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

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

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)

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

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

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!

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