December 2006

This article is the second of a two-part series on methylene blue. Methylene blue has important uses in both biology and chemistry. Last month's Chlorine Compound of the Month article addressed the use of methylene blue in biology.


The chemical structure of Methylene blue, C16H18N3ClS,
includes three joined rings of atoms. Each unlabeled corner
(vertex) of each hexagonal ring represents the location of a
carbon atom--can you locate all 16 carbon atoms in the structure?

Methylene blue, a common stain used by biologists to help them see bacteria and other forms of life under the microscope, is also a tool of the chemist. Through color, methylene blue indicates the presence or absence of oxygen. Oxygen is necessary for many forms of life on Earth, so any compound that can help detect its presence is bound to be useful.

Methylene Blue: Oxygen Indicator

Water containing the methylene blue indicator is blue when oxygen is present. A deep blue color can be achieved by capping a solution of water and methylene blue and shaking it vigorously to mix oxygen from the air into the water. If oxygen is removed from the solution, the blue color disappears.

Oxygen-rich environments are said to be "oxidizing." Oxidizing chemicals, such as oxygen and chlorine, strip electrons away from atoms of other elements. Chemical elements that have such a strong attraction for electrons that they can "rip" electrons away from other types of atoms are known as oxidizing agents, and are very useful in chemistry. Methylene blue indicates the presence of oxidizing agents because it is oxidized itself by these compounds. When electrons are stripped from methylene blue, the resulting molecule imparts a blue color to the solution--giving a clear sign of a chemical change.

Milk: Got Oxygen?

Foods contaminated with bacteria are a serious threat to public health. Raw milk is heated to destroy any bacteria that may be present in a process called pasteurization.

At one time methylene blue was added to pasteurized milk samples to be sure that milk was bacteria-free. If the blue color of the sampled milk disappeared over the test period, it was likely that bacteria were consuming oxygen, and the milk had not been pasteurized properly. If the milk sample remained blue in color over the test period, bacteria were assumed not to be present.

Oxidizing and Reducing Environments in Nature

Earth's oxygen-rich atmosphere is, not surprisingly, a naturally oxidizing environment, and so are the flowing and churning waters of our planet's oceans and rivers. Surface waters contain dissolved oxygen from the atmosphere (a fact that fish appreciate, as they obtain all the oxygen they need from water taken in through their gills). Scientists call oxygen-rich environments "aerobic."

Oxygen-poor waters, such as those found in bogs and swamps, are "reducing" environments, also known as "anaerobic." Anaerobic waters are rich in decaying organic matter such as dead plant leaves, stems and insects. Oxygen, which would normally help break down organic matter chemically, is missing from anaerobic environments.

Anaerobic swamp environments of the past are responsible for today's coal deposits. Coal forms over millions of years as once-living organic matter accumulates, decays and compresses into the hard black material that becomes a useful fuel.


Follow-up Questions:

  1. Learn more about pasteurization and describe the steps. For whom is this process named and why?

  2. Look it up: How is methylene blue helpful to people who keep tropical fish?

  3. Invent another use for methylene blue besides the ones described in this article series.

For a list of previous "Chlorine Compound of the Month" features, click here.


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