Why does methanol (an alcohol) make methyl orange (an indicator) turn yellow?

1 Answer
Apr 20, 2016

See the explanation below.

Explanation:

Methyl orange is an intensely-coloured indicator that is red below "pH 3.1" and orange-yellow above "pH 4.4".

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The red (acid) form has an "H"^+ attached to one of the "N" atoms.

The yellow (basic) form has lost the "H"^+.

The "pH" of pure water ("p"K_"a" = 15.7) is 7.00. That is in the yellow region of the indicator range.

Methanol ("p"K_"a" = 15.2) is an even weaker acid than water.

Methanol is even less able than water to protonate the "N" atom of methyl orange, so the indicator is yellow in methanol.