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.