Advanced Chemistry: PART
uv and visible light absorption spectroscopy
- methylene blue dye
Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK IB
KS5 A/AS GCE advanced A level organic chemistry students US K12 grade 11 grade 12 organic chemistry
of analysis and molecular structure determination
SPECTROSCOPY INDEXES *
All Advanced Organic
Chemistry Notes *
The origin of colour, the wavelengths of
visible light, our perception!
theory, spectrometer, examples of absorption & reflectance spectra
spectra - index of examples: uses, applications, more on the
chemistry of colour
brown * Use
mobile phone/ipad etc. in 'landscape' mode *
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The uv and visible
absorption spectrum of some aromatic compounds
Methylene blue is a dark green powder that yields a blue
solution in water.
A solution of methylene blue (e.g. as chloride salt) shows
strong absorption in the green-yellow-orange-red region of the visible
blue colour results,
which would be expected from a complimentary colour argument (see below).
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
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
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 ∆Eelec 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 ∆Eelec required for excitation of the pi
electrons, well beyond the energy range of visible light
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!
Key words & phrases: interpreting the uv-visible absorption
spectrum of methylene blue, identifying the maximum absorption peaks in the uv-visible
absorption spectrum of methylene blue, explaining the uv-visible absorption spectrum of
methylene blue, how to use the visible absorption spectra of methylene
blue to explain the colour of
methylene blue, applications of the uv-visible absorption spectrum of
methylene blue describing and explaining the uses of methylene blue
molecular structure of the leucomethylene blue oxidised and reduced
forms of methylene blue microscope slide tissue staining agent clinical
use of methylene blue in methemoglobinemia
UV and visible spectroscopy index
Chemical properties of phenylamine, derivatives including diazonium
ions and azo dyes
All Advanced Organic
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