I know that the universal indicator is a solution of various pH indicators used to find out the pH of a substance by displaying different colors.

But, what is the pH of the universal indicator itself?

I found this line on the Wikipedia page for pH indicator:

In and of themselves, pH indicators are frequently weak acids or weak bases.

A quick google search indicates that a universal indicator is usually composed of thymol blue, methyl red, bromothymol blue and phenolphthalein.

How can this information be used to infer the actual pH of the universal indicator itself?

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  • 1
    $\begingroup$ I think you should read about pKa... $\endgroup$
    – Mithoron
    22 hours ago

Interesting question. You successfully found the components but forgot the solvent(s)? Do you remember that pH is defined for aqueous solutions only. All these indicator molecules are large relatively hydrophobic organic molecules so they are not water soluble. Thymol blue and phenolphthalein are such examples. One needs to add alcohol to dissolve these indