CHEM 101-Introductory Chemistry I-Lab 3 - Cations and

QUALITATIVE ANALYSIS OF CATIONS

We can separate the ions in a mixture by selectively precipitating them with intermediate centrifuging steps. In this 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.

DATA SHEET:

KNOWN SOLUTION

Addition of Sulfate

PPT NO PPT CONCLUSION

____X____ ___________ barium ion

Addition of Oxalate

PPT NO PPT CONCLUSION

____X____ ___________ calcium

Addition of Hydrogen Phosphate

PPT NO PPT CONCLUSION

____X____ ___________ magnesium

UNKNOWN SOLUTION

Addition of Sulfate

PPT NO PPT CONCLUSION

_________ ____X______ magnesium

Addition of Oxalate

PPT NO PPT CONCLUSION

_________ ____X______ magnesium

Addition of Hydrogen Phosphate

PPT NO PPT CONCLUSION

____X____ ___________ magnesium

IDENTITY OF CATION OR CATIONS IN THE UNKNOWN

UNKNOWN SOLUTION #2

Addition of Sulfate

PPT NO PPT CONCLUSION

____X____ ___________ barium ion

Addition of Oxalate

PPT NO PPT CONCLUSION

_________ _____X_____ magnesium

Addition of Hydrogen Phosphate

PPT NO PPT CONCLUSION

____X____ ___________ magnesium

IDENTITY OF CATION OR CATIONS IN THE UNKNOWN

QUALITATIVE ANALYSIS OF ANIONS

We can separate the ions in a mixture by selectively precipitating them with intermediate centrifuging steps. In this experiment we will separate the anions chloride, iodide and sulfate. We will first observe the reactions for a known mixture containing all three anions and then use those results to analyze an unknown mixture.

Procedure

SEPARATION OF A KNOWN ANION MIXTURE

A. IODIDE IDENTIFICATION

1. Place 10 drops of the known mixture of iodide, chloride and sulfate in a test tube. Add 20 drops of silver nitrate and stir with a glass stirring rod. The presence of a yellow precipitate suggests that iodide ion is present.

2. Centrifuge the sample. 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 #3 and save for the sulfate analysis.

3. Add 10 drops of ammonium hydroxide to the precipitate in test tube 1 and thoroughly stir with a stirring rod. Centrifuge the precipitate. Pour the supernate into test tube #2 as save for the chloride analysis. A yellow precipitate indicates the presence of iodide.

B. CHLORIDE IDENTIFICATION

Add dilute nitric acid dropwise into test tube #2 until the solution turns blue litmus paper red. The presence of a white precipitate indicates that chloride is present.

C. SULFATE IDENTIFICATION

1. Add 10 drops of barium nitrate to the supernate in test tube #3. The presence of a white precipitate indicates that sulfate ion is present.

2. Dispose of the test tube contents in the barium waste container in the hood.

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 anions that were previously examined. Add 20 drops of silver nitrate and stir with a glass stirring rod. The presence of a yellow precipitate suggests that iodide ion is present. The presence of a white precipitate indicates that chloride ion is present and iodide ion is not present. Go to step 5. If there is no precipitate iodide and chloride ions are not present. Go to step 6.

2. Centrifuge the sample. Pour the supernate into test tube #3 and save for the sulfate analysis.

3. If the precipitate in test tube 1 was YELLOW add 10 drops of ammonium hydroxide to the precipitate and thoroughly stir with a stirring rod. Centrifuge the precipitate. Pour the supernate into test tube #2 and save for the chloride analysis. A yellow precipitate indicates the presence of iodide.

4. Add dilute nitric acid dropwise into test tube #2 until the solution turns blue litmus paper red. The presence of a white precipitate indicates that chloride is present. Go to step 6.

5. Centrifuge the sample. Pour the supernate into test tube #3.

6. Add 10 drops of barium nitrate to the supernate in test tube #3. The presence of a white precipitate indicates that sulfate ion is present.

7. Dispose of the test tube contents in the barium waste container in the hood.

DATA SHEET:

KNOWN SOLUTION

Addition of silver nitrate

PPT NO PPT CONCLUSION

_YELLOW PPT ___________ iodide

Addition of Ammonium hydroxide

PPT NO PPT CONCLUSION

_YELLOW PPT ___________ iodide

Addition of Nitric Acid

PPT NO PPT CONCLUSION

_WHITE PPT ___________ chloride

Addition of Barium nitrate

PPT NO PPT CONCLUSION

_ WHITE PPT ___________ sulfate

UNKNOWN SOLUTION

Addition of Silver nitrate

PPT NO PPT CONCLUSION

_________ _____X_____ sulfate

Addition of Ammonium Hydroxide

PPT NO PPT CONCLUSION

_________ ____X_______ sulfate

Addition of Nitric Acid

PPT NO PPT CONCLUSION

_________ _____X______ sulfate

Addition of Barium nitrate

PPT NO PPT CONCLUSION

_ WHITE PPT ___________ sulfate

IDENTITY OF ANION OR ANIONS IN THE UNKNOWN

UNKNOWN SOLUTION

Addition of Silver nitrate

PPT NO PPT CONCLUSION

WHITE PPT ___________ chloride

Addition of Ammonium Hydroxide

PPT NO PPT CONCLUSION

_________ ____X_______ chloride

Addition of Nitric Acid

PPT NO PPT CONCLUSION

_________ _____X ______ chloride

Addition of Barium nitrate

PPT NO PPT CONCLUSION

WHITE PPT_ ___________ sulfate

IDENTITY OF ANION OR ANIONS IN THE UNKNOWN