Qualitative Analysis Identifying Ions in Solution

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

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

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

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

Qualitative analysis using precipitation

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.

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.

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+ --- ---- --

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.

Practice Exercises Draw a qualitative analysis scheme showing how you would separate a mixture of Mg+2, Pb+4, and Sr+2 ions. Cl- SO42- S2- OH- PO43- Mg+2 Pb+4 Sr+2

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

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

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

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-

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-

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+