Qualitative Analysis Identifying Ions in Solution 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 IonsSymbolColour 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 Anions Chromate CrO 4 2- Yellow Dichromate Cr 2 O 7 2- Orange Permanganate MnO 4 2- purple

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.

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 (CaCO 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.

1. Look at the Solubility Table to find anions which can precipitate Ag + and Sr 2+. ► We can use: Cl -, SO 4 2-, S 2-, OH -, and PO Set up a table of solubilities. Cl - SO 4 2- S 2- OH - PO 4 3- Ag + PptpptpptpptPpt Sr ppt

Ag + and/or Sr +2 Add NaCl (aq) No PPt Thus no Ag + in solution PPt AgCl (s) Ag + present in solution Filter and keep filtrate Add Na 3 PO 4 (aq) PPt Sr 3 (PO 4 ) 2 (s) Sr +2 present in solution No PPt Thus no Sr +2 in solution

Procedure: 1. Start by adding Cl -, S 2-, or OH - to try to precipitate Ag + (we do not use SO 4 2- or PO 4 3- 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 4 2- or PO 4 3- 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 +2, and Sr +2 ions. Cl - SO 4 2- S 2- OH - PO 4 3- Mg+2 Pb+2 Sr+2

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 4 2- S 2- OH - PO 4 3- Mg+2 ppt Pb+2 ppt Sr+2 ppt

Mg +2 Pb +2 and/or Sr +2 Add NaCl (aq) No PPt Thus no Pb +2 in solution PPt PbCl 2 (s) Pb +2 present in solution Filter and keep filtrate 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 Na 3 (PO 4 ) 2 (aq) PPt Sr 3 (PO 4 ) 2 (s) Sr +2 present in solution No PPt Thus 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. Cl - SO 4 2- S 2- OH - PO 4 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. Cl - SO 4 2- S 2- OH - PO 4 3- Cr+3, ppt Cu+2, ppt Ba+2 ppt 2.

► Flowchart ► First add sulphate…like sodium sulphate solution  If there is a ppt, BaSO 4 (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

Examples ► A solution which contains only one of the following anions: I-, SO 4 2-, or OH - is tested with various reagents and the following results are obtained: ► Reagent Results ► 0.2M AgNO 3 precipitate ► 0.2M Fe(NO 3 ) 2 no precipitate ► 0.2M 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 4 2-, or CO 3 2- ► is tested with various reagents and the following results are obtained: ► Reagent Results ► 0.2M AgNO 3 precipitate ► 0.2M Fe(NO 3 ) 2 precipitate ► 0.2M Sr(NO 3 ) 2 no precipitate ► Which anion does the solution contain? ____________________________ OH -

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 ► 0.2M Na 2 SO 4 precipitate ► 0.2M Na 2 S precipitate ► 0.2M NaNO 3 no precipitate ► Which cation does the solution contain? Pb 2+

Examples cont’d A reagent that may be used to separate Cl - from S 2- by precipitation is A. KNO 3 B. AgNO 3 C. Pb(NO 3 ) 2 D. Al(NO 3 ) 3 D

Which of the following could be used to separate Pb 2+ from Ba 2+ by precipitation? A. Na 2 S B. NaNO 3 C. Na 2 CO 3 D. Na 3 PO 4 Examples cont’d A

Which sequence of chemicals, in the order listed, would successfully separate Sr 2+, Pb 2+, and Ag + ? A.NaCl (aq), Na 2 CO 3(aq), and Na 2 SO 4(aq) B.NaOH (aq), Na 2 SO 4(aq), and Na 2 CO 3(aq) C.Na 2 CO 3(aq), NaCl (aq), and Na 2 SO 4(aq) D.NaOH (aq), NaCl (aq), and Na 2 CO 3(aq) Examples cont’d D