then mercury must have been absent from the original unknown sample.

Figure 8.14: Separation of Group I ions

Group 1 Cation Precipitates

left: cation goup 1 ppt: PbCl2, PbCl2, AgCl (all white)

middle: product from test for Hg2

2+: mix of Hg (black) and HgNH2Cl (white)

right: product from test for Pb2+: PbCrO4 (yellow) when K2CrO4(aq) is reacted with saturated PbCl2

PbCl2 precipitates when HCl is added.

The presence of lead is confirmed by adding chromate ion; yellow PbCrO4 precipitates.

Hg2Cl2 reacts with NH3 to form black Hg metal and HgNH2Cl.

Slide 17 of 34

Chloride Group Precipitates (Pb, Hg and Ag)

(a) Group precipitate

Wash ppt with hot water PbCl2 is slightly soluble. Test aqueous solution with CrO4

2-.

(c) Pb2+(aq) + CrO42- → PbCrO4(s)

Test remaining precipitate with ammonia.

(b) AgCl(s) + 2 NH3(aq) → Ag(NH3)2 (aq) + Cl-(aq)

(b) Hg2Cl2(a) + 2 NH3 → Hg(l) + HgNH2Cl(s) + NH4

+(aq) + Cl-(aq)

• Once the Group 1 cations have been precipitated, hydrogen sulfide is used as the next reagent in the qualitative analysis scheme.

• H2S is a weak diprotic acid; there is very little ionization of the HS– ion and it is the precipitating agent.

• Hydrogen sulfide has the familiar rotten egg odor that is very noticeable around volcanic areas.

• Because of its toxicity, H2S is generally produced only in small quantities and directly in the solution where it is to be used.

Hydrogen Sulfide in theQualitative Analysis Scheme

General Chemistry: Chapter 19Prentice-Hall © 2002 Slide 19 of 34

Hydrogen Sulfide EquilibriaH2S(aq) + H2O(l) → HS-(aq) + H3O+(aq) Ka1 = 1.0×10-7

HS-(aq) + H2O(l) → S2-(aq) + H3O+(aq) Ka2 = 1.0×10-19

S2- is an extremely strong base and is unlikely to be theprecipitating agent for the sulfide groups.

General Chemistry: Chapter 19Prentice-Hall © 2002 Slide 20 of 34

Lead Sulfide EquilibriaPbS(s) + H2O(l) → Pb2+(aq) + HS-(aq) + OH-(aq)

Ksp = 3×10-28

H3O+(aq) + HS-(aq) → H2S(aq) + H2O(aq) 1/Ka1 = 1.0/1.0×10-7

H3O+(aq) + OH-(aq) → H2O(l) + H2O(l) 1/Kw = 1.0/1.0×10-14

PbS(s) + 2 H3O+ (l) → Pb2+(aq) + H2S(aq) + 2 H2O(l)

Kspa = = 3×10-7Ksp

Ka1 Kw

3×10-28

1.0×10-7 ×1.0×10-14

=

• The concentration of HS– is so low in a strongly acidic solution, that only the most insoluble sulfides precipitate.

• These include the eight metal sulfides of Group 2.• Five of the Group 3 cations form sulfides that are soluble

in acidic solution but insoluble in alkaline NH3/NH4+.

• The other three Group 3 cations form insoluble hydroxides in the alkaline solution.

• The cations of Groups 4 and 5 are soluble.• Group 4 ions are precipitated as carbonates.• Group 5 does not precipitate; these must be determined

by flame test.

Cation Groups 2, 3, 4, and 5

Copyright © Cengage Learning. All rights reserved

22

Separation of Cu2+ and Hg2+ from Ni2+ and Mn2+ using H2S

A Qualitative Analysis

Scheme for Ag+, Al3+, Cu2+ and

Fe3+

General Chemistry: Chapter 19Prentice-Hall © 2002 Slide 24 of 34

Dissolving Metal Sulfides

• Several methods exist to re-dissolve precipitated metal sulfides.– React with an acid.

• FeS readily soluble in strong acid but PbS and HgS are not because their Ksp values are too low.

