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Methyl orange indicator changes color to red and yellow with an acid and base respectively. I am unable to understand why this color change takes place.

Searched on Google about the same with litmus paper and some answers said that litmus paper contains an acid an a base with color red and blue respectively. But what about phenolphthalein it does not show any color change with acid.

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    $\begingroup$ They have different structures in different media, phenolphthalein exists in quinonoid form in basic media (which is pink), in phenolic form in acidic media, which is colorless. Methyl orange exists in quinonoid form in acidic media (red), whereas in basic media remains in original form (yellow). $\endgroup$
    – Zenix
    Commented Apr 12, 2020 at 9:53
  • $\begingroup$ Red mixed with yellow looks orange, while pink mixed with colorless looks pink-ish. $\endgroup$
    – Karsten ♦
    Commented Apr 12, 2020 at 11:59
  • $\begingroup$ see section on Methyl orange on the following webpage chemguide.co.uk/physical/acidbaseeqia/indicators.html $\endgroup$
    – MaxW
    Commented Apr 13, 2020 at 15:55

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Phenolphtalein is formed of three phenyl rings which are attached to the same Carbon atom. In the basic form, this central Carbon atom is part of a double bond joining it to one of these phenyl rings, which has a quinonic structure. The delocalization of the pi electrons is possible over all rings simultaneously. The rings are coplanar, and define is a huge domain. This is a huge "box" for the electrons, and the electronic transitions are low energy, that is in the visible.

In acidic solution, the central Carbon atom is related to four simple bonds. The phenyl rings are not coplanar. Their pi systems are not even delocalized over the whole molecule. The domain available to the pi electrons is limited to the phenyl rings, which are about three times smaller than in basic solutions. So the electronic transitions are high energy, in the UV region. And the phenolphtalein is colorless.

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  • $\begingroup$ That's basically correct, but at more extreme pH conditions phenolphthalein can undergo still more changes. Check it out. $\endgroup$
    – Oscar Lanzi
    Commented Apr 12, 2020 at 18:15

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