QUALITATIVE ANALYSIS OF CATIONS
We can separate the ions in a mixture by selectively precipitating them with intermediate centrifuging steps. In this
... [Show More] experiment we will separate the alkaline earth metal ions, Group IIA (2), magnesium, calcium and barium. We will first observe the reactions for a known mixture containing all three cations and then use those results to analyze an unknown mixture.
Procedure
SEPARATION OF A KNOWN CATION MIXTURE
A. BARIUM REMOVAL
1. Place 10 drops of the known mixture of barium, calcium and magnesium in a test tube. Add 20 drops of ammonium sulfate and stir with a glass stirring rod. The presence of a white precipitate indicates that barium ion is present.
2. Centrifuge the sample and add another drop of ammonium sulfate to insure that all the barium has been removed. The liquid above the solid material at the bottom of the test tube after centrifuging is called the supernate. Pour the supernate into test tube #2.
3. Wash the test tube and pour the wash water with the barium sulfate precipitate into the waste container in the hood.
B. CALCIUM REMOVAL
1. Add 10 drops of ammonium oxalate to the supernate in test tube #2 and stir with a glass stirring rod. The presence of a white precipitate indicates that calcium is present.
2. Centrifuge the sample and add another drop of ammonium oxalate to insure that all the calcium has been removed. Pour the supernate into test tube #3. Wash the test tube and this wash water can go down the drain.
Chemistry 101 Spring 2020
C. MAGNESIUM REMOVAL
1. Add 10 drops of sodium hydrogen phosphate to the supernate in test tube #3. Add one drop of sodium hydroxide to the test tube and stir with a glass stirring rod. The presence of a white precipitate indicates magnesium is present. Wash the test tube and this wash water can go down the drain.
ANALYSIS OF AN UNKNOWN MIXTURE
1. Place 10 drops of the unknown mixture in a test tube. The unknown will be a solution containing one, two or three of the alkaline earth metals that were previously examined. Add 20 drops of ammonium sulfate and stir with a glass stirring rod. The presence of a white precipitate indicates that barium ion is present. If there is not a precipitate go to Step 4.
2. Centrifuge the sample and add another drop of ammonium sulfate to insure that all the barium has been removed. Pour the supernate into test tube #2.
3. Wash the test tube and pour the wash water with the barium sulfate precipitate into the waste container in the hood.
4. Add 10 drops of ammonium oxalate to the liquid in test tube #2 and stir with a glass stirring rod. The presence of a white precipitate indicates that calcium is present. If there is no precipitate go to step 6.
5. Centrifuge the sample and add another drop of ammonium oxalate to insure that all the calcium has been removed. Pour the supernate into test tube #3. Wash the test tube and this wash water can go down the drain.
6. Add 10 drops of sodium hydrogen phosphate to the supernate in test tube #3. Add one drop of sodium hydroxide to the test tube and stir with a glass stirring rod. The presence of a white precipitate indicates magnesium is present. Wash the test tube and this wash water can go down the drain.
7. Based on the observations in steps 1, 4 and 6 identify the cation(s) present in the unknown. [Show Less]