1. Qualitative Analysis Identifying Ions in Solution

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

3. Solution Colors Cations Anions Ions Symbol Colour Chromium(II)
Copper(II)
Cr2+
Cu2+
Blue
Chromium(III)
Copper(I)
Iron(II)
Nickel(II)
Cr3+
Cu+
Fe2+
Ni2+
Green
Iron(III)
Fe3+
Pale yellow
Cobalt(II)
Manganese(II)
Co2+
Mn2+
Pink
Anions
Chromate
CrO42-
Yellow
Dichromate
Cr2O72-
Orange
Permanganate
MnO42-
purple 3

4. Flame Colours of Solutions
If a flame test produces one of the these colours, that ion is in the solution. Other colors are possible, but these are the most common. 4

5. Some Precipitate Colours
Al3+, Mg2+,Ca2+  white
Cu2+  blue/green
Fe2+  grey/green
Fe3+  red/brown
Precipitated calcium carbonate (CaCO3)
Precipitated copper(II) hydroxide, Cu(OH)2 5

6. Qualitative analysis using precipitation

7. For Example: Ag+ and Sr+2
We try to find some anion which could form a precipitate with only one of our two cations at a time. Assume one or both of these cation is in solution
If a precipitate is formed, we can then assume that the ion we are looking for is in fact present; if no precipitate forms, the ion is absent.
If a precipitate forms, then we filter it off and add another anion to precipitate the second ion. If a precipitate forms, then the second ion is present. If a precipitate does not form, then the second ion is not present in the solution.

8. Developing a Qualitative Analysis Scheme
For many ionic substances the decision can be made using the solubility table.
Looking at the previous example, let’s develop a qualitative analysis scheme.

9. 2. Set up a table of solubilities.
Look at the Solubility Table to find anions which can precipitate Ag+ and Sr2+.
We can use: Cl-, SO42-, S2-, OH-, and PO43-.
2. Set up a table of solubilities.
Cl-
SO42-
S2-
OH-
PO43-
Ag+
Ppt
ppt
Sr2+
---
---- --

10. Ag+ and/or Sr+2
Add NaCl (aq)
PPt
AgCl (s)
Ag+ present in solution
Filter and keep filtrate
No PPt
Thus no Ag+ in solution
Add Na3PO4 (aq)
PPt
Sr3(PO4)2 (s)
Sr+2 present in solution
No PPt
Thus no Sr+2 in solution

11. Add SO42- or PO43- to try to precipitate Sr2+.
Procedure:
Start by adding Cl-, S2-, or OH- to try to precipitate Ag+ (we do not use SO42- or PO43- as they could also ppt Sr2+).
If a precipitate forms, then there is Ag+ present.
Filter off and discard the precipitate. Keep the left over solution for the next part.
Add SO42- or PO43- to try to precipitate Sr2+.
If a precipitate forms, then there is Sr2+ present.

12. Practice Exercises
Draw a qualitative analysis scheme showing how you would separate a mixture of Mg+2, Pb+2, and Sr+2 ions.
Cl-
SO42-
S2-
OH-
PO43-
Mg+2
Pb+2
Sr+2

13. Practice Exercises
Draw a qualitative analysis scheme showing how you would separate a mixture of Mg+2, Pb+2, and Sr+2 ions.
Cl-
SO42-
S2-
OH-
PO43-
Mg+2
ppt
Pb+2
Sr+2

14. Mg+2 Pb+2 and/or Sr+2
Add NaCl (aq)
PPt
PbCl2 (s)
Pb+2 present in solution
Filter and keep filtrate
No PPt
Thus no Pb+2 in solution
Add NaOH (aq)
PPt
Mg(OH)2 (s)
Mg+2 present in solution
Filter and keep filtate
No PPt
Thus no Mg+2 in solution
Add Na3(PO4)2 (aq)
PPt
Sr3(PO4)2 (s)
Sr+2 present in solution
No PPt
Thus no Sr+2 in solution

15. Draw a qualitative analysis scheme showing how you would separate a mixture of Cr+3, Cu+2, and Ba+2 ions. 2.
Cl-
SO42-
S2-
OH-
PO43-
Cr+3,
Cu+2,
Ba+2

16. Draw a qualitative analysis scheme showing how you would separate a mixture of Cr+3, Cu+2, and Ba+2 ions. 2.
Cl-
SO42-
S2-
OH-
PO43-
Cr+3,
ppt
Cu+2,
Ba+2

17. First add sulphate…like sodium sulphate solution
Flowchart
First add sulphate…like sodium sulphate solution
If there is a ppt, BaSO4(s) present, thus Ba+2 ions in solution
If there is no ppt, Cu+2 and Cr+3 ions could be present
Both Cu+2 and Cr+3 precipitate with the same anions.
Differentiate by the colour of the solution
If the solution is green, there is Cr+3 present
If the solution is blue, there is Cu+2 present

18. Examples
A solution which contains only one of the following anions: I-, SO42-, or OH- is tested with various reagents and the following results are obtained:
Reagent Results
0.2M AgNO3 precipitate
0.2M Fe(NO3)2 no precipitate
0.2M Sr(NO3)2 no precipitate
Which anion does the solution contain? I-

19. Examples cont’d
A solution which contains only one of the following anions: OH-, SO42-, or CO32-
is tested with various reagents and the following results are obtained:
Reagent Results
0.2M AgNO3 precipitate
0.2M Fe(NO3)2 precipitate
0.2M Sr(NO3)2 no precipitate
Which anion does the solution contain? ____________________________
OH-

20. Examples cont’d
A solution which contains only one of the following cations: Mg2+, Pb2+, or NH4+ is tested with various reagents and the following results are obtained:
Reagent Results
0.2M Na2SO4 precipitate
0.2M Na2S precipitate
0.2M NaNO3 no precipitate
Which cation does the solution contain?
Pb2+

21. Examples 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

22. Examples cont’d
Which of the following could be used to separate Pb2+ from Ba2+ by precipitation?
A. Na2S
B. NaNO3
C. Na2CO3
D. Na3PO4 A

23. Examples cont’d
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) D