6 Identification of Anions and Cations in Solutions

Background:

Detectives in mystery novels often rush evidence from the crime scene to the lab for analysis. In this experiment you will become a chemical detective. You will conduct laboratory analysis to determine the ionic composition of an unknown solution. The process of determining the composition of a sample of matter by conducting chemical tests is called qualitative analysis. Solutions of unknown ions can be subjected to chemical tests and the results can be compared to the results given by known ions, in the same tests. By conducting the appropriate tests and applying logic, the identities of the ions present in an unknown solution can be determined.

The analysis you perform are based upon the data that no two ions produce the same set of chemical reactions. Each ion reacts in its own characteristic way. In this experiment, you will observe several types of chemical reactions commonly used as tests in qualitative analysis. These reactions include a color change, the evolution of a gas, and the formation of a precipitate---a solid product. As you do this experiment, remember that careful observation and logical reasoning are the keys to being a good detective. Who knows what ions lurk in your unknown solution?

Goals:

· Observe the reactions of common anions and cations in solution to simple chemical tests.

· Predict the ions present in an unknown solution based on the logical application of chemical tests.

Equipment:

  
safety goggles 1 cobalt - blue glass
9 small test tubes 1 test - tube holder
1 test tube rack 1 ring stand
1 25-mL graduated cylinder 1 ring support
1 250-mL beaker 1 wire gauze
1 gas burner 1 crucible tong
1 dropper pipet 1 plastic wash bottle
1 nichrome wire ( 10-cm length ) 1 forceps
  Materials:

Part A. Anions

  
0.1 M silver nitrate, AgNO3 0.05 M sodium phosphate, Na3PO4
0.1 M sodium sulfate, Na2SO4 6 M nitric acid, HNO3
0.1 M barium chloride, BaCl2 6 hydrochloric acid, HCl
0.1 M sodium hydrogen carbonate, NaHCO3 distilled water
0.1 M ammonium molybdate, (NH4)2MoO4 unknown solutions (anions)
0.1 M sodium chloride, NaCl Ý
  Part B. Cations  
0.1 M iron (III) sulfate, Fe2(SO4)3 3 M sulfuric acid, H2SO4
0.1 M sodium chloride, NaCl 6 M sodium hydroxide, NaOH
0.1 M potassium thiocyanate, KSCN 6 M hydrochloric acid, HCl
0.1 M potassium chloride, KCl distilled water
0.1 M calcium nitrate, Ca(NO3)2 red litmus paper, 1 cm x 10 cm
0.1 M sodium oxalate, Na2C2O4 unknown solutions (cations)
0.1 M ammonium nitrate, NH4NO3 Ý
  Safety:

· Note the Safety Symbols used here and in the Procedure section. Review safety information on pages 7-10.

· Always wear safety goggles when working in lab.

· Nitric acid, hydrochloric acid , sulfuric acid, and sodium hydroxide are corrosive and can cause severe injury.

· Silver, barium, and oxalate compounds are poisonous. Avoid contact with these chemicals.

· Silver nitrate will stain skin and clothing.

· Never pick up a dropper bottle by its cap. Always hold a dropper with the tip lower than the rubber bulb, so that liquid does not run into the bulb.

· Never cover the opening of a test tube with your finger when mixing chemicals in the tube. To mix the contents, "flick" the tube as demonstrated by your teacher.

Procedure:

Copy Table 6.1 into your laboratory notebook. As you perform the experiment, record your observations in this table.

Procedure note: In testing for different ions in steps 3-13 you will always begin with two test tubes. For each step you must add 2 mL of the known solution to one test tube, and 2 mL of your unknown solution to a second test tube. The name of the known solution to be used is given in each step.

Note that 1 mL is approximately 20 drops. Count out 40 drops of water in a test tube and make note of the level of water in the tube. Throughout the experiment, fill a test tube to this same level whenever a 2 mL sample is called for. This practice will save you considerable time. Always clean the medicine dropper after each use.

Part A. Testing for Anions (Day 1)

1. Set up a boiling water bath for use in step 6.

2. Thoroughly clean all the test tubes in this experiment, rinsing them well with distilled water. Record the number of the unknown solution you will be testing.

3. Test for chloride ion, Cl-. Known solution is sodium chloride. CAUTION: Nitric acid is corrosive. Do not out your finger over the top of the tube to cover it. Add 2 mL of 6 M nitric acid to each tube and gently flick the tubes to mix. Add 2 mL of 6 M nitric acid to each tube, and flick to mix. Record observations.

4. Test for sulfate ion, SO2-4. Known solution is sodium sulfate. CAUTION: Nitric acid is corrosive. Do not put your finger over the top of the tube to cover it. Add 2 mL of 6 M nitric acid to each tube and gently flick the tubes to mix. Add 10 drops of silver nitrate to each tube, and flick to mix. Record observations.