– React with an oxidizing acid.3 CuS(aq) + 8 H+(aq) + 2 NO3

-(aq) →

3 Cu2+(aq) + 3 S(s) + 2 NO(g) + 4 H2O(l)

General Chemistry: Chapter 19Prentice-Hall © 2002 Slide 25 of 34

A Sensitive Test for Copper(II)

[Cu(H2O)4]2+(aq) + 4 NH3(aq) → [Cu(NH3)4]2+(aq) + 4 H2O(l)

IDENTIFICATION OF ANION

Identifying Anions:

CO32, Cl, I, SO4

2, NO3, OH

add dilute HCl

solution

litmus remains redSO4

2, Cl, I, NO3

white precipitateSO4

2

no precipitate

new sample

litmus goes blue

OH, CO32

bubbles of gas

CO32

no bubblesOH

add red litmus

add Ba(NO3)2

solution

precipitateCl, I

precipitate disappears

Cl

precipitate remains

I

no precipitateNO3

add AgNO3

solution

new sample

add dilute NH3

solution

Identifying Anions I• Use the anion flowchart to identify the anion

used below:

Add Ba(NO3)2

Add AgNO3

Add NH3Add red litmus

Identifying Anions II• Use the anion flowchart to identify the anion used

below:

Add HClAdd red litmus

Identifying Cations I• Use the cation flowchart to identify the cation used

below:

Add NaOH

Add KSCN

Identifying Cations II• Use the cation flowchart to identify the cation used

below:

Add NaOH

Add NH3

Separation into Ion Groups

Ion Group 1: Insoluble chloridesAg+, Hg22+, Pb2+

Ion Group 2: Acid-insoluble sulfidesCu2+, Cd2+, Hg2+, As3+, Sb3+, Bi3+, Sn2+, Sn4+, Pb2+

Ion Group 3: Base-insoluble sulfides and hydroxidesZn2+, Mn2+, Ni2+, Fe2+, Co2+ as sulfides, and Al3+, Cr3+

as hydroxidesIon Group 4: Insoluble phosphates Mg2+, Ca2+, Ba2+

Ion Group 5: Alkali metal and ammonium ions Na+, K+, NH4

+

Tests to Determine the Presence of Cations in Ion Group 5

Na+ ions K+ ionsOH - + NH4+ NH3 + H2O

plus litmus paper

Silver Precipitates and Complexes

• 2 AgNO3(aq) + NaHCO3(aq) Ag2CO3(s) + NaNO3(aq) + HNO3(aq) (white)

• Ag2CO3(s) + 2 NaOH(aq) 2 AgOH(s) + Na2CO3(aq) (brown)

• AgOH(s) + NaCl(aq) AgCl(s) + NaOH(aq) (white)

• AgCl(s) + 2 NH3(aq) Ag(NH3)+2(aq) + Cl-(aq) (clear)

• Ag(NH3)+2(aq) + NaBr(aq) AgBr(s) + 2 NH3(aq) (cream)

• AgBr(s) + 2 Na2S2O3(aq) Ag(S2O3)-32(aq) + Br-

(aq) + 4 Na+(aq) (clear)

• Ag(S2O3)-32(aq) + KI(aq) AgI(s) + K+

(aq) + 2 S2O3-2

(aq) (yellow)

• 2AgI(s) + Na2S(aq) Ag2S(s) + 2 NaI(aq) (black)

Selective Precipitation (Mixtures of Metal Ions)

• Use a reagent whose anion forms a precipitate with only one or a few of the metal ions in the mixture.

• Example: Solution contains Ba2+ and Ag+ ions. Adding NaCl will form a precipitate with Ag+

(AgCl), while still leaving Ba2+ in solution.

Separation of Cu2+ and Hg2+ from Ni2+ and Mn2+ using H2S

• At a low pH, [S2–] is relatively low and only the very insoluble HgS and CuS precipitate.

• When OH– is added to lower [H+], the value of [S2–] increases, and MnS and NiS precipitate.

