Stubborn Blue
Glucose is a chemical compound most widely distributed in the world of living organisms. It is estimated that plants produce about 100 billion tons of it per year through photosynthesis!
Glucose molecules are also part of numerous compounds, such as sucrose, starch, cellulose. Glucose in aqueous solution is in the ring form (two isomers differing in configuration) with a small admixture of the chain form. Both ring forms are transformed through a chain form - this phenomenon is called mutation (From Lat. Mutare = change).
In the state of equilibrium, the content of all forms of the glucose molecule is as follows (for clarity, the carbon atoms with the corresponding number of hydrogen atoms are omitted at the junctions of the bonds):
The low content of the chain form causes characteristic glucose reactions (after consumption, it is restored from ring forms), for example, Trommer and Tollens tests. But these are not the only colorful reactions involving this compound.
In experiment we will use glucose, sodium hydroxide, NaOH, and methylene blue dye (photo 1), used, among other things, as a preparation for the aquarium. Add some NaOH solution (photo 2) of the same concentration and a few drops of dye (photo 3). The contents of the flask turn blue (photo 4), but it quickly disappears (photos 5 and 6). After shaking, the solution turns blue again (photos 7 and 8), and then discoloration again after a while. The process can be repeated several times.
It happens during the experiment oxidation of glucose to gluconic acid (the aldehyde group of the chain form -CHO turns into a carboxyl group -COOH), more precisely, into the sodium salt of this acid, which is formed in a strongly alkaline reaction medium. Glucose oxidation is induced by methylene blue, the oxidized form of which is oxidized from the reduced form (leukoprinciples, gr. leukemia = white), differs in color:
The current process can be represented as follows:
glucose + oxidized dye ® gluconic acid + reduced dye
The above reaction is responsible for the disappearance of the blue color of the solution. After shaking the contents of the flask, water-soluble oxygen in the air oxidizes the reduced form of the dye, as a result of which the blue color reappears. The process is repeated until the glucose is depleted. Thus, methylene blue acts as a catalyst for the reaction.
Watch the experience in the video:
