Why do our cosmetics change over time?
Sometimes it’s because of oxidation, the loss of electrons. Ascorbic acid is easily oxidized, losing two electrons, becoming dehydroascorbic acid. Enzymes and antioxidants in our body can give dehydroascorbic acid back those two electrons, reducing it back into ascorbic acid.
Your foundation getting darker after it dries isn’t oxidation, even though it’s often called that. It’s just the water or solvent in the foundation evaporating. If you’ve ever painted, you’ll know that wet paints tend to dry darker.
Methylene blue is a deeply blue organic dye. It is can be used in analytical chemistry as a redox (reduction-oxidation) indicator.
In oxidizing environments, methylene blue is a bright blue, in reducing environments the methylene blue accepts electrons and becomes leukomethylene blue which is colourless.
In the vial is an alkaline solution of glucose. Glucose is a reducing sugar, which means it can donate electrons. This creates a reducing environment for methylene blue, so the methylene blue is reduced to leukomethylene blue.
When the vial is shaken, oxygen is dissolved into the solution – this oxidizes the leukomethylene blue causing it to lose electrons, turning it back into bright blue methylene blue. In turn, the oxygen is reduced to water.
As the oxygen is consumed by the reaction, the glucose reduces the methylene blue – turning it back into the clear and colourless leukomethylene blue. In turn, the glucose is oxidized to gluconic acid.
In the video, you’re seeing excess oxygen dissipating out of the solution, as well as oxygen being consumed by the reaction.
This experiment is repeatable by shaking in more oxygen, but won’t go on forever – eventually, all of the glucose will have been oxidized to gluconic acid and the glucose is needed to reduce the methylene blue.
Oxidation was first observed with oxygen, hence its name. However, the modern definition of oxidation is the loss of electrons. Anything that can gain electrons, causing something else to lose electrons, is an oxidizing agent.
Some common oxidizing agents that aren’t oxygen are elemental halogens like fluorine, chlorine, bromine, and iodine. Fluorine (F2) is such a strong oxidizer it can oxidize water into oxygen!
2F2 + 2H2O → 4HF + O2