1. Identifying Cations in Solution
Objectives
Observe chemical behavior of barium, calcium and magnesium ions
To analyze an unknown solution for one or more of the following cations Ba2+, Ca2+ and Mg2+
To develop skills in centrifuging, flame testing and using litmus paper

2. Qualitative Analysis
A systematic procedure for separating and identifying ions present in an unknown solution
No two ions behave identically in all situations
With a comprehensive knowledge of the behavior of all ions in chemical reactions it is possible to figure out schemes to identify what ions are present

3. Qualitative Analysis: Ba2+,Ca2+ and Mg2+
One key method used in qualitative analysis is to know what anions when coupled with a cation of interest causes it to precipitate while other ions stay in solution. The presence of the precipitate along with its color confirm the presence of that metal ion
For example, if I have a mixture of Ba2+, Ca2+ and Mg2+, I can add ammonium sulfate [(NH4)2SO4]. The sulfate anion SO42- reacts with Ba2+ to make a white precipitate of BaSO4
Mg2+ and Ca2+ do not react with (NH4)2SO4
Supernatant (Ca2+ and Mg2+)
Precipitate (BaSO4)

4. Flame Testing
Flame testing is a technique that can be used to confirm the presence of an ion (remember last week?)
Dip wire into you solution of interest then hold the wire in a bunsen flame
Copper
Lithium

5. Testing Acidity: Litmus Paper
Litmus paper can be used to determine whether a solution is acidic or basic
A glass stirring rod is placed in solution and then on a piece of litmus paper
Acidic solutions turn blue litmus pink
Basic solutions turn red litmus blue
basic
acidic

6. Qualitative Procedure
Step 1
Ba2+(aq), Ca2+(aq), Mg2+(aq) + (NH4)2SO4  BaSO4(s) + Mg2+(aq) + Ca2+(aq)
Centrifuge to separate the precipitate of BaSO4(s)
Flame test on BaSO4(s)  apple green flame confirms Ba!
Step 2
Mg2+(aq) + Ca2+(aq) + (NH4)C2O4(aq)  CaC2O4(s) + Mg2+(aq)
Centrifuge to separate the precipitate of CaC2O4(s)
Flame test on CaC2O4(s) brick red flame confirms Ca!
Step 3
Mg2+(aq) + Na2HPO4(aq) + NaOH(aq)  MgNH4PO4.6H2O(s)
Centrifuge and separate MgNH4PO4.6H2O(s)
MgNH4PO4.6H2O(s) + HCl and magnesium indicator (magneson II) + NaOH dropwise till basic  Blue gel – confirms Mg!

7. Experimental Procedure
Start by performing a cation analysis with a known mixture containing Ba2+, Ca2+ and Mg2+
Starting with Step 1 to isolate and confirm Barium, Step 2 to isolate and confirm Calcium and Step 3 to isolate and confirm Magnesium
Repeat Steps 1-3 with an unknown solution and identify which of these 3 ions you have