Qualitative Analysis Identifying Ions in Solution Qualitative Analysis

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 Ions Cations Anions Symbol Colour Chromium(II) Copper(II) Cr 2+ Cu 2+ Blue Chromium(III) Copper(I) Iron(II) Nickel(II) Cr 3+ Cu+ Fe 2+ Ni 2+ Green Iron(III) Fe 3+ Pale yellow Cobalt(II) Manganese(II) Co 2+ Mn 2+ Pink Chromate Cr. O 42 - Yellow Dichromate Cr 2 O 72 - Orange Permanganate Mn. O 42 - purple

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

Some Precipitate Colours Al 3+, Mg 2+, Ca 2+ white Cu 2+ blue/green Fe 2+ grey/green Fe 3+ red/brown Precipitated calcium carbonate (Ca. CO 3) Precipitated copper(II) hydroxide, Cu(OH)2

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 ► Qualitative Analysis: ► 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.

Look at the Solubility Table to find anions which can precipitate Ag+ and Sr 2+. ► We can use: Cl-, SO 42 -, S 2 -, OH-, and PO 43 -. 1. 2. Set up a table of solubilities. Ag+ Cl. Ppt SO 42 ppt Sr 2+ ---- --- OHppt -- PO 43 Ppt ppt

Procedure: 1. Start by adding Cl-, S 2 -, or OH- to try to precipitate Ag+ (we do not use SO 42 - or PO 43 - as they could also ppt Sr 2+). § If a precipitate forms, then there is Ag+ present. Filter off and discard the precipitate. Keep the left over solution for the next part. 2. Add SO 42 - or PO 43 - to try to precipitate Sr 2+. ► If a precipitate forms, then there is Sr 2+ present. ►

Practice Exercises 1. Draw a qualitative analysis scheme showing how you would separate a mixture of Mg+2, Pb+4, and Sr+2 ions. Cl. Mg+2 Pb+4 Sr+2 SO 42 - S 2 - OH- PO 43 -

Practice Exercises 1. Draw a qualitative analysis scheme showing how you would separate a mixture of Mg+2, Pb+2, and Sr+2 ions. Cl- SO 42 - S 2 - OH- Mg+2 Pb+2 Sr+2 ppt ppt PO 43 - ppt ppt ppt

Draw a qualitative analysis scheme showing how you would separate a mixture of Cr+3, Cu+2, and Ba+2 ions. 2. Cl. Cr+3, Cu+2, Ba+2 SO 42 - S 2 - OH- PO 43 -

Draw a qualitative analysis scheme showing how you would separate a mixture of Cr+3, Cu+2, and Ba+2 ions. 2. Cl- SO 42 - S 2 - OH- PO 43 - Cr+3, ppt ppt Cu+2, ppt ppt Ba+2 ppt

Examples ►A solution which contains only one of the following anions: I-, SO 42 -, or OH- is tested with various reagents and the following results are obtained: ► Reagent Results ► 0. 2 M Ag. NO 3 precipitate ► 0. 2 M Fe(NO 3)2 no precipitate ► 0. 2 M Sr(NO 3)2 no precipitate ► Which anion does the solution contain? I-

Examples cont’d ►A solution which contains only one of the following anions: OH-, SO 42 -, or CO 32► is tested with various reagents and the following results are obtained: ► Reagent Results ► 0. 2 M Ag. NO 3 precipitate ► 0. 2 M Fe(NO 3)2 precipitate ► 0. 2 M Sr(NO 3)2 no precipitate OH ► Which anion does the solution contain? ______________

Examples cont’d ►A solution which contains only one of the following cations: Mg 2+, Pb 2+, or NH 4+ is tested with various reagents and the following results are obtained: ► Reagent Results ► 0. 2 M Na 2 SO 4 precipitate ► 0. 2 M Na 2 S precipitate ► 0. 2 M Na. NO 3 no precipitate ► Which cation does the solution contain? Pb 2+