Qualitative Qualitative AnalysisAnalysis

Identifying Ions in Identifying Ions in SolutionSolution

Qualitative AnalysisQualitative Analysis involves involves the use of experimental the use of experimental procedures to determine procedures to determine what elements or ions are what elements or ions are present in a substance. present in a substance.

Solution ColorsSolution Colors

CationsCations

IonsIons SymbolSymbol ColourColour

Chromium(II)Chromium(II)

Copper(II)Copper(II)CrCr2+2+

CuCu2+2+

BlueBlue

Chromium(III)Chromium(III)

Copper(I)Copper(I)

Iron(II)Iron(II)

Nickel(II)Nickel(II)

CrCr3+3+

CuCu++

FeFe2+2+

NiNi2+2+

GreenGreen

Iron(III)Iron(III) FeFe3+3+ Pale yellowPale yellow

Cobalt(II)Cobalt(II)

Manganese(II)Manganese(II)CoCo2+2+

MnMn2+2+

PinkPink

AnionsAnions

ChromateChromate CrOCrO442-2- YellowYellow

DichromateDichromate CrCr22OO772-2- OrangeOrange

PermanganatePermanganate MnOMnO442-2- purplepurple

Flame Colours of SolutionsFlame Colours of Solutions

If a flame test produces one of the these If a flame test produces one of the these colours, that ion is in the solution. Other colours, that ion is in the solution. Other

colors are possible, but these are the most colors are possible, but these are the most common.common.

Some Precipitate ColoursSome Precipitate Colours

AlAl3+3+, Mg, Mg2+2+,Ca,Ca2+2+ white white

CuCu2+2+ blue/green blue/green

FeFe2+2+ grey/green grey/green

FeFe3+3+ red/brown red/brown

Precipitated calcium carbonate (CaCO3)

Precipitated copper(II) hydroxide, Cu(OH)2

Qualitative analysis using Qualitative analysis using precipitationprecipitation

For Example: For Example: AAgg++ and Sr and Sr+2+2

We try to find some We try to find some anionanion which could form a which could form a precipitateprecipitate with only with only oneone of our two cations at of our two cations at a time. Assume one or both of these cation is a time. Assume one or both of these cation is in solutionin solution

If a If a precipitateprecipitate is formed, we can then assume is formed, we can then assume that the ion we are looking for is in factthat the ion we are looking for is in fact presentpresent; if ; if no precipitateno precipitate forms, the ion is forms, the ion is absentabsent..

If a precipitate forms, then we filter it off and If a precipitate forms, then we filter it off and add another anion to precipitate the second add another anion to precipitate the second ion. If a precipitate forms, then the second ion ion. If a precipitate forms, then the second ion is present. If a precipitate does not form, then is present. If a precipitate does not form, then the second ion is not present in the solution.the second ion is not present in the solution.

Developing a Qualitative Developing a Qualitative Analysis SchemeAnalysis Scheme

►Qualitative Analysis:Qualitative Analysis:►For many ionic substances the For many ionic substances the

decision can be made using the decision can be made using the solubility table.solubility table.

►Looking at the previous example, Looking at the previous example, let’s develop a let’s develop a qualitative qualitative analysis schemeanalysis scheme..

1.1. Look at the Solubility Table to find Look at the Solubility Table to find anions which can precipitate Aganions which can precipitate Ag++ and and SrSr2+2+..

► We can use: ClWe can use: Cl--, SO, SO442-2-, S, S2-2-, OH, OH--, and , and

POPO443-3-..2. Set up a table of 2. Set up a table of

solubilities.solubilities.ClCl-- SOSO44

2-2- SS2-2- OHOH-- POPO443-3-

AgAg++ PptPpt pptppt pptppt pptppt PptPpt

SrSr2+2+ ------ -------- ------ ---- pptppt

Ag+ and/or Sr+2

Add NaCl (aq)

No PPtThus no Ag+ in solution

PPtAgCl (s)

Ag+ present in solutionFilter and keep filtrate

Add Na3PO4 (aq)

PPtSr3(PO4)2 (s)

Sr+2 present in solution

No PPtThus no Sr+2 in solution

Procedure:Procedure:1.1. Start by adding ClStart by adding Cl--, S, S2-2-, or OH, or OH-- to try to to try to

precipitate Agprecipitate Ag++ (we do not use SO (we do not use SO442-2- or or

POPO443-3- as they could also ppt Sr as they could also ppt Sr2+2+).).

If a precipitate forms, then there is Ag+ If a precipitate forms, then there is Ag+ present. present.

► Filter off and discard the precipitate. Filter off and discard the precipitate. Keep the left over solution for the next Keep the left over solution for the next part. part.

2.2. Add SOAdd SO442-2- or PO or PO44

3-3- to try to precipitate Sr to try to precipitate Sr2+.2+.

► If a precipitate forms, then there is SrIf a precipitate forms, then there is Sr2+2+ present. present.

Practice Exercises Practice Exercises 1.Draw a qualitative analysis scheme

showing how you would separate a mixture of Mg+2, Pb+2, and Sr+2 ions.

