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Doc Brown's
Advanced Chemistry: PART
15.5
uv and visible light absorption spectroscopy - methyl red and methyl
orange indicator molecules
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Dr
Phil Brown PhD:
Doc Brown's advanced level organic chemistry exam revision notes
suitable for students of UK A level chemistry courses & US K12 grade 11-12 & AP honors chemistry courses:
The uv-visible absorption
spectroscopy of methyl red and methyl orange indicators
[uv-visible
spectra page updated
Mar 9th 2026 *]
SPECTROSCOPY INDEXES
All Advanced Organic
Chemistry Notes
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15.5.1
The origin of colour, the wavelengths of
visible light, our perception!
15.5.2
uv-visible spectroscopy
theory, spectrometer, examples of absorption & reflectance spectra
explained
15.5.3
uv-visible absorption
spectra - index of examples: uses, applications, more on the
chemistry of colour
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Acid-alkali titration indicators
Acid-alkali titration indicators work on the
principal that they change molecular form, hence change colour,
over a pH range that allows an accurate end-point to be
observed. Below are the two forms of two such
indicators, namely methyl orange and methyl red.
In both cases, the change is brought about by protonation of one
form of the indicator molecule.
The small change
in molecular structure is sufficient to cause a significant
colour change i.e. the different chromophores have a different
range of
∆Eelec electron
excitation energies, therefore different absorption pattern and
different colours observed.
I've noticed on the internet several different
structures for the protonated form of the indicator, but you
always get the same colour change!
With respect to the -N=N- azo linkage, the main difference
in structure is that methyl orange has a sulfonic acid group on the 4
position of a benzene ring and methyl red has a carboxylic acid group on a 2
position of a benzene ring.
In either case, for these two acid-alkali titration
indicators, on protonation, the small change in molecular structure, is
sufficient to change the range of
∆Eelec
excitation energies, resulting in a different visible light absorption
spectrum and observed colour.
The visible light absorption spectrum
for methyl red indicator is shown below.
Image adapted from
https://www.researchgate.net/figure/UV-visible-absorption-spectra-of-methyl-red-solution-measured-in-basic-pH9-and-acid_fig1_259757642
In alkaline conditions e.g. pH 9, methyl red
indicator has a
λmax of
435 nm and coloured yellow.
In acid
conditions e.g. pH 2, methyl red indicator has a
λmax of
520 nm and coloured red.
Methyl red indicator is red coloured in pH
<4.4 solutions and yellow in solutions of pH >6.2.
The
end-point, irrespective which is added to what in the
acid-alkali titration, the end-point is an intermediate orange colour
at ~pH 5, when
both forms exist in a ~1 : 1 ratio.
See also
The uv-visible absorption spectrum of
methylene blue which is used as a redox
titration indicator.
and
theory of indicators
used for acid-alkali titrations, calculations and pH curves
Key words & phrases: interpreting the uv-visible absorption spectra of
methyl red indicator,
identifying the maximum absorption peaks in the uv-visible absorption spectra of
methyl red indicator, explaining the uv-visible absorption spectra of methyl red
indicator, how to use the visible
absorption spectra of methyl red indicator to explain the different colours of
methyl red indicator, applications of the uv-visible absorption spectra of
methyl red indicator uv-visible light absorption spectrum of methyl orange
indicator, identifying the maximum absorption peaks in the
uv-visible absorption spectrum of methyl orange indicator, explaining the uv-visible absorption
spectrum of methyl orange indicator, how to use the visible absorption spectra of
methyl orange indicator to explain the
colour of methyl orange indicator, applications of the uv-visible absorption
spectrum of methyl orange indicator
Associated links
UV and visible spectroscopy index
SPECTROSCOPY
INDEXES
Acid-alkali titrations and
indicators
The theory of acid-alkali titration
indicators
All Advanced Organic
Chemistry Notes
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