Experiment 5 - Qualitative Analysis

Procedure:

Part D - Analysis of an Unknown Solution

1. To about 20 drops of unknown solution add about 20 drops of dilute HCl in a small test tube. Record your observations.
2. Centrifuge and separate the precipitate from the solution. Discard the solution.
3. Add about 2 mL of distilled water and stir to wash the precipitate. Centrifuge and separate the precipitate from the solution and again discard the solution.
4. Check for Pb²⁺: Add about 2 mL of distilled water to the test tube which contains the precipitate and place it in a beaker of boiling water for about 3 minutes. Mix occasionally. Record your observations. No change may be observed.
5. Centrifuge and separate the precipitate from the solution. Transfer the solution to a clean test tube. Keep the precipitate for step 7.
6. Add a few drops of K₂CrO₄ to the solution from step 5. A yellow precipitate confirms the presence of Pb²⁺ ions in the unknown solution. Record your observations.
7. Check for Hg₂²⁺: Add about 10 drops of concentrated ammonium hydroxide, NH₄OH, (located in the fume hood) to the precipitate from step 5. A black precipitate confirms the presence of Hg₂²⁺ ions in the unknown solution. Record your observations.
8. Stir the substances in the test tube from step 7 and then centrifuge to separate the precipitate from the solution. Transfer the solution to a clean test tube.
9. Check for Ag⁺: Add nitric acid, HNO₃, drop=wise to the solution from step 8, until the solution becomes acidic. Place a small drop of the test tube solution on the blue litmus paper. If the blue litmus paper turns pink the test tube solution is acidic. If a white precipitate forms, Ag⁺ ions are in the unknown solution. Record your observations.