(Reference:  CH112 General Chemistry II Laboratory Manual, Department of Chemistry , Michigan Technological University, Wiley, 1998)
 

PREPARATION
 

  • 50 mL      0.1 M NaCl
  • 150 mL    Standard
  •                 · 50 mL 0.1 M ZnNO3
  •                 · 50 mL 0.1 M CuNO3
  •                 · 50 mL 0.1 M MnNO3
  • 150 mL    Unknown (same as standard)
  • 100 mL    6 M HCl (at a reagent bench or about 20 mL in dropper bottles for each group)
  • 25 mL      0.1 M K4Fe(CN)6 (at a reagent bench or about 5 mL in dropper bottles for each group)
  • 10 g         NaBiO3 (at a reagent bench)
  • 100 mL    6M H2SO4 (at a reagent bench or about 20 mL in dropper bottles for each group)
  • 100 mL    6 M NH3 (at a reagent bench or about 20 mL in dropper bottles for each group)
  • 100 mL    6 M HNO3 (at a reagent bench or about 20 mL in dropper bottles for each group)
  • 100 mL    6 % H2O2 (at a reagent bench or about 20 mL in dropper bottles for each group)
  • 100 mL    6 M NaOH (at a reagent bench or about 20 mL in dropper bottles for each group)
  • Hot plates for boiling water bath; a 250 mL beaker and a test tube holder.
  • 10 Test tubes for each group
  • Red litmus paper
  • Lots of disposable transfer pipets (unused)
  • Filled distilled water bottles for each group
  • BACKGROUND

    In this experiment you will systematically separate cations from a mixture of substances.  There are several types of reactions you will use to allow you to separate these cations: precipitation, acids and bases, oxidation and reduction.  You will compare a standard solution to a waste-water solution to determine which cations are present

     PROCEDURE
    · Standard and unknown are located at your bench.  Transfer 22 drops of each into separate test tubes- LABEL each tube!
    · All of the reagents you will need are at the bench also.  Make absolutely sure not to mix any of the transfer pipets with multiple solutions!
    · Remember, you must do each test to both the standard and the sample!

    A.  TEST FOR SILVER ION

    Ag+  +   Cl  AgCl (s)

    1. To each solution add 10 drops of 0.1 M sodium chloride (NaCl) solution.  Mix well!  If silver ions are present you should see a white cloudy formation.  Centrifuge each tube, standard and unknown, for 1 minute or until the supernatant is clear.
    2. Carefully draw off the supernatant with your transfer pipet and put into the next test tube and label it C.  Save the precipitate for step B.

    B.  CONFORMATION OF SILVER ION

    AgCl (s)    +   2 NH3 (aq)   Ag(NH3)2+   +   Cl-

    1. Add 10 drops of distilled water to precipitate, mix well.
    2. Centrifuge, then draw off supernatant, discard this supernatant.
    3. To confirm that the precipitate is silver chloride (AgCl) and 6M ammonia (NH3) and mix until solid is dissolved
    4. Now add 6M nitric acid (HNO3) until precipitate forms again.

    We know this is a SILVER ion because it is the only chloride which is not soluable in water, but is in ammonia.

    C.  PRECIPITATION OF C

    1. To the tube labeled C add 5 drops of 6% hydrogen peroxide (H2O2)
    2. While stirring add 6M sodium hydroxide (NaOH) until test to make sure solution is basic

    To test basicity, place one drop of solution on a piece of red litmus paper.  If it is basic the red litmus will turn blue.

    3. Add one more drop of NaOH
    4. Stir solution, then place in boiling water bath.
    5. A dark precipitate will form

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

    6. Boil the solution for at least 2 minutes, then centrifuge
    7. Draw off the supernatant with your transfer pipet and save it in a tube labeled E

    D.  CONFIRMATION OF MANGANESE

    1. To the precipitate from step C, add 3 drops of water and 3 drops of 6M sulfuric acid (H2SO4) , then stir
    2. Add 1 drop of H2O2 and boil for 2 minutes, or until solid dissolves
    3. Add 5 drops HNO3 and stir
    4. Add small amounts of sodium bismuthate (NaBiO3), a few granules at a time, until no more dissolves.  A few granules should remain in the bottom of the test tube.
    5. Centrifuge

    If solution is purple then the (MnO4-) is  present, and this confirms the presence of manganese.
    14H+  + 2Mn2+  +  5BiO3   2 MnO4-  +  5Bi3+  +  7H2O

    E.  CONFIRMATION OF ZINC

    1. To tube labeled E add 6M HCl until acidic when tested adding ONE DROP AT A TIME and testing.
    2. Add 2 drops 0.1 M K4Fe(CN)6 and stir.

    If solution is light green then Zinc is present.

    Centrifuge to see color more clearly.

    3Zn2+  +  2K 2Fe(CN)64-   K2Zn3[Fe(CN)6]2(s)

    DATA
    Record information for each step, for both the standard and the sample
      
    Step Observations: Standard Observations: Sample Interpretations
    1 0 0 0
    2 0 0 0
    3 0 0 0
    4 0 0 0
    5 0 0 0
    QUESTIONS

    Which ions did your unknown sample contain?
     
     
     
     
     
     

    What are the definitions of a supernatant and a precipitate?
     
     
     
     
     
     

    Why did you use RED litmus to test if the solution in step C was basic?
     
     
     
     
     
     

    DISPOSAL

     · Unused portions of acids and bases can be saved for other labs and demos
     · Hydrogen peroxide solution should be allowed to decompose, in a hood, and made fresh each year
     · The unused unknowns and standards can also be saved
     · Student solutions can all go down the drain with lots of water
     
     

    Ginger Chateauneuf, 2000.
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