Advanced Level Organic Chemistry: UV and visible light absorption spectroscopy - methyl red/orange

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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

Doc Brown's 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 courses Spectroscopic methods of analysis and molecular structure determination

SPECTROSCOPY INDEXES  *  All Advanced Organic Chemistry Notes  *  [WEBSITE SEARCH BOX]

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.

molecular structure of methyl orange indicator in acid or alkali

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.

molecular structure of methyl red indicator in acid or alkali

 

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.

uv-visible light absorption spectra of methyl red indicator in acid or alkali wavelengths of maximum absorbance 

Image adapted from https://www.researchgate.net/figure/UV-visible-absorption-spectra-of-methyl-red-solution-measured-in-basic-pH9-and-acid_fig1_259757642

diagram image of the visualisation of the wavelengths of visible light in nanometres nm

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



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UV and visible spectroscopy index

SPECTROSCOPY INDEXES

Acid-alkali titrations and indicators

The theory of acid-alkali titration indicators

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