Water Quality Lab -- TOPS

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	.	.	.	.	.	.	.	.	.
   1B	.	.	.	.	.	.	.	.	.
   2A	.	.	.	.	.	.	.	.	.
   2B	.	.	.	.	.	.	.	.	.
   3A		.	.	.	.	.	.	.	.
   3B	.	.	.	.	.	.	.	.	.

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?

 

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