Separation and Qualitative Analysis of Cations. Evaluation copy

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1 Separation and Qualitative Analysis of Cations Computer 14A When a chemist conducts a qualitative analysis, the primary goal is to identify the substances that are present in a mixture. It is important to separate substances from each other in a mixture, so that a confirming test can be as definitive as possible. After a thorough qualitative analysis investigation, it would be nice to say, for example, that potassium is present instead of saying merely that an alkali metal is present. Conducting a qualitative analysis, or qual scheme as it is sometimes nicknamed, is a methodical process that requires great care. A qual scheme can be used to identify a wide variety of substances; in this experiment, you will focus on ions. It is fascinating to seek and identify a substance based on an established pattern of chemical behavior. You will see some very interesting things during this experiment, things that will help you understand more about the nature of cations and anions. Consider this experiment to be more of a research project than a simple series of tests. In this experiment, you will place almost all of your emphasis on conceptual and theoretical information about the chemical behavior of ions. Other than careful measuring, there is no data to gather, graph, or examine in a qual scheme. OBJECTIVE In this experiment, you will Prepare and analyze a solution that contains ten selected cations. Analyze an unknown solution that contains a selection of cations. MATERIALS Test Solutions and Substances Cation Solutions 6 M hydrochloric acid, HCl 0.2 M silver nitrate, AgNO 3 6 M nitric acid, HNO M mercury (I) nitrate, Hg 2 (NO 3 ) 2 6 M ammonia, NH M copper (II) nitrate, Cu(NO 3 ) 2 6 M sulfuric acid, H 2 SO M zinc nitrate, Zn(NO 3 ) 2 6 M acetic acid, CH 3 COOH 0.2 M cobalt (II) nitrate, Co(NO 3 ) 2 6 M sodium hydroxide, NaOH 0.5 M lead (II) nitrate, Pb(NO 3 ) M potassium hexacyanoferrate (II), K 4 [Fe(CN) 6 ] 0.2 M iron (III) nitrate, Fe(NO 3 ) M aluminum nitrate, Al(NO 3 ) M tin (II) chloride, SnCl M bismuth (III) nitrate, Bi(NO 3 ) M potassium thiocyanate, KSCN 0.2 M manganese (II) nitrate, Mn(NO 3 ) 2 3% hydrogen peroxide, H 2 O 2 Safety Chemicals for Spills 0.1% aluminon baking soda, NaHCO 3 (s) potassium nitrite, KNO 2 (s) vinegar (5% acetic acid solution) sodium bismuthate, NaBiO 3 (s) Evaluation copy Advanced Chemistry with Vernier 14A - 1

2 Computer 14A Equipment 10 test tubes, mm plastic Beral pipets, graduated test tube holder several stirring rods test tube rack or racks ring stand, ring, wire gauze, burner or hot plate corks to fit test tubes spatula 400 ml beaker for rinsing stirring rods ph paper or Vernier ph Sensor 250 ml beaker for hot water bath capillary pipet centrifuge PRE-LAB EXERCISE 1. Prepare a chart for recording your observations as you test for the presence of cations. A sample chart, with the test results for a known solution, is shown below. Known Solution Unknown Solution Test Procedure Results Conclusions Results Conclusions Add HCl to solution in White ppt forms. Ag +, Pb 2+, and/or 1 Test Tube 1. Centrifuge. Other ions in the Pour the supernatant into liquid in Test Hg 2+ 2 present. Test Tube 2 Tube Label your test tubes clearly, and arrange your chemical reagents in an orderly manner, so that you will be certain of the results of each test in the qual scheme. 3. Wash and rinse all of your glassware with distilled water. Tap water will contain some of the ions for which you are conducting the tests. If you use a glass stirring rod, make sure that it is rinsed between uses. You may store your glass stirrers in a 400 ml beaker that is about ¾ full with distilled water. 4. Prepare a hot-water bath in a 250 ml beaker, which will be used intermittently during the experiment. 5. You will be testing two solutions, one that you will prepare and one unknown solution. Decide, before starting this experiment, whether or not you will be testing the solutions simultaneously. There are advantages and disadvantages either way, and it depends largely on how comfortable you are with this long procedure. 6. At certain points in the testing, you will be checking the ph of a solution. You may use conventional ph paper to check ph or you may use a Vernier ph Sensor. If you use a ph Sensor, follow the steps below to set up the sensor. a. Connect a LabPro or CBL 2 interface to the computer or handheld with the proper cable. b. Connect the ph Sensor to Channel 1 of the interface. c. Start the data collection program and set up the program for a ph Sensor. d. The ph readings will be displayed on the computer screen. 14A - 2 Advanced Chemistry with Vernier

