Experiment 7. Qualitative Analysis of Anions

LEARNING OBJECTIVES

The students will be able

1. to observe the chemical behavior of various anions in solutions.

2. to apply various laboratory techniques.

BACKGROUND

This experiment is an exercise in qualitative analysis where you will be analyzing solutions to determine the presence of anions. Certain techniques will be used to separate the ions from one another. A good starting point for this type of analysis is the observation of the color and solubility of a substance. Therefore, we will rely on more sophisticated analyses such as precipitation, acid-base, and electron transfer reactions. Again, the goal will be to correctly identify the anions (two of the five investigated) present in an unknown solution.

· In the first part, you will analyze a “known” solution which contains all five of the anions; these procedures will enable you to see the color changes, gas evolution, precipitation, and any other special effects of each reaction.

· In the second part, you will analyze an “unknown” solution using the same techniques, to determine the presence of two of these anions.

· The plan should be simple.

First, you will investigate the precipitation reactions given in Table 1. Remarkably, the AgNO3 solution will give precipitates with all of the anions present in the solution. The only anion that gives gas evolution is carbonate. At some point during the analysis each anion will react specifically with the reagent added to the sample. Therefore, you will develop a schematic flowchart called “Flowcharting Qualitative Analysis”.

MATERIALS

Known and unknown solutions: 0.10 M NaCl, 0.10 M KI, 0.50 M Na2CO3, 0.10 M Na2SO4, and 0.10 M

NaBr.

Test Reagent:

Silver nitrate, 0.10 M AgNO3, nitric acid, 6.0 M HNO3, Ammonia, 6.0 M NH3, iron(III) nitrate, 0.10 M Fe(NO3)3 in 6.0 M HNO3, Potassium permanganate, 0.020 M KMnO4, Barium chloride, 0.10 M BaCl2, mineral oil or Hexane, baking soda (to neutralize spills)

Equipment: small test tubes (12), test tube holder, test tube rack, Beral pipets, centrifuge.

SAFETY ALERT:

· Most of the acids and bases are very concentrated and can cause chemical burns.

· Acid spills can be neutralized with baking soda, NaHCO3.

· Neutralize base spills with vinegar solution (dilute acetic acid, HC2H3O2)  Some compounds are poisonous; wash your hands.

· Solutions containing silver and potassium permanganate can cause stains which do not appear immediately.

NOTE: DISPOSE OF THE MINERAL OIL or HEXANE LAYER IN THE ORGANIC WASTE CONTAINER.

PRELAB QUESTIONSNAME:

1. Identify the five possible anions which are to be identified in this experiment.

2. How many of these five anions will appear in your unknown sample?

3. Four of the chemicals used to make the known and unknown samples contain the sodium ion. How will this affect your results in this experiment?

4. What purpose does the centrifuge have in this experiment?

5. Four of positive tests for an unknown ion involve the formation of a precipitate. Which ion has a positive test that involves the formation of gas?