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Chemistry LibreTexts

Traffic Light Reaction

  • Page ID
    131428
  • Required Training

    Required PPE

    UC Lab Safety Fundamentals

    Lab coat, safety glasses/goggles, nitrile gloves

    Performers Required: 1

    Equipment

    Chemicals

    500 mL Florence/Erlenmeyer flask (or larger) or 50 mL falcon tube

    Dextrose (D-glucose, C6H12O6)

    Rubber stopper to fit flask

    Potassium hydroxide (KOH)

    Indigo carmine (I.C.), 0.025g

    Lecture Demo Procedure: (Prep 5 minutes early due to waiting in step 2)

    1. Add 300 mL water, 8 g KOH, and 10 g dextrose to the flask, swirling the solution until everything has dissolved.
    2. Add the vial of I.C. indicator, swirling again to mix. The solution will initially turn green, but after a few moments it will become yellow. (5 minute wait)
    3. To perform the demo, stopper the flask and shake the solution vigorously, making sure to hold the stopper in place with one hand. The solution will turn red, and then green after sufficient shaking. After the solution has turned green, stop shaking and let the solution rest; it will transition back through red and become yellow again.
    4. This reaction can be repeated several times (~10) before the colors fade significantly. Adding a few more drops of the indicator will allow for an additional 5-10 cycles, and this can be repeated to get >50 cycles from the solution.

    Table Demo Procedure:

    1. Add 30 mL water, 0.8 g KOH, and 1 g dextrose to a 50 mL tube, swirling the solution until everything has dissolved.
    2. Add 2-3 drops of the I.C. indicator solution, swirling again to mix. The solution will initially turn green, but after a few moments it will become yellow.

    Clean-up: The waste solution should be neutralized, and then rinsed down the drain with copious amounts of water.

    Hazards: KOH is a strong base and its solutions are highly corrosive, causing immediate chemical burns on contact.

    Principle: This demonstration involves a reversible oxidation-reduction reaction between I.C., oxygen (O2), and a reducing sugar. I.C. is a both a pH and a redox indicator; the oxidized form is blue below pH 11.4, yellow above pH 13, and green in the intermediate range, while the reduced form is yellow at all pH levels. When the flask is shaken, atmospheric O2 is dissolved in the solution and oxidizes the I.C. to its green form. Dextrose is a reducing sugar, and in alkaline solution it is converted to an enolate which reduces the I.C. first to a red semiquinone intermediate and finally to the yellow reduced form; the dextrose is ultimately oxidized into arabinonic acid and formate anions. Shaking the flask again introduces more oxygen, which repeats the cycle until no dextrose remains.

    Notes:

    • It takes 45 secs to 1min & 15 secs to change from greeen to red on the first 5 minutes of mixing the solution.
    • This is essentially the same as the Blue Bottle demonstration, only a different indicator is used. In this case, the colors changes are strongly dependent on the pH of the system; above pH 13, the oscillation is between yellow, red, and yellow, whereas below pH 11.4 the color shifts through blue, purple, red, orange, and yellow, though the reaction rate is drastically reduced. The structures of the reduced forms of the I.C. indicator are not fully determined, and it is thought that the green color comes from a mixing of the blue and yellow forms. Unlike the Blue Bottle demonstration, the Traffic Light solution takes several days to become cloudy, such that it can be prepared significantly in advance; this may be preferable, as the color transitions become faster as the solution ages. Additionally, the colors become less intense with each successive transition, but the solution can be refreshed with a few additional drops of the I.C. indicator, allowing upwards of 50 transitions from a single preparation.
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