Advanced Chemistry: PART
uv and visible light absorption spectroscopy - methyl red and methyl
orange indicator molecules
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 *
This is a BIG website, take time to explore it
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
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
In alkaline conditions e.g. pH 9, methyl red
indicator has a
435 nm and coloured yellow.
conditions e.g. pH 2, methyl red indicator has a
520 nm and coloured red.
Methyl red indicator is red coloured in pH
<4.4 solutions and yellow in solutions of pH >6.2.
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.
The uv-visible absorption spectrum of
methylene blue which is used as a redox
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
UV and visible spectroscopy index
Acid-alkali titrations and
The theory of acid-alkali titration
All Advanced Organic
Use My Google search site box
Website map buttons below
|Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted. Exam revision summaries & references to science course specifications