5. Test for hydrogen carbonate ion, HCO-3. Known solution is sodium hydrogen carbonate. Carefully observe the test tubes as you add 2mL of 6M hydrochloric acid to each tube. Record observations.

6. Test for phosphate ion , PO3-4. Known solution is sodu\ium phosphate. Add 1 mL of 6M nitric acid and 10 drops of ammonium molybdate solution to each tube and mix. Place the tubes in a boiling water bath and heat for 5 minutes. Allow tubes to cool in a test tube rack for 10 minutes. Record observations. (Retain the water bath for Part B, step 12.)

7. Follow your teacher's instructions for proper disposal of the materials.

Part B. Testing for Cations: ( Day 2)

8. Review the general directions at the beginning of the Procedure section. Prepare ypour test tubes as in Part A, step 2.

9. Test for ion (III) ion, Fe3+. Known solution is iron(III) sulfate. CAUTION: sulfuric acid is corrosive. Add 5 drops of 3 M sulfuric acid and 5 drops of potassium thiocyanate solution to each tube. Flick gently to mix. Record observations.

10. Flame test solution for sodium ion, Na+. Known soltuion is sodium chloride. Add 3 drops of 6 M hydrochloric acid to each tube. Flick gently to mix.

Add 3-4 mL of 6 M hydrochloric acid to a small test tube in a test-tube rack. Heat the end of a 10-cm length of nichrome wire in a hot burner flame, as shown in Figure 6.1. While it is still hot, dip the end of the wire into the hydrochloric acid in the test tube. Remove the wire from the acid and immediately reheat it in the burner flame. Repeat this aid cleaning of the wire until the flame remains unchanged when the wire is heated.

Dip the acid-cleaned wire into the sodium chloride solution. Immediately hold it in the hpot burner flame. Observe the color of the flame. Acid-clean the wire and then test the unknown solution. Record the color of the flame. (A faint colored flame is not considered a positive test for sodium.) Save the prepared unknown solution for the next test.

11. Flame test for potassium ion , K+. Known solution is potassium chloride. Add 3 drops of 6 M hydrochloric acid to each tube and flick gently to mix. Acid-clean the nichrome wire. Flame-test the potassium chloride solution. Acid-clean the wire and test your unknown. f your unknown contains sodium, the color of the sodium flame will mask the color that is characteristic of potassium . You will be able to see the potassium color, if it is there, by looking at the flame through a piece of cobalt - blue glass. Record ytou results.

12. Test for calcium ion, Ca2+. Known solution is calcium nitrate. To each tube, add 10 drops of sodium oxalate solution. Warm the tubes in the boiling waterbath for a few minutes. Record your observations.

(optional) you can also performa flame test for calcium. Add 3 drops of 6 M hydrochloric acid to fresh 2- mL samples of the calcium nitrate solution and the unknown solution. Perform flame tests as in step 10. Record the color of the calium flame and the results for your unknown.

13. Test for ammonium ion, NH+4. Known solution is ammonium nitrate. CAUTION: Sodium hydroxide can cause burns. To each tube, add 3 drops of 6M hydroxide. Hold the tube containing the ammonium nitrate solution in a test tube holder. Gently warm the tube along its sides using a back and forth motion through a burner flame. Do not allow the solution to boil . CAUTION: At all times make sure that the opening of the tube is pointed away from other people. Hold a moistened piece of red litmus paper near the mouth of the test tube, as shown in Figure 6.2. The test will be spoiled if the solution contacts the litmus paper. Record the changes you observe. Fan the vapors coming out of the tube towards your nose with your hand. Cautiously sniff the vapors. Reocrd your observstions. Repeat the procedure for your unknown solution.

14. Follow your teacher's instructions for proper disposal of the materials.Ý

Data Record:

  
Table 6.1 Tests for Common Ions
Ions Test Reagents/

Test Procedure

 		Test Results for

Solution with Ion

 		Test Results for

Unknown solution No.____

 		Is Ion Present

in Unknown?
Cl- Ý Ý Ý Ý
SO2-4 Ý Ý Ý Ý
HCO-3 Ý Ý Ý Ý
PO3-4 Ý Ý Ý Ý
Fe3+ Ý Ý Ý Ý
Na+ Ý Ý Ý Ý
K+ Ý Ý Ý Ý
Ca2+ Ý Ý Ý Ý
NH+4 Ý Ý Ý Ý
Ý QUESTONS:

1. List the anions present in your unknown.

2. List the cations present in your unknown.

3. It is possible to get a false-positive or a false-negative result when testing for ions. Propose a situation that could lead to a false positive for a particular ion. Choose a different ion and show how a false negative could result. Which do you think is more likely to happen, a false positive or a flase negative result? Explain your reasoning.