This experiment continues the qualitative analysis begun in Experiment 19. Here we will be analyzing solutions to determine the presence of anions. The same techniques that were used for the cation analysis must be used for the anions. If you have not carried out Experiment 19, read the introductory section before starting this experiment. The major difference between cation and anion analysis is that in anion analysis, a series of separations of the ions from one another is usually not the most efficient way to determine their presence. Instead, only some separations will be made, and the initial test solution will be used to test many of the ions. Refer to the flow chart at the end of the experimental directions as you proceed.
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First you will prepare and analyze a “known” solution which contains all six of the anions. Then you will analyze an “unknown” solution using the same techniques, to determine the presence or absence of each anion.
Most of the acids and bases used are very concentrated and can cause chemical burns if spilled. Handle them with care. Wash acid or base spills off of yourself with lots of water. Small spills (a few drops) can be cleaned up with paper towels. Larger acid spills can be neutralized with baking soda, NaHCO3, and then safely cleaned up. Neutralize base spills with a vinegar solution (dilute acetic acid). Some of the compounds are poisonous. Wash your hands when finished.
Solutions containing silver ions and potassium permanganate solutions cause stains which do not appear immediately. If you suspect that you spilled any of these solutions on yourself, wash off with soap and water.
Wear Chemical Splash Goggles and a Chemical-Resistant Apron.
Preparation of a Solution for Analysis.
Prepare a known solution containing 1 mL of each of the anions to be tested. This solution will be referred to as the original test solution. Your teacher will provide you with an “unknown” solution to be analyzed.
Note that the following directions are written for a “known” solution that contains all of the anions. An “unknown” solution will probably not form all of the products described in this procedure. You should make note of any differences as you analyze your “unknown” solution.
Aqueous solutions of all of the anions to be tested are colorless. The positive ion associated with each of the anions will be either sodium or potassium ion.
1. Separation of the Halides (Cl-, Br-, I-); Confirmation of Chloride.
The halides all form insoluble silver compounds. Silver chloride is a white solid, silver bromide is pale cream-colored solid, and the solid silver iodide is light yellow in color.
Cl-(aq) + Ag+(aq) AgCl(s)
Br-(aq) + Ag+(aq) AgBr(s)
I-(aq) + Ag+(aq) AgI(s)
Silver chloride is the only silver halide that dissolves in 6 M ammonia, NH3, forming the colorless ion Ag(NH3)2+. If nitric acid, HNO3, is added to a solution containing this ion, the ammonia in the complex reacts with hydrogen ions to form ammonium ions, and the silver recombines with the chloride ions which are still present in solution.
AgCl(s) + 2 NH3(aq) Ag(NH3)2+(aq)+ (aq)
Ag(NH3)2+ (aq) + (aq) + 2 H+(aq) AgCl(s) + 2 NH4+(aq)
Place 10 drops of the original test solution (or unknown solution) in a test tube. Test to see if the solution is acidic. If it is not, add 6 M acetic acid, HC2H3O2, dropwise with stirring until the solution is acidic. Add 10 drops of 0.1 M silver nitrate, AgNO3. A precipitate of AgCl, AgBr, and AgI will form. Centrifuge and pour off the supernatant liquid. Wash the solid with 0.5 mL distilled water, centrifuge and discard the wash water.
Add 0.5 mL 6 M ammonia, NH3, to the precipitate. Stir to dissolve any AgCl.
Centrifuge, and pour the supernatant liquid into another test tube to test for chloride ion. Discard the precipitate of AgBr and AgI in a container provided for disposal of waste solutions.
Add 1 mL 6 M nitric acid, NHO3, to the solution containing the dissolved silver chloride. The solution will get hot and smoke from the reaction with the excess ammonia whether or not silver chloride is present. Test with litmus or pH paper to see if the solution is acidic. If it is not, add more HNO3 until the solution is acidic. The appearance of the white precipitate of AgCl in the acidic solution confirms the presence of chloride. 2. Separation and Confirmation of Bromide and Iodide.
In acid solution, iron(III) ion, Fe3+, is a weak oxidizing agent capable of oxidizing the easily oxidized iodide ion to iodine. Bromide and other ions present will not interfere. The nonpolar iodine will preferentially dissolve in nonpolar mineral oil, where it can be identified by its pink to violet color.
2 I-(aq) + 2 Fe3+(aq) I2(aq) + 2 Fe2+
KMnO4 is a stronger oxidizing agent than the iron (III) nitrate and will oxidize bromide, Br-, to bromine, Br2. Other ions present will not interfere. The nonpolar bromine can be extracted into nonpolar mineral oil where it can be identified by its characteristic yellow to brown color.
10 Br-(aq) + 2 MnO4-(aq) + 16 H+(aq) 5 Br2(aq) + 2 Mn2+(aq) + 8 H2O(l)
Place 10 drops of the original test solution (or unknown solution) in a test tube. Add 6 M HNO3 dropwise with stirring until the solution is acidic. Add 1 mL 0.1 M Fe(NO3)3 in 0.6 M HNO3 solution and stir. Then add 1 mL of mineral oil, stopper the test tube with a cork stopper and shake for 30 seconds. The presence of a pale pink to purple color in the mineral oil layer (the top layer) due to dissolved iodine confirms the presence of I- in the original solution.