Separation of Cu2+ and Hg2+ from Ni2+ and Mn2+ using H2S

4219 Heterogeneous & complex equilibria

Qualitative analysis of metal ionsMixture of metal ions add HCl

group I Ag+, Hg22+, Pb2+

filtrate of soluble chloride chloride | add 0.3 M H+ & H2Sgroup II Cu2+, Cd2+, Hg2+, Pb2+ filtrate of soluble metal sulfide

As3+, Sb3+, Bi3+, Sn4+ | | add OH- & H2S acid insoluble sulfide | (NH4OH) group III Mn2+, Fe2+, Co2+, Ni2+, Zn2+ filtrate of soluble metal ions base insoluble sulfide | | add CO3

2- or PO43-

Al(OH)3, Cr (OH)3 | |hydroxide |

group IV Mg2+, Ca2+, Sr2+, Ba2+ filtrate of soluble metal ions ppt as carbonates or phosphates | K+, Na+ group V

Selective Precipitation of IonsOne can use differences in solubilities of salts to separate ions in a mixture.

Qualitative Analysis of

Cations

16.11

Qualitative Analysis SchemeQualitative Analysis Outline

In acid, H2S produces very little S2–, so only the most-insoluble sulfides precipitate.

In base, there is more S2–, and the less-insoluble sulfides also precipitate.

Some hydroxides also precipitate here.

7)The qualitative analysis of the Group I cations illustrates complex ion equilibria

Solution of Ag+, Hg2

2+, Pb2+

Solution of Pb2+Precipitate ofAgCl, Hg2Cl2

Precipitate ofPbCrO4

Solution ofAg(NH3)2+, Cl-

Precip. of AgCl

Precipitate ofHg, HgNH2Cl

Precipitation of AgCl, Hg2Cl2, PbCl2

Add cold HCl

Heat

Add CrO42-

Add H+

Add NH3

4)Qualitative Analysis = scheme to separate and identify mixtures of cations by precipitation

– Group I Insoluble Chlorides:

Add HCl. AgCl, PbCl2, Hg2Cl2 precipitate

– Group II Sulfides Insoluble in Acid Solution:

Add H2S. Low [S2-] due to [H+]. HgS, CdS, Bi2S3, CuS, and SnS2 precip.

– Group III Sulfides Insoluble in Basic Solution:

Add OH-. CoS, ZnS, MnS, NiS, FeS, Cr(OH)3, and Al(OH)3 precipitate.

d)Group IV Insoluble Carbonates:

Add CO32-. BaCO3, CaCO3, and MgCO3 precipitate.

e)Group V Alkali Metal and Ammonium

These ions (Na+, K+, NH4+) are soluble with common ions

f) Further tests would tell us which specific ions in each group are present

new sample

precipitate disappearsAl3+, Zn2+, Pb2+

precipitate remainsMg2+, Ba2+

add 2 drops, thenexcess NH3

solution

white precipitate formsAl3+, Zn2+, Pb2+, Mg2+,

Ba2+

add excessNaOH solution

add 2 drops of dilute NaOH

solution.

no precipitateNH4

+, Na+

add NaOH solution, heat, test gas with

red litmus.

litmus stays red Na+

litmus goes blue NH4

+

blue precipitate then deep blue solution

Cu2+

blue precipitate formsCu2+

brown precipitate formsAg+

brown precipitate then colourless solution Ag+

add 2 drops, then excess

NH3 solution

add 2 drops, then excessNH3 solution

orange precipitate formsFe3+

green precipitate formsFe2+

white precipitate formsAl3+, Pb2+

white precipitate forms and disappears, Zn2+

add dilute H2SO4 solution

colourless solutionAl3+

white precipitatePb2+

add dilute H2SO4 solution

colourless solutionMg2+

white precipitateBa2+

add 2 drops KSCN solution

dark red solution confirms Fe3+

Identifying Cations: NH4+, Na+, Mg2+, Ag+, Fe2+, Fe3+, Cu2+, Al3+, Pb2+, Zn2+, Ba2+

new sample

newsample

newsample

newsample

new sample

General Chemistry: Chapter 19Prentice-Hall © 2002 Slide 52 of 34

Focus On Shells, Teeth and Fossils

Ca2+(aq) + 2 HCO3-(aq) →

CaCO3(s) + H2O(l) + CO2(g)

Calcite

Ca5(PO4)3OH(s) + 4H3O+(aq) → 5Ca2+(s) + 5H2O(l) + 3HPO42-(aq)

Fluoroapatite

Ca5(PO4)3F(s)

Hydroxyapatite

Ca5(PO4)3OH(s)