EXPERIMENT 6

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EXPERIMENT 6

• SPECTROPHOTOMETRIC ANALYSIS OF COBALT IONS IN
  SOLUTION

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PURPOSE

• To study the visible absorption spectrum of
  Co(NO3)2 . 6H2O, to test the validity of the
  Beer-Lambert Law for Co(II) and to determine the
  Co2 two solutions of unknown Co2
  concentration.

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Weigh the calculated quantity of Co(NO3)2 . 6H2O
to 0.001 g, into a 50 mL beaker.
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Add about 10 mL water and swirl the beaker with
your hand to dissolve the solid.
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Transfer the solution quantitatively to a 50 mL
volumetric flask.
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Rinse the beaker with water.
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And also the funnel.
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To obtain an accurate reading, you should be at
eye level with the meniscus (i.e. your line of
sight should be parallel with the mark).
Fill to the mark with water.
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Mix well.
This is the stock solution that will be used to
prepare the following standard solutions.
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Before proceeding, first rinse a 50.00 mL burette
using 5 mL or less of the stock solution. Make
sure all the inside walls, including the tip of
the burette, are rinsed!
Next, fill the burette with the stock Co2
solution.
Discard the rinse solution.
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Label a small beaker and rinse it with 5 mL of
this solution. Discard the rinse solution. Pour
the remaining solution into the beaker.
Prepare the first standard solution (0.0250 M) by
adding the previously calculated volume of the
stock solution to a 25.00 mL volumetric flask.
Fill the volumetric flask to the mark with water
and mix well.
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Prepare each of the remaining standard solutions
in the same manner, delivering the calculated
amount of the stock solution to the 25.00 mL
flask. Add water to make up the 25.00 mL volume,
and transfer each solution to a small beaker
previously rinsed with the respective solution .
Always rinse the volumetric flask with water
before preparing the next solution.
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Transfer your standard solutions to labeled test
tubes. Make sure you rinse each of them with the
corresponding solution before filling them .
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The absorbance of the A solution can be read
directly.
The D solution
has to be diluted.
You are going to have two solutions of unknown
cobalt concentration
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Rinse a 10 mL volumetric pipette with a small
amount of this solution.
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Rinse the inside walls with the D solution, and
discard it.
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Make sure to use your index finger to handle the
pipette.
Fill the pipette with the solution above the
pipettes mark.
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Dry the outside walls of the pipette with a
tissue.
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To obtain an accurate reading, you should be at
eye level with the meniscus (i.e. your line of
sight should be parallel with your reading).
Release the solution to the mark.
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Deliver the solution into a 25 mL volumetric
flask and dilute to the mark with water.
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The tip of the pipette will have a small amount
of the solution. DO NOT shake or blow the pipette
to force it out.
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Transfer your unknown solutions to labeled test
tubes. Make sure you rinse each of them with the
corresponding solution before filling them .
Take the rack containing the test tubes and the
cuvette to the spectrophotometer. DO NOT take
beakers or flasks!
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Handle the cuvette only by the top edge of the
ribbed sides.
Rinse the cuvette with distilled water.
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The liquid should be free of bubbles.
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Always position the cuvette the same way.
Insert the cuvette into the sample holder.
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• Rinse the cuvette with a small amount of the
  standard to be used. Discard the rinse solution
  in the waste container provided.

• Use the remaining solution to fill the cuvette
  about 80 full.

• The solution should be free of bubbles.

Starting with the lowest concentration
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• Follow the instructions given in your lab manual
  to record the absorbance data. Record it directly
  onto the Data and Observations sheet.

• Blot the outside of the cuvette then insert the
  sample into the sample holder.

• Remove the cuvette and pour the solution back
  into the original test tube.

• Repeat all steps until all your standards and the
  unknown solutions are done.

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Calibration Graph and Analysis

• Bring at least one sheet of millimeter-ruled
  graph paper for this experiment.
• Prepare the calibration graph by plotting the
  absorbance of the solutions versus the Co2 and
  draw the best straight line passing through the
  origin.
• Determine the concentrations and report the
  original concentration of the two unknown
  solutions.