In this experiment, the students will be required to develop a scientific method for testing The System to determine what it can do. The procedure is open-ended, allowing for a variety of results.
(Must be made on the day of the experiment.)
The 0.36M potassium hydroxide solution used is moderately corrosive. Avoid contact with the skin and eyes. Methylene blue is a moderately toxic biological stain that can turn the skin blue.
QUESTION ---> TEST ---> OBSERVE ---> HYPOTHESIZE ---> RETEST.You may have to modify your hypotheses and retest again, based upon how The System responds to each test. The tests that you decide to use should be able to clearly indicate whether your hypotheses are correct or not.
TEST OBSERVATIONS HYPOTHESES
Half-fill a large beaker with water. Pour the solution of The System into the beaker to dilute it. Check the pH of the solution and slowly add 1M HCl(aq) until it is neutralized. Pour the neutral mixture down the drain with an excess of water.
The System is a clear solution that changes blue in color when shaken, purple when exposed to bright sunlight and yellow when allowed to stand for a day. The duration of the color change is dependent upon both the length of shaking and temperature of the system. After a day or two, the yellow color persists and no change occurs by shaking. If the container is opened, allowing oxygen into the system, then it will be temporarily re-charged and form a green color upon being shaken.
This can be an entirely open-ended experiment or you can suggest that the students test the effects of shaking, temperature changes, or exposure to a strong light source. If the containers are securely sealed, the students can take The System home with them and observe the changes in behavior over a twenty-four hour period. Other variables in The System can be introduced by the instructor by
A possible explanation of the reactions involved is suggested below. The methylene blue is an organic dye that is reduced to a colorless compound by a basic solution or a reducing sugar. When the tube is shaken, the colorless solution is reoxidized by the oxygen above the liquid into the blue dye, methylene blue. When left standing, the dye is slowly reduced again, back into the colorless solution. After several hours, the oxygen above the liquid is depleted and the solution will no longer change color. The oxygen can be replenished by opening the container to the air for a few minutes, and the reaction will continue to work. After several days, the solution will turn brown in color and no further changes will occur.
In simple terms, the changes between the blue and colorless solutions are due to a redox cycle between the two forms of methylene blue:
Submitted by Mark Case. Experiment based upon a laboratory exercise presented by Dr. Stephen Thompson of Colorado State University while at the 1988 Woodrow Wilson Foundation Chemistry Institute held at Princeton University.
Fruton & Simmonds, General Biochemistry. Wiley & Sons, 1958, pp. 302-303.
Campbell, J. A., J. Chem. Ed., 40, 578, 1963.
Breyer, Arthur C., Oxidation and Reduction Experiments and Demonstrations, Beaver College, Glenside, PA 19038.
Liem, Tik L. Invitations to Science Inquiry, 2nd ed., Ginn Press, Lexington, MA 02173, 1987.
Structural formulas were obtained from the CRC Handbook of Chemistry and Physics.