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

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Answer 1 · 2016-04-20T22:16:21

See the explanation below.

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

www.chemguide.co.uk

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.