ClCl-- SOSO442-2- SS2-2- OHOH-- POPO44

3-3-

Mg+2

Pb+2

Sr+2

Practice Exercises Practice Exercises 1.Draw a qualitative analysis scheme

showing how you would separate a mixture of Mg+2, Pb+2, and Sr+2 ions.

ClCl-- SOSO442-2- SS2-2- OHOH-- POPO44

3-3-

Mg+2 ppt ppt

Pb+2ppt ppt ppt ppt ppt

Sr+2 ppt

Mg+2 Pb+2 and/or Sr+2

Add NaCl (aq)

No PPtThus no Pb+2 in solution

PPtPbCl2 (s)

Pb+2 present in solutionFilter and keep filtrate

Add NaOH (aq)

PPtMg(OH)2 (s)

Mg+2 present in solutionFilter and keep filtate

No PPtThus no Mg+2 in solution

Add Na3(PO4)2 (aq)

PPtSr3(PO4)2 (s)

Sr+2 present in solution

No PPtThus no Sr+2 in solution

Draw a qualitative analysis scheme showing how you would separate a

mixture of Cr+3, Cu+2, and Ba+2 ions.

ClCl-- SOSO442-2- SS2-2- OHOH-- POPO44

3-3-

Cr+3,

Cu+2,

Ba+2

2.

Draw a qualitative analysis scheme showing how you would separate a

mixture of Cr+3, Cu+2, and Ba+2 ions.

ClCl-- SOSO442-2- SS2-2- OHOH-- POPO44

3-3-

Cr+3, ppt ppt ppt

Cu+2, ppt ppt ppt

Ba+2ppt ppt

2.

►FlowchartFlowchart►First add sulphate…like sodium sulphate solutionFirst add sulphate…like sodium sulphate solution

If there is a ppt, BaSOIf there is a ppt, BaSO44(s) present, thus Ba(s) present, thus Ba+2+2 ions in ions in solutionsolution

If there is no ppt, CuIf there is no ppt, Cu+2+2 and Cr and Cr+3+3 ions could be present ions could be present Both CuBoth Cu+2+2 and Cr and Cr+3+3 precipitate with the same anions. precipitate with the same anions. Differentiate by the colour of the solutionDifferentiate by the colour of the solution If the solution is green, there is CrIf the solution is green, there is Cr+3+3 present present If the solution is blue, there is CuIf the solution is blue, there is Cu+2+2 present present

ExamplesExamples

►A solution which contains only one of the A solution which contains only one of the following anions: I-, SOfollowing anions: I-, SO44

2-2-, or OH, or OH-- is is tested tested with various reagents and the following with various reagents and the following results are obtained:results are obtained:

►Reagent ResultsReagent Results►0.2M AgNO0.2M AgNO33 precipitate precipitate

►0.2M Fe(NO0.2M Fe(NO33))22 no precipitate no precipitate

►0.2M Sr(NO0.2M Sr(NO33))22 no precipitate no precipitate

►Which anion does the solution contain?Which anion does the solution contain? I-

Examples cont’dExamples cont’d►A solution which contains only one of the A solution which contains only one of the

following anions: OHfollowing anions: OH--, SO, SO442-2-, or CO, or CO33

2-2-

► is tested with is tested with various reagents and the various reagents and the following results are obtained:following results are obtained:

►Reagent ResultsReagent Results►0.2M AgNO0.2M AgNO33 precipitate precipitate

►0.2M Fe(NO0.2M Fe(NO33))22 precipitate precipitate

►0.2M Sr(NO0.2M Sr(NO33))22 no precipitate no precipitate

►Which anion does the solution contain? Which anion does the solution contain? ________________________________________________________ OH-

Examples cont’dExamples cont’d►A solution which contains only one of A solution which contains only one of

the following cations: Mgthe following cations: Mg2+2+, Pb, Pb2+2+, or , or NHNH44

++ is tested with is tested with various reagents various reagents and the following results are obtained:and the following results are obtained:

►Reagent ResultsReagent Results►0.2M Na0.2M Na22SOSO44 precipitate precipitate

►0.2M Na0.2M Na22S precipitateS precipitate

►0.2M NaNO0.2M NaNO33 no precipitate no precipitate

►Which cation does the solution contain? Which cation does the solution contain? Pb2+

Examples cont’dExamples cont’d

A reagent that may be used to separate Cl- from S2- by precipitation is

A. KNO3

B. AgNO3

C. Pb(NO3)2

D. Al(NO3)3

D

Which of the following could be used to separate Pb2+ from Ba2+ by precipitation?

A. Na2S

B. NaNO3

C. Na2CO3

D. Na3PO4

Examples cont’dExamples cont’d

A

Which sequence of chemicals, in the order listed, would successfully separate Sr2+, Pb2+, and Ag+?

A. NaCl(aq), Na2CO3(aq), and Na2SO4(aq)

B. NaOH(aq), Na2SO4(aq), and Na2CO3(aq)

C. Na2CO3(aq), NaCl(aq), and Na2SO4(aq)

D. NaOH(aq), NaCl(aq), and Na2CO3(aq)

 

Examples cont’dExamples cont’d

D