COLORED BOTTLES

Materials:

three 1 liter florence flasks with rubber stoppers
solid KOH
solid glucose.

This should be prepared only 10-15 minutes before use.

KOH: Prepare KOH by dissolving 50 g into 2 liters of water or 25 g into 1 liter of water. One liter of KOH is enough for all three bottles or for three trials.

You may try just one or the three bottles.

BLUE BOTTLE:
Dissolve 3 g of glucose in 300 mL KOH. Then add 5-10? drops of methylene blue indicator (enough to get a nice dark blue color). Allow it to stand for about 3-5 minutes until it becomes colorless.

RED BOTTLE:
Dissolve 10 g of glucose in 300 mL KOH. Then add dropwise 5-10? drops of resazurin indicator until the solution appears reddish blue, which turns to a fluorescent red in less than 2 minutes, and to colorless in another minute or so.

TWO COLOR BOTTLE:
Dissolve 10 g glucose in 300 mL KOH. Then add a spatula tip of indophenol as the sodium salt, until the solution is dark blue. Allow it to stand until colorless and then add 5-10? drops of resazurin until it is bright red. Again let stand until colorless.

DEMONSTRATION:
To carry out the demonstration, put a rubber stopper on the flask and clamp your hand over it. Give each bottle one vigorous shake. Show the color to the audience and let it stand until colorless. For the two color bottle, the first shake should give red and several more vigorous shakes should give a blue color. The color will fade to red and then colorless. This demo may be repeated for 1-2 hours before all the chemicals are used up.

INTRODUCTION:
Suggested Commentary: Everyone else has to mix chemicals or pour a chemical into another chemical in order to get a color change. I am not going to add any new chemicals to this flask. All that I am going to do is shake the bottle one time and produce a color. You see that it is now colorless. Here goes... What do you know just by shaking this bottle it turned blue! Can you beat that?
Repeat with the red bottle. (Then start to look more closely as the color fads. Express some disappointment, then shake them again.
Now work with the two color bottle. Shake gently first, show the color and then shake it more vigorously. Watch it as it fads to red and then colorless.

DISCUSSION:

Even though it didn't look like I added anything to cause this reaction, when I shook the bottle I actually did. Can anyone guess what it was?? (Yes, Oxygen or Air).

This experiment is based on the fact that the indicators are oxidation-reduction rather than acid-base as we have been mostly using. The indicator has a color in the oxidized state and colorless in the reduced state. When the flask is shaken, the oxygen in the air above the solution is dissolved and reacts with the indicator. As the solution is allowed to stand, the indicator is gradually reduced by the glucose in the basic solution to the colorless state. Reshaking the bottle repeats the reactions. These may be repeated many times over a period of hours until the glucose is used up.

Reactions:

oxygen gas + shaking ===> oxygen dissolved in solution
oxygen + methylene blue (colorless) ===> Blue color
glucose + KOH ===> glucoside ion
glucoside ion + methylene blue (blue color) ===> colorless

The other indicators react in a similar fashion.

Source: Chen, Philip S,Entertaining and Educational Chemical Demonstrations, Chemical Elements Publishing Co, 1974, p. 37-40.

Other examples based upon the same principles can be found in:

Vandaveer, Walter R. IV; Mosher, Mel. The Blue Bottle Revisited J.
Chem. Educ. 1997 74 402.