I came across the fact that alcohols cannot turn blue litmus red. The fact confuses me as they are acidic and acids do turn blue litmus solution red. What prevents alcohols from turning blue litmus solution red?
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asked Jan 21, 2015 at 18:41
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3$\begingroup$ I'm not sure but I think because of two reasons: 1- Aqua in aqueous solution (the solution I assume you mean) is amphoteric. 2- Alcohols aren't very strong acids. $\endgroup$– M.A.R.Commented Jan 21, 2015 at 18:54
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3 Answers
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Litmus is a weak acid, with a $\mathrm{p}K_\mathrm{a}$ of about 6.5.
Alcohols are extremely weak acids, with $\mathrm{p}K_\mathrm{a}$'s (typically) around 17.
The litmus itself is a much stronger acid than the alcohol. Dissociation of the alcohol won't produce enough protons to shift the equilibrium between the two colored forms of the indicator, so you'll see no color change.
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Alcohol in water solution is not acidic enough to change the solution pH. So, it doesn't turn blue litmus red.
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The pH of ethanol ($\ce{C2H5OH}$) or pure alcohol is $7.33$. This means that ethanol is slightly basic. And the colour of litmus paper change at $\mathrm{pH} =7$. Consider the ionization of indicator in solution represented by this equation:
$$\ce{HIn <--> H+ + In-}$$
And the acidic colour of $\ce{HIn}$ is red, the basic colour of $\ce{In-}$ is blue. According to Le Chatelier’s principle, if we put indicator in basic solution, $\ce{H+}$ reacts with base, more $\ce{HIn}$ ionized, more $\ce{In-}$ formed, so the colour of blue litmus unchanged.
answered Mar 21, 2017 at 13:18