Doc B's Advanced Chemistry: PART 15.5 uv and visible light absorption spectroscopy - methylene blue dye

SPECTROSCOPY INDEXES

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15.5.1 The origin of colour, the wavelengths of visible light, our perception!

The uses of methylene blue

Note that methylene blue is blue in an oxidising environment but can be reduced to a colourless form.

(1) A methylene blue injection is used to treat methemoglobinemia, a condition in which the blood loses its ability to efficiently carry oxygen through the bodies blood vessels.

It works by converting (reducing) the iron(III) ion (ferric, Fe³⁺) into the iron(II) ion (ferrous, Fe²⁺) in the hemoglobin (haemoglobin)  molecule.

This is the reduction of methemoglobin back to hemoglobin, the oxygen carrier, so methylene blue is a reducing agent and oxidised in the process, the Fe³⁺ ion is the oxidising agent.

 

(2) As a blue dye or stain

(i) Methylene blue is used as a staining agent in certain types of medical examination.

e.g. endoscopic polypectomy to highlight a polyp to be surgically removed from the gut system.

(ii) It is used in biological staining of slides of tissue prepared for examination with a microscope.

It helps to observe the state of DNA and RNA in cells.

(iii) Methylene blue is used as an indicator to determine whether eukaryotic cells such as yeast are alive or dead.

The methylene blue is reduced in viable cells, leaving them unstained because the reduced form of methylene blue is colourless.

However dead cells are unable to reduce the oxidized methylene blue and the cells are stained blue.

Methylene blue can interfere with the respiration of the yeast as it picks up hydrogen ions made during the process

 

(3) Methylene blue is used as a redox indicator in certain volumetric titrations in analytical chemistry.

Methylene blue turns colourless on exposure to a reducing agent.

It is colourless in a reducing environment and blue in an oxidising environment (see diagram below).

Image adapted from https://www.ld-didactic.de/documents/en-US/EXP/C/C2/C2222_e.pdf

Why the 'drastic' change in colour for the oxidation/reduction of methylene blue?

For a small change in molecular structure, there is a drastic change in colour.

The change is due to a change in the chromophore - a big change in the conjugated system.

In the oxidised form the conjugated system extends over three rings - two of which are aromatic (benzene) rings, so ∆E_elec required for excitation of the pi electrons is within the green-red energy range of visible light photons.

You can see a continuous alternation of single and double bonds. e.g. in the middle of the molecule you have a -C=C-N=C-C=C- bond network the connects the two benzene rings.

BUT, in the reduced form of methylene blue (leucomethylene blue), the two benzene ring are not conjugated together, giving two smaller conjugated systems, causing a significant increase in the ∆E_elec required for excitation of the pi electrons, well beyond the energy range of visible light photons.

Note on the left of the diagram above, one of the nitrogen atoms is protonated rather than the alternative structure shown on the absorption spectrum of methylene blue. I'm afraid you do find, on the internet, alternative structures to molecules, but if you look carefully, they are NOT structural isomers - it is all about where you put the double bonds!

UV and visible spectroscopy index

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Chemical properties of phenylamine, derivatives including diazonium ions and azo dyes

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