Draw the mineral oil layer off the solution with a capillary dropper and discard in the container provided for waste solutions. Add 0.1 M KMnO4 solution dropwise with stirring until the solution remains pink. Again add 1 mL mineral oil, cork and shake the test tube for 30 seconds. The presence of a yellow to brown color in the mineral oil layer due to dissolved bromine confirms the presence of Br- in the original solution. Discard the solution in the container provided. 3. Confirmation of Carbonate.
In acid solution, carbonate forms carbon dioxide gas and water. The carbon dioxide may be seen as a slight effervescence. Carbon dioxide is less soluble in hot water than cold water.
When carbon dioxide gas is passed through a saturated solution of barium hydroxide, it readily forms a
precipitate of white barium carbonate.
CO3 2-(aq) + 2 H+(aq) CO2(g) + H2O(l)
CO2(g) + Ba2+(aq) + 2 OH-(aq) BaCO3(s) + H2O(l)
If any bubbles were formed when acid was added to the original solution, carbonate is probably present and carbon dioxide is being formed. A confirmation of the presence of carbonate involves reacting evolving carbon dioxide with barium hydroxide to form white, insoluble barium carbonate.
Place 2 mL of clear, saturated Ba(OH)2 solution in a test tube to be available for the test with carbon dioxide. Place 1 mL of the original test solution (or unknown solution) in a different test tube. Acidify this solution by adding 0.5 mL of 6 M HNO3. Place the tube in a hot water bath and observe to see if any gas bubbles form. Take a dry Beral pipet and squeeze the bulb closed. Place the tip of the pipet close to (but not touching) the surface of the liquid in the test tube and slowly release the bulb to draw escaping carbon dioxide into the pipet. Put the pipet into the barium hydroxide solution, and slowly squeeze the bulb, causing the gas in the pipet to bubble through the barium hydroxide solution. This procedure may be repeated. The formation of a cloudy white precipitate of barium carbonate confirms the presence of carbonate ion in the original solution. 4. Confirmation of Sulfate.
The test for sulfate is the formation of white, insoluble barium sulfate. This solid is insoluble even in
SO4 2-(aq) + Ba2+(aq) BaSO4(s)
Place 0.5 mL of the original test solution (or unknown solution) in a test tube. Add 6 M nitric acid, HNO3, dropwise until the solution is acidic. Then add 0.5 mL 0.1 M BaCl2 solution. The formation of a white precipitate of BaSO4 confirms the presence of sulfate. 5. Confirmation of Nitrate.
The test for nitrate involves the reduction of nitrate ions in basic solution to ammonia, NH3, using solid aluminum as the reducing agent. When the solution is heated, ammonia gas is liberated. The evolving
ammonia gas will turn litmus paper from pink to blue.
3 NO3 –(aq) + 8 Al(s) + 5 OH -(aq) + 18 H2O(l) 3 NH3(g) + 8 Al(OH)4 –(aq)
Place 1 mL of the original test solution (or unknown solution) in a test tube. Add 6 M NaOH dropwise until the solution is basic, and then add 6 drops in excess. Use a Beral pipet to transfer the solution to the bottom of a dry test tube without getting the walls of the test tube wet with solution. Add the tip of a spatula of aluminum granules. Place a small cotton wad loosely about halfway down the test tube, but not touching the solution. This is to prevent spattering of the solution onto the litmus paper. Hang a piece of moist red litmus paper (or pH paper) in the tube so that the bottom of the paper is close to (but not touching) the cotton. Now warm the solution in a hot water bath until it starts bubbling strongly. Be sure that the solution and the cotton do not touch the litmus paper. Allow the solution to cool. A slow color change (within 3 to 5 minutes) of the litmus from pink to blue, starting at the bottom and spreading to the top, indicates the evolution of ammonia and confirms the presence of nitrate in the original solution.
Your teacher will provide a waste container for the solutions used in this experiment. The teacher will add solid zinc and some sodium sulfate to the waste collected. The substances may be safely disposed of using the method in the Flinn Chemical Catalog / Reference Manual, suggested disposal method #11 (procedure B). See the appendix.
In your laboratory discussion include answers to the following questions:
1. The confirmatory test for chloride ion with silver ion is the same chemical reaction used to confirm silver in the cation analysis scheme. Explain what the reaction is and how the initial precipitate is dissolved and reprecipitated. Use equations in your explanation.
2. The procedure for chloride analysis makes use of the fact that AgCl can be dissolved in ammonia, but neither AgBr nor AgI will dissolve in ammonia. Look up the solubility products of AgCl, AgBr and AgI and show how their relative solubilities agree with this fact.
3. Refer to a table of standard reduction potentials to find the values for the reduction of Cl2, Br2, I2, MnO4-, and Fe3+. List the reduction reactions according to the Eº values. From the listing determine which of the halides can be oxidized by Fe3+ and which can be oxidized by acidic MnO4-.
4. Explain why it is necessary to test for iodide by oxidation with Fe3+ before the test for bromide by oxidation with MnO4- is done.
5. Write separate oxidation and reduction half-reactions for the procedure used in the test for nitrate ions.
6. In the nitrate test, why must care be taken to keep the moist litmus from coming in contact with the cotton or the solution?