1. Identifying Cations in Solution
Objectives
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 knowledge of the behavior of all ions in chemical reactions it is possible to figure out schemes to separate identify what ions are present

3. Qualitative Analysis: Ba2+,Ca2+ and Mg2+
In qualitative analysis we know what anions precipitate our cation of interest while leaving all other cations 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
Barium
Calcium

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+
Next perform a cation analysis on an unknown mixture.
Start with Step 1 (adding ammonium sulfate) to isolate Barium sulfate by centrifuging and confirm Barium (flame test)
Step 2 isolate Calcium Oxalate from the supernatant by adding ammonium oxalate and centrifuging. Then confirm the calcium with a flame test.
Step 3 isolate Magnesium from the supernatant in step 2 by adding sodium hydrogen phosphate and 1 drop of NaOH