Qualitative Analysis

Introduction

One procedure for analyzing a mixture of several substances involves a systematic separation of cations.

You will first analyze a solution known to contain specific ions before proceeding to the analysis of an unknown solution. The purpose of this experiment is to familiarize you with techniques involved in separations and the use of confirmation tests to identify an unknown.

Study the diagram of the procedure carefully. The purpose of such a diagram is to help you understand what is happening in a given reaction so that you will know more than just that the solution "turned blue." It is important to keep good records of your observations.

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Cleanliness and care in working with chemicals must be exercised at all times. Consult your instructor if any step is not clear.

Your instructor will tell you how to obtain your sample containing the following ions: Ag⁺, Fe³⁺, Cu²⁺,Mn²⁺, Zn²⁺, and Al³⁺. Use 22 drops of unknown unless instructed otherwise.

When checking solutions with test papers, use a glass stirring rod. Place a small piece of test paper on a clean surface (such as a watch glass.) Place the rod in the liquid to be tested. Remove the rod, and touch the rod to the test paper. The small amount of liquid adhering to the rod is usually sufficient to run the test.

Part A: Separation of silver ions from solution.

Step 1-Separation of Silver:

To the solution containing the ions, add 15 drops of 0.1ÊM sodium chloride, NaCl, solution. Shake well. You should see a white precipitate, silver chloride, form. Centrifuge until the liquid on top of the precipitate (the supernatant) is clear (approximately 1 minute).

Silver ions are precipitated with the addition of any soluble chloride to silver chloride, AgCl: Ag⁺ + Cl^- --> AgCl(s)

The AgCl is a white precipitate quite insoluble in water and dilute acids. Test to see if the Ag± ion is completely precipitated by adding another drop or two of 0.1 M NaCl. If more precipitate forms, add 0.1M NaCl until no more precipitate occurs, then centrifuge and test again. (Results on later tests will be more clear cut when the removal of each cation at its appropriate step is complete.)

Carefully draw off the supernatant with a Pasteur pipet and save in a test tube labeled "Step 3." The supernatant contains the remainder of the ions except for silver. It is better to leave a small amount of the supernatant in the tube than to draw any of the precipitate into the pipet. Set the tube with the clear liquid aside. Save the precipitate for step 2.

Step 2-Confirmation of Silver:

Wash the white precipitate from step 1 with 20 drops of distilled water by stirring, centrifuging, and drawing off the wash liquid with a pipet. Discard the wash. Be careful not to draw up the precipitate into the pipet. To confirm that the precipitate is AgCl, carefully add 6 M aqueous NH₃ with shaking until the solid is completely dissolved (about 20 drops). The AgCl can be distinguished from other insoluble chlorides by its solubility in ammonia which occurs by the formation of the silver complex ion: AgCl + 2NH₃(aq) --> Ag(NH₃)²⁺ + Cl^-

Now add 6 M nitric acid, HNO₃ (Caution!) until the solution is acidic (about 20 drops). Caution!: the test tube may get very warm. Check with litmus to see that the solution is acidic. A white precipitate of AgCl confirms the presence of silver. Discard this precipitate.

Part B: Separation of iron, manganese, and copper from solution.

Step 3-Precipitation of Iron, Manganese, and Copper:

To the solution saved from step 1, labeled "Step 3," add 10 drops of 6% hydrogen peroxide, H₂O₂ (Caution!). While stirring, add 6M sodium hydroxide, NaOH (Caution!), until the solution is basic (about 10 drops), and then add 3 drops more. Stir the solution and place the test tube in a boiling water bath. A dark precipitate should form of iron hydroxide, Fe(OH)₃, manganese dioxide, MnO₂, and copper hydroxide, Cu(OH)₂. Boil the solution for 2 minutes or more and centrifuge out the solid. Draw off the supernatant with a Pasteur pipet, which is known as "decanting," and save it in a test tube labeled "Step 9." Save the precipitate for step 4. The solution should contain Al(OH)₄^-, aluminum hydroxide ions, and Zn(OH)₄^2--, zinc hydroxide ions, still in solution. The excess NaOH is needed to form the hydroxide complex ions of Al and Zn and insure that they stay in solution. Possible solids are:

Fe³⁺ + 3OH^- --> Fe(OH)₃(s) red-brown solid

Mn(OH)₂(s) + H₂O₂ --> MnO₂(s) + 2H₂O dark brown

Cu²⁺ + 2OH^- --> Cu(OH)₂(s) blue (masked by dark precipitate)

Step 4-Separation of Manganese:

Wash the solid from step 3 with 10 drops of water and 10 drops of NaOH. Centrifuge and use a Pasteur pipet to discard the wash. The washing will insure that all of the aluminum and zinc ions have been removed from the precipitate. To the precipitate, which may contain Fe(OH)₃, MnO₂, and Cu(OH)₂, add 5 drops of water. Add 6 M sulfuric acid, H₂SO₄ (Caution!) until acidic when tested with litmus paper (about 5 drops). Stir the solution for a minute and centrifuge out any undissolved solid, which should be brown to black MnO₂. Centrifuge. Decant the liquid with a pipet into a test tube. The liquid may contain Fe³⁺ and Cu²⁺. Save the supernatant for "Step 6" and set aside for future tests. Save the precipitate. Wash the precipitate with 20 drops of 6 M H₂SO₄, centrifuge, and discard the wash.

Step 5-Confirmation of Manganese:

Add 20 drops of 6 M HNO₃ (Caution!) to the precipitate from step 4 and stir. Slowly dissolve small amounts of sodium bismuthate, NaBiO₃, until no more will dissolve. Make sure there is a small excess of solid on the bottom of the test tube. Centrifuge. The solution will be purple in the presence of MnO₄^-, which confirms the presence of manganese. Discard.

Step 6-Separation of Iron and Indication of Copper:

The solution from step 4, labeled "Step 6" should contain Fe³⁺ and Cu²⁺ ions. Add aqueous NH₃ until precipitation is complete. A brown precipitate should form of Fe(OH)₃. Centrifuge. Decant the supernatant, which should contain Cu²⁺. A slight blue tinge indicates that copper is present in solution. Save the liquid for later in a test tube labeled "Step 8". Save the precipitate for step 7.

Cu(OH)₂ + 4NH₃ --> Cu(NH₃)₄²⁺ + 2OH^-

Step 7-Confirmation of Iron:

Wash the precipitate from step 6 with 20 drops of water and 20 drops of aqueous NH₃. Discard the wash. Add 15 drops of H₂O and 15 drops of 6 M H₂SO₄ to dissolve the precipitate. Add a few drops of 0.5 M KSCN solution to the Fe³⁺ ion to produce the intensely red FeSCN²⁺ ion in solution. Discard.

Step 8-Confirmation of Copper:

To the solution from step 6 add 6 M HCl until the blue color fades and the solution is acidic when tested with litmus paper. Add 2 or 3 drops of 0.1 M potassium ferrocyanide, K₄Fe(CN)₆, which forms a red-brown to pink precipitate. You may need to centrifuge to see the color clearly. This is a sensitive test for copper, giving a red coloration even in extremely dilute solutions. Discard.

2Cu²⁺ + [Fe(CN)₆]^4- --> Cu₂[Fe(CN)₆](s) (reddish brown)

Part C: Separation of aluminum and zinc from solution.

Step 9-Preparation of Solution:

Return to the solution from step 3. Since copper ions will interfere with tests for aluminum and zinc, it is important to have removed them thoroughly. If there is a blue tinge to the supernatant, there are still copper ions in solution and you will need to do an additional step. If no color is present, use the liquid for step 10.

If a blue color is present, it is likely that the solution is deficient in hydroxide ions. Add 3 drops of 6 M NaOH. Boil the solution for 2 minutes or more, then centrifuge. A precipitate indicates that there were other ions present. Centrifuge. Decant. Save the supernatant for step 10. Discard the solid. If the solution is still not clear, repeat this procedure.

Step 10-Separation of Aluminum:

Once the solution containing aluminum and zinc ions is colorless, add about 10 drops of 6 M HNO₃ (Caution!) until acidic when tested with litmus. Place the solution in a boiling water bath for a minute or two. Add 6 M aqueous NH₃ (Caution!), drop by drop, until the solution is basic when tested with litmus. Add 3 more drops of 6 M aqueous NH₃. Stir the mixture for a minute. If aluminum is present, a light, translucent, gelatinous precipitate of Al(OH)₃ should be present. It may be hard to see, so look closely. Centrifuge and save the supernatant that contains zinc ions in a tube labeled "Step 12." Save the precipitate for step 11.

Step 11-Confirmation of Aluminum:

Dissolve the precipitate from step 10 with 6 M HNO₃. Add 5Êdrops of water and 2 drops of aluminon reagent and stir thoroughly. At this point, the solution may be colored because of the aluminon. Add 6 M aqueous NH₃ (Caution!) drop by drop, stirring well, until the solution is basic when tested with litmus. If Al³⁺ is present, the precipitate of Al(OH)₃ forms and adsorbs the aluminon from the solution producing a red precipitate of Al(OH)₃. This is called a "lake." The supernatant is essentially colorless. The test for aluminum is not the red solution but rather the red precipitate (Al(OH)₃ and adsorbed aluminon dye). Centrifuge to concentrate the precipitate and check to see if the color of the precipitate is red. Discard all.

Step 12-Confirmation of Zinc:

Make the solution from step 10 slightly acidic when tested with litmus with 6 M HCl (Caution!) added drop by drop. Add 2 or 3 drops of 0.1 M K₄Fe(CN)₆ and stir. If zinc is present, a white to greenish precipitate of K₂Zn₃[Fe(CN)₆]₂ forms. Centrifuge to make the precipitate more compact for examination. Discard all.

(Zinc may also be confirmed by adding a few drops of NaOH to the original sample until the solution tests basic with litmus and testing it with dithizone test paper. A red-violet color to the test paper is produced if Zn²⁺+ is present. NaOH gives an orange color to the test paper that must not be mistaken for the red-violet color with Zn²⁺. A known sample of Zn²⁺ should always be used side by side with the unknown when using the dithizone test paper. Discard the test paper.)

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1. Define the following terms:
   1. precipitate
   2. complex ion
   3. supernatant
   4. decant
2. Consult a solubility table. Suggest other ions which could be precipitated when chloride ion is added.
3. Write an outline scheme for the separation of Ag⁺, Cu²⁺, and Al³⁺ ions.
4. Write the balanced net ionic equations for the reactions that occur when
   1. solutions of AgNO₃ and KCl are mixed
   2. solutions of Fe(NO₃)₃ and KOH are mixed
   3. solutions of NaNO₃ and KCl are mixed
5. Give the chemical formula for each of the following
   1. a blue complex ion
   2. a colorless complex ion
   3. a purple complex ion
   4. a white precipitate
6. Why is H₂O₂ needed in the separation of manganese?

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Several of the chemicals used are toxic and corrosive. Do not ingest chemicals. Avoid contact with the chemicals. Wash spills immediately using large amounts of water. Wash hands before leaving the laboratory.

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1. 1. An insoluble (solid) that settles to the bottom of a solid/liquid mixture.
   2. An ion in which a central atom is surrounded by other atoms, molecules, or ions. The term is not universally applied, however. FeCl₄^- is described as a complex ion, but MnO₄^- is not.
   3. The supernatant is the liquid above a precipitate or that which was removed from above a precipitate.
   4. To decant is to pour of the supernatant from above a precipitate.
2. Lead(II) and mercury(II) can be separated as chlorides.
3. Add dilute HCl; decant. Add excess 6 M NH₃ to the supernatant; decant.
4. Ag⁺ + Cl^- --> AgCl

   Fe³⁺ + 3 OH^- --> Fe(OH)₃

   no reaction

5. blue -- Cu(NH₃)₄²⁺; colorless -- Zn(NH₃)₄²⁺;

   purple --MnO₄^-; white precipitate -- AgCl.

6. Hydrogen peroxide is added to separate the manganese as manganese dioxide.

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When the student is finished with the known solution, an unknown with 2 or 3 ions should be assigned. A few drops of distilled water may be added so samples appear to be of equal volume. It would be better not to give a sample containing the Cu²⁺ and the Zn²⁺ together because the copper ion imparts a slight pink tinge to the zinc if the separation is not complete. This may confuse the student.

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• The following solutions may be delivered in Beral pipet containers:
  • 0.1 M Fe(NO₃)₃•9H₂O (2 g / 50 mL)
  • 0.1 M AgNO₃ (0.85 g/ 50 mL)
  • 0.1 M Cu(NO₃)₂• 3H₂O (1.2 g / 50 mL)
  • 0.3 M Zn(NO₃)₂• 6H₂O (4.47 g / 50 mL)
  • 0.3 M Al(NO₃)₃ (5.7 g / 50 mL)
  • 0.1 M Mn(NO₃)₂ (0.8 mL of 50% solution diluted to make 50 mL)
  • 0.1 M NaCl (2.9 g / 50 mL)
• The following solutions may be delivered in bottles with dropping dispensers. Caution students about contamination of these bottles if they contain droppers (as opposed to dropping caps). The dropper should not touch any chemical or object other than the solution in the reservoir bottle.
  • 6 M NaOH (12 g / 50 mL solution
  • 6 M HCl (25 mL / 50 mL solution)
  • 6 M HNO₃ (20 mL / 50 mL solution)
  • 6 M H₂SO₄ (16.5 mL / 50 mL solution)
  • 6 M NH₃ (20 mL / 50 mL solution)
  • 6% H₂O₂ (beauty supply store; drug store)
  • Aluminon Reagent (0.05 g / 50 mL)
  • NaBiO₃ (solid)
  • K₄Fe(CN)₆•3H₂O (4.8 g / 50 mL solution)
  • 0.5M KSCN (2.5 g / 50 mL solution)
• dithizone paper

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The Woodrow Wilson microscale version of this lesson was developed by:

Carol Chen
3297 W. 250 South
Kokomo, IN. 46902

Richard Ingels
4134 Svedlund
Homer, Alaska 99603

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