3 SAFETY Separation and Qualitative Analysis of Cations The acid and base solutions that you will use in this experiment have high molar concentrations. Even though you will use only a few drops of these solutions, they must be used very carefully. Use the normal precautions and processes for diluting and cleaning up any spills of the acids and bases. Locate the containers of vinegar and baking soda that have been placed in the lab for use in neutralizing spills. PROCEDURE Separation and Analysis for Cations 1. Obtain and wear goggles. 2. Set up 10 test tubes in a test-tube rack. Label the test tubes #1-10. Prepare a hot water bath in a 250 ml beaker. 3. Prepare a known solution in Test Tube 1 by mixing three drops of each ion that you wish to test. You will have about 2 ml of solution. Analyze the Solution for the Presence of Pb 2+, Hg 2 2+, and Ag + ions 4. Add 8 drops of 6 M HCl solution to Test Tube 1 and stir the mixture. If a white precipitate forms, then one or more of the following ions are present: Ag +, Pb 2+, and/or Hg Centrifuge the test tube and add one more drop of 6 M HCl. No additional precipitate should form. If more precipitate does form, continue adding HCl one drop at a time until the supernatant remains clear. 5. Centrifuge the mixture and transfer the supernatant liquid to Test Tube Add 1 ml of distilled water to Test Tube 1, containing the precipitate. Stir the mixture, centrifuge the test tube, and decant the wash water down the sink. 7. Test the precipitate for Pb 2+ ions by first adding 1 ml of distilled water to Test Tube 1. Place Test Tube 1 in the hot water bath, which should be at or very near boiling, for about 3 minutes. Stir the test tube every thirty seconds or so. Centrifuge the hot liquid and quickly decant the supernatant into Test Tube Add 5 drops of 6 M H 2 SO 4 to Test Tube 3. If there are lead (II) ions present, a white precipitate will form in the test tube. If you find Pb 2+ ions, repeat Steps 7 and 8 with Test Tube 1 to remove all of the lead (II) ions from the precipitate. Continue until the addition of sulfuric acid does not form a white precipitate. Dispose of the lead (II) sulfate precipitate, from Test Tube 3, as directed. Status Check At this point in your analysis, you should have three test tubes in use. Test Tube 1 contains a precipitate, which you have tested for Pb 2+ ions. Test Tube 2 contains a liquid, which is the supernatant from Test Tube 1. Test Tube 3 has been used to conduct the test for the presence of Pb 2+ ions. Rinse and clean Test Tube Test the precipitate for Hg 2 2+ ions by adding 1 ml of 6 M NH 3 solution to Test Tube 1. If a dark gray precipitate forms, then Hg 2 2+ ions are present. Stir the mixture and centrifuge the Advanced Chemistry with Vernier 14A - 3

4 Computer 14A test tube. Decant the supernatant into Test Tube 4. Dispose of the mercury compound, if one formed, as directed. You are now finished with Test Tube 1; it may be rinsed and cleaned. 10. Test the liquid in Test Tube 4 for Ag + ions by adding 15 drops of 6 M HCl solution to the test tube. The reaction produces smoke and heat, regardless of the presence of Ag + ions. Stir the mixture. Check the ph of the liquid and, if it is not acidic, add more HCl dropwise until the liquid is acidic. If a white precipitate forms in the acidic liquid, then Ag + ions are present. Dispose of the silver compound, if it forms, as directed. Rinse and clean Test Tube 4. You have now completed the analysis for the presence of Pb 2+, Hg 2 2+, and Ag + ions. You should have one test tube of liquid left to analyze, Test Tube 2. Analyze the Solution for the Presence of Mn 2+, Co 2+, Fe 3+, Bi 3+, and Cu Add 10 drops of 3% hydrogen peroxide solution to Test Tube 2. Add three drops of 6 M NaOH solution, mix, and check the ph of the liquid. If it is not basic, add more NaOH dropwise until the liquid is basic. Once the liquid becomes basic, add 3 additional drops of 6 M NaOH. 12. Stir the mixture in the test tube, and place it in a hot water bath for three minutes. A precipitate will form; the darker the precipitate, the more different ions are present. Centrifuge the mixture and decant the supernatant into Test Tube 5. Save the supernatant for later analysis. 13. Wash the precipitate in Test Tube 2 with 10 drops of 6 M NaOH and 10 drops of distilled water. Centrifuge the test tube and discard the wash water. 14. Add 5 drops of water to Test Tube 2 and mix. Add 6 M H 2 SO 4 dropwise until the solution is acidic. Centrifuge the test tube and transfer the supernatant to Test Tube 6. Wash the precipitate in Test Tube 2 with 10 drops of distilled water, centrifuge it, and discard the supernatant. Test Tube 2 may contain either or both Mn 2+ and Co 2+ ions. Test Tube 6 may contain one or more of the following ions: Fe 3+, Bi 3+, and Cu Add 1 ml of water and 1 ml of 6 M H 2 SO 4 solution to Test Tube 2. Add 1 ml of 3% H 2 O 2 solution and place the test tube in a hot water bath, brought to gentle boiling. Stir Test Tube 2 occasionally, and keep it in the hot water bath until all of the precipitate dissolves. When the precipitate dissolves, transfer half of the solution to Test Tube To test for Mn 2+, add 1 ml of 6 M HNO 3 to Test Tube 7. Mix, and add a very small scoop of solid sodium bismuthate, NaBiO 3. There should be an excess of sodium bismuthate in the test tube; add more if necessary to create an excess. Stir the contents of the test tube and centrifuge it. If the supernatant liquid is a purple color, then Mn 2+ ions are present. Dispose of the contents of Test Tube 7 as directed and clean the test tube. 17. Take Test Tube 2 to a fume hood. To test for Co 2+, slowly add a small scoop of solid KNO 2 to Test Tube 2. If a yellow precipitate forms, then Co 2+ ions are present. Possible products of this reaction are NO and NO 2 gases, which are toxic. Fill Test Tube 2 with water and dispose of the contents as directed. Rinse and clean Test Tube To test for Cu 2+, add 6 M NH 3 dropwise to Test Tube 6 until the solution is basic, and then add 1 ml more of the 6 M NH 3. Stir the mixture. Centrifuge the test tube and transfer the supernatant to Test Tube 8. If the liquid in Test Tube 8 is a blue color, then there are Cu 2+ 14A - 4 Advanced Chemistry with Vernier

5 Separation and Qualitative Analysis of Cations ions present. Dispose of the contents of Test Tube 8, unless you wish to conduct the optional test in Step (optional) Here s a second test for the presence of Cu 2+ ions: add 6 M HC 2 H 3 O 2 solution dropwise to Test Tube 8 until the blue color disappears. Then add 2 drops of 0.1 M K 4 [Fe(CN) 6 ]. A reddish-brown precipitate indicates the presence of Cu 2+ ions. 20. Add 10 drops of distilled water to the precipitate in Test Tube 6. Stir the mixture, and add 6 M H 2 SO 4 dropwise until the precipitate dissolves. Transfer half of the solution to Test Tube To test for Fe 3+ add 5 drops of 0.1 M KSCN solution to Test Tube 6. If the liquid in the test tube turns a deep red color, then Fe 3+ ions are present. Dispose of the contents as directed. Rinse and clean Test Tube To test for Bi 3+ add 2 drops of 6 M HCl solution to Test Tube 9 and stir the mixture. Obtain a 400 ml or 600 ml beaker and fill it with about 300 ml of tap water. Add a few drops of the solution in Test Tube 9 to the beaker of tap water. If a white cloudiness appears in the beaker, then Bi 3+ ions are present. Dispose of the contents of Test Tube 9, unless you wish to conduct the optional test in Step (optional) Here s a second test for the presence of Bi 3+ ions: add 6 M NaOH to Test Tube 9 dropwise until a white precipitate forms, and then add 10 more drops of 6 M NaOH. Add 5 drops of 0.1 M SnCl 2 solution. The appearance of a black precipitate indicates the presence of Bi 3+ ions. Status Check At this point, you have successfully separated and analyzed your known solution for the presence of the following ions: Pb 2+, Hg 2 2+, Ag +, Mn 2+, Co 2+, Fe 3+, Bi 3+, and Cu 2+. You should have one test tube of liquid to analyze, Test Tube 5. Analyze the Solution for the Presence of Zn 2+ and Al To test for Al 3+, add 10 drops of 6 M HNO 3 solution to Test Tube 5. Check the ph of the solution. If it is not acidic, add more 6 M HNO 3 dropwise until the solution becomes acidic. Add 6 M NH 3 solution dropwise until the solution is basic, and then add an additional 3 drops of 6 M NH 3. Stir the mixture. If a gelatinous precipitate forms, then Al 3+ ions are present. This precipitate is difficult to see. Centrifuge the test tube and check again for the precipitate. After centrifuging the test tube, transfer the supernatant to Test Tube To confirm the presence of Al 3+ ions, add 6 M HCl solution to Test Tube 5 dropwise until the precipitate dissolves. Stir the solution, and add 3 drops of aluminon solution. Add 6 M NH 3 solution dropwise until the mixture is basic. Centrifuge the test tube. If the precipitate is now a red color, then Al 3+ ions are indeed present. 26. To test for Zn 2+, and complete the qual scheme for cations, add 6 M HCl solution dropwise to Test Tube 10 until the liquid is acidic. Add 3 drops of 0.1 M K 4 [Fe(CN) 6 ] and stir the mixture. Centrifuge the test tube. If there is a precipitate (its color could be white or light green or blue green) in the test tube, then there are Zn 2+ ions present. Dispose of the contents of Test Tubes 5 and 10 as directed. Advanced Chemistry with Vernier 14A - 5

6 Computer 14A DATA ANALYSIS Identify the cations that were present in your unknown solution. Describe, in detail, how you used the results of your testing to identify the contents of your unknown solution. As you prepare your concluding remarks, consider the following questions: 1. Describe the process by which Ag + ions are precipitated out of solution. 2. A disproportion reaction occurs when NH 3 solution reacts with Hg 2 Cl 2. Write a balanced reaction equation for this event. 3. Write half-reactions that show how H 2 O 2 can act as either an oxidizing agent or a reducing agent, and describe where each of these situations occurred in your testing. 4. In your testing, several precipitates are formed, and then dissolved as complexes. Select three of these situations for which to write a balanced reaction equation. 5. When Mn 2+ ions are separated from the mixture, they go through a series of oxidizing and reducing steps. Write the reaction equations that describe this process. 6. The NH 3 solution is used, in three separate instances, to separate two ions. In each of these cases, one ion forms a precipitate and the other ion remains in solution as a complex. Describe each of these instances, using reaction equations. 7. Al 3+ and Zn 2+ ions are amphoteric. Describe how the amphoteric nature of these ions is shown in your testing. 14A - 6 Advanced Chemistry with Vernier

7 Vernier Lab Safety Instructions Disclaimer THIS IS AN EVALUATION COPY OF THE VERNIER STUDENT LAB. This copy does not include: Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs The complete Advanced Chemistry with Vernier lab manual includes 35 labs and essential teacher information. The full lab book is available for purchase at: Vernier Software & Technology S.W. Millikan Way Beaverton, OR Toll Free (888) (503) FAX (503)