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I'm wondering about how does the litmus solution work as a pH indicator.

And another question:

When you put drops of litmus solution into an colorless acid (e.g. HCl), it turns red. But then if you add colorless base (e.g. NaOH) to the mixture, it turns blue. How come?

I thought that the litmus solution doesn't change color after it's changed color once already after a chemical change with an acid or a base. In this case, I thought that the mixture wouldn't turn blue after adding the NaOH into the mixture. But it seems that it still works well after used once.

How come?

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1 Answer 1

This explanation is true for most organic indicators. Litmus is a complex mixture of organic molecules. The molecule responsible for the color change (the chromophore), is 7-hydroxyphenoxazone (image taken from Wikipedia, public domain):

7-hydroxyphenoxazone

See that $OH$ group at the lower left? That's where the so-called "acidic proton" is. This molecule is actually a weak acid, meaning that in the presence of a strong base, the $OH$ group can lose a proton, which reacts with an hydroxide ion ($OH^-$) to form a water. The 7-hydroxyphenoxazone is now a negatively charged ion: it looks the same except it is missing the $H$ on the $OH$ group, and it leaves behind an electron.

To think about the equation that describes what's going on, pretend that the entire 7-hydroxyphenoxazone molecule is represented by the formula $HLit$, where $H$ is the hydrogen on the $OH$ group and $Lit$ is everything else (this is common procedure: only the hydrogen matters in the acid-base equation). Then the acid-base equation is:

$OH^-+HLit \leftrightarrow H_2O+Lit^-$

As more strong base is added, more hydroxide ion is present, driving the equilibrium to the right and producing more $Lit^-$ ion. And now, the color change: $HLit$ is red, but $Lit^-$ is bue. So the more acidic your solution, the more the equilibrium is to the left and thus there is more $HLit$ than $Lit^-$.

Litmus is pretty good for checking rough acidity or basicity, because its midpoint pH is close to 7 (neutral). Litmus is as red as it gets at a pH of 4.5, and is as blue as it gets at a pH of 8.3, so the midpoint is about 6.4, a bit more acidic than pure water.

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