Water Quality Lab -- TOPS


MATERIALS

clock (that indicates seconds)

600 mL beaker

3 100 mL beakers

six test tubes

test tube rack

thermometer

top loader balance (0.01 g)

10 mL pipettes

10 mL pipette pumps

hot plate

stirring rod

yeast

25 mL nonfat milk (per group)

0.1% methylene blue solution or bromthymol blue

25 mL graduated cylinder

Permanent Marking Pen

PROCEDURE

  1. Obtain a 600 mL beaker and a thermometer. Fill the beaker half full of tap water and place the thermometer in it. Record the initial water temperature on the answer sheet (Question #1).. 
  2. Now place your beaker of water used in the previous step on the hot plate and gently warm it to 40ūC. It doesn't have to be exactly 40ūC. Caution: The surface of the hot plate will be much warmer than 40ūC..
  3. While the water is heating, obtain six test tubes, a test tube rack, three 100 mL beakers, and a 10 mL pipette & green pipette pump. Label the beakers: yeast, 100% milk, and 25% milk. Label the test tubes 1A, 1B, 2A, 2B, 3A, 3B with the Sharpie. (Note: Do NOT use a grease pencil.).
  4. Place 1.0 g of yeast in the beaker labeled yeast and add 20 mL of tap water using the 25 mL graduated cylinder. Gently stir the mixture with a stirring rod, then rinse the stirring rod..
  5. Pour 20 mL of nonfat milk into the beaker labeled 100% milk, Caution: When transferring a small quantity of a solution from one small container to another, both containers should be held. If not held, a slight jar could cause one of the containers to spill. .
  6. Pipette 5 mL of nonfat milk from the beaker in Step 5 into the beaker labeled 25% milk. Rinse the pipette. Using the graduated cylinder, add 15 mL of tap water. Stir with a glass rod. Rinse the glass rod after stirring..
  7. Prepare the test tubes as indicated in the chart on the next page, using a 10 mL pipette to measure the liquids. Record their color.
     

    TABLE I. OXYGEN LOSS DATA TABLE

    Test
    Tube
    H2O
    100% milk
    25%
    milk
    Methylene blue
    Initial Color
    Time Yeast is Added
    New Color
    Time of Color Change
    Seconds taken
    1A

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    1B

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    2A

    .

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    .

    .

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    .

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    2B

    .

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    .

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    .

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    3A

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    3B

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    .

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    .

    .

  8. Place tubes #1A, #2A, and #3A in the 40ūC water bath. Leave tubes #lB, #2B, and #3B in the rack..
  9. Using clean equipment, pipette 2 mL of yeast solution into each of the tubes and stir each one. Record the time, to the second, that you add the yeast to each tube in the chart above. Warning: Do not shake the test tubes once you have mixed in the yeast. (Why wouldn't you want to shake the test tube?).
  10. Record the time it takes for each tube to turn from blue to white. A blue ring may remain at the top of the solution or a very light greenish-tan color may remain in the solution..
  11. Tubes 1A and 1B should not change color. They are the control or comparison. What might cause a color change in the controls?