Shaking Solution

Chemistry:	Redox Reactions
Grade Range:	Elementary School, Middle School, High School
Format:	Hands-on

This is a common demonstration for Reduction/Oxidation reactions. It is easy to prepare, easy to perform, and after preparation it can be used for several hours! Be careful with the indicators, since both of them will stain clothing.

Materials

• Sodium Hydroxide
• Dextrose (Sugar)
• Water
• 500 mL Erlenmeyer flask
• Stopper for flask
• Indicator: Methylene Blue or Resazurin

Safety Precautions

Please read the Chemical Safety section of the Demonstration Safety page before performing this demonstration.

During preparation, wear safety goggles and gloves. During the presentation, make sure whomever is shaking the solution keeps a hand on the stopper, to prevent it from coming out.

Demonstration

Preparation:

1. In the Erlenmeyer flask, combine 300 mL of water with 8g of Sodium Hydroxide and 10g of Dextrose. Add the solids individually, and allow the first to dissolve fully before adding the next one.
   • Alternatively, if there are prepared solutions for the NaOH and sugar, mix them in the Erlenmeyer flask as according to their labels.
2. If your indicator is not already in a water solution, then combine 0.5-1.0 g of the indicator with 50mL of water. Stir thoroughly, store in a bottle and label it. Add 8-10 drops of the indicator solution to the Erlenmeyer flask, and then stopper the flask.
3. The solution will initially be blue (Methylene blue) or purple/pink (Resazurin). After you let it sit, the solution will turn clear (Methylene) or light teal (Resazurin), and it is now ready to use.

Presentation:

1. Show the solution to the students, and swirl the bottle gently. A very slight color change will happen. Let it sit, and after a few seconds the color will disappear!
2. Ask a student to hold it and shake it. The solution will now turn colors dramatically! the more they shake it, the more pronounced the color will become, and the longer it will take for the color to dissipate.
3. Ask the students why it changes colors. What is happening when we shake it? Is there more than one thing going on here?

Why This Works

Short Explanation

This demonstration works because it is a Redox Reaction. Redox stands for Reduction/Oxidation, which are reactions that happen due to an exchange of electrons. Reduction is when a molecule gains electrons, and Oxidation is when a molecule loses electrons. A helpful way to remember this is with this phrase: "LEO says GER". The letters in LEO stand for Loss Equals Oxidation, and GER stands for Gain Equals Reduction.

Remember: There is air all around us, including inside of the flask, and part of that air is oxygen. So when the flask is shaken, some of that oxygen will dissolve into the solution. The oxygen will oxidize the indicator, and removes some of the electrons. This is what makes it change colors! Then, the glucose in the solution will reduce the indicator, and give it some electrons. This is what makes it turn back. Since there is so much glucose in the solution and very little indicator, this can happen a lot, and can be repeated for several hours!

Full Explanation

Reduction-Oxidation reactions, or Redox Reactions, are a type of reaction that relies on the transfer of electrons. Reduction is when electrons are gained, usually by the loss of an oxygen. Oxidation is when electrons are lost, usually by the gain of an oxygen. A helpful way to remember this is with the phrase "LEO says GER". The letters in LEO stand for Loss Equals Oxidation, and GER stands for Gain Equals Reduction.

In this experiment, we are seeing a color change when the oxidation of the indicator takes place. For this to happen, however, we need to first have our indicator be reduced. What is often missed during this demonstration is that, when we shake the solution, we are effectively reversing a step of the reaction that took place. This is why it will always go back to clear at the end, since the reaction will then go forward. There are four reaction steps to this demonstration, as listed here:

Shaking Solution Reaction Steps

1. Sugar Oxidation	C6H12O6(aq) + 2NaOH(aq) ⇒ C6H12O7(aq) + 2Na+(aq) + H2O(l) + 2e-
2. Gluconic Acid Ionization	C6H12O7(aq) ⇒ C6H11O7-(aq) + H+(aq)
3. Indicator Reduction (via Gluconic Acid)	Methylene Blue: C16H18N3S+(aq) (Blue) + H+(aq) + 2e- ⇒ C16H19N3S(aq) (Colorless) Resazurin: C12H7NO4-(aq) (Purple/Pink) + 2H+(aq) +e- ⇒ C12H7NO3(aq) (Teal) + H2O(l)
4. Indicator Oxidation (via dissolved O2)	Methylene Blue: 2C16H19N3S(aq) (Colorless) + O2-(aq) ⇒ 2C16H18N3S+(aq) (Blue) + H2O(l) Resazurin: C12H7NO3(aq) (Teal) + O2-(aq) ⇒ C12H7NO4-(aq) (Purple/Pink) + e-

• In step one, the initial reaction is between the sodium hydroxide and dextrose turns the dextrose into gluconic acid, and frees two electrons. This means it was oxidized, since it lost electrons, and the NaOH was reduced, since it gained electrons. The gluconic acid also happened to gain an oxygen, which is another indicator for oxidation. What is important to note for this step is that it is catalyzed by the indicator; that is to say, the indicator actually plays an active role in this reaction, which is unusual. This catalysis is explained in step three.
• In step two, we see the gluconic acid being ionized into gluconate and a free hydrogen. Gluconic acid is a weak acid, so although it does react much with the solution it is in, it will act as a buffer and ionize slowly. This produces a free hydrogen, and the gluconate ion will partner with the sodium ion from the first step.
• In step three, the hydrogen, combined with the two extra electrons, changes the color of the solution. The indicators we use is either Methylene Blue or Resazurin, both of which are redox indicators, meaning that they will have a color shift when a redox reaction happens. However, in this case the indicator is playing an active role in the reaction, and causes the sugar oxidation to be catalyzed. Both of the indicators will bond with a positive ion, so they will initially try to partner with the free Na⁺. This causes the pH of the solution to change, which means the gluconic acid buffer will ionize faster to prevent the indicator from bonding with the free Na⁺. This results in the indicator bonding instead with the H⁺ and removing electrons from the solution, which encourages the sugar oxidation to happen faster.
• Step four is after the initial reduction, and when the solutions are ready for presenting. When you shake the solution, oxygen will be dissolved into it. The dissolved Oxygen will react with the indicators, Oxidizing them and causing a color change. When this happens, the now-oxidized indicators will again try to bond with the Na⁺ ion. This triggers another gluconic acid buffer response, and will accelerate the ionization until the solution is once again reduced. As long as there is enough gluconic acid in solution, and so long as there are some extra electrons in solution, then you will be able to shake the solution and cause the color change!

Due to the amount of indicator being so low compared to the other reactants, This reaction is able to happen a lot, and for a long while. After a few hours, the solution will change color slightly, and then stop working. It stops working due to the lack of free electrons and H⁺ to make the reverse reaction happen. The color change happens because, after expending all of the extra Hydrogen and electrons, the indicators will bond with some of the other ions in solution, such as the gluconate and Na⁺, and become unable to do their proper color changes.

Additional Information

• Redox Reactions are all around us! Combustion is a redox reaction, and rust on metals is one as well!
• Methylene Blue is used to check the quality of milk produced. After added to some milk, the rate that it turns blue determines what rating the milk gets. A slow change is good, and a fast change is poor.
• This demonstration pairs well with the Potassium Permanganate And Glycerin and Elephant Toothpaste demonstrations.
• This demonstration is a part of the Chemistry Show!