Successive Dilution of a Toxic Solution
from Green Chemistry Colloquy sponsored by the Burroughs Wellcome Fund

Purpose

  • to examine dilution as a method for disposal of toxic waste (copper sulfate)
  • to discover the relationship between absorption of light and concentration
  • to develop a sensitive test for the presence of copper ions
    •

Materials

Each group:

tray

dropper

stirring stick

paper

LabPro

calculator

colorimeter

pipette system

Each table:

copper sulfate solution

distilled water

ammonia

Procedure

In the five cups in the tray, mix 5 different concentrations of copper sulfate starting with the 50,000 ppm solution given. (Do not dilute below 5000 ppm.) Fill out the table below.

Cup #

Parts 50000 ppm CuSO4

Parts H20

CuSO4Concentration (ppm)

A

1

0

50,000

B

C

D

E

 

Part I - Determine the relationship between absorption and

concentration (Beer's Law)
  1. Connect the colorimeter to Channel 1 on the LabPro using an adapter cable.
  2. Plug the LabPro in using the AC adapter and turn on the calculator and the LabPro (listen for the beeps)
  3. Press APPS and choose CHEMBIO. Press ENTER to begin the program.
  4. SETUP PROBES - choose 1 probe, colorimeter in channel 1
  5. The calculator will ask for a first calibration point. Turn the dial on the colorimeter to 0%T. Press the + key when the voltage stabilizes. The reference point is 0.
  6. For the second calibration point turn the dial on the colorimeter to Red (635 nm). Place a clean empty cuvette in the colorimeter so that the light shines through the smooth sides. Make sure that the cuvette is clean (no fingerprints, lint etc.). Close the colorimeter and press the + key when the voltage stabilizes. The reference is 100.
  7. Press ENTER to return to the main menu.
  8. COLLECT DATA - choose TRIGGER/PROMPT. This will allow you to take a data point for each concentration of copper sulfate.
  9. Use a clean dropper or pipette to fill the cuvette 3/4 full with the solution in cup A. Be careful to keep the clear sides of the cuvette clean. Cap the cuvette and place it in the colorimeter so the light shines through the clear sides of the cuvette.
  10. Close the colorimeter and press + when the voltage stabilizes. Enter the concentration in ppm as the value.
  11. Clean and dry the cuvette.
  12. Choose MORE DATA on the calculator and repeat steps 9-11 for each solution in your tray.
  13. When you have finished choose STOP. Observe the graph. Describe the shape of the curve.
  14. Press ENTER and choose NO when asked to repeat.
  15. Return to the main menu and choose FIT CURVE. If your graph appears linear choose 1 (LINEAR L1, L2), if not choose 5 (POWER L1, L2). Record the equation and the R value.
    16.

Part I Questions

1. What is the relationship between concentration and absorption of red light?

2. What is the equation that fits your data? What do each of the quantities in the equation represent (i.e. C = concentration)?

3. Find the concentration of the mystery solution. Show your calculations here.

4. How much water would be needed to dilute 1 liter of the 50,000 ppm solution to 5 ppm?

Part II - Using Ammonia as a Test for Copper Ions

1. Make sure each cup in your tray is at least 1/3 full. Add 10 drops of ammonia to each cup. Record your observations in the table below.

Cup

Color before Ammonia

Color after Ammonia

Concentration (ppm)

A

B

C

D

E

 

2. Investigate the sensitivity of the ammonia test. Clean out your tray and mix 4 new concentrations of copper sulfate from 500 ppm to 100 ppm as described below

  • Add 5 drops of 50,000 ppm copper sulfate to 45 drops of water in cup A and stir well. This is a 5000 ppm solution.
  • Use the dropper to transfer 5 drops of solution from cup A to cup B. Add 45 drops of water. Stir well. This is a 500 ppm solution
  • Mix drops of 500 ppm solution from cup B and drops of water to prepare solutions according to the following chart

Cup

1

2

3

4

5

6

Concentration of copper sulfate (ppm)

500

400

300

200

100

0

Drops of 500 ppm copper sulfate (Cup B)

10

8

6

4

2

0

Drops of water

0

2

4

6

8

10

Record the color before and after adding 5 drops of ammonia to cups 1-6. Check your results with those of the groups around you.

Part II Questions

What is the lowest concentration of copper ions that can be detected by the ammonia test?

How do you explain any differences in your results from those of other groups?

Why was water used alone in cup 6?

 

 
 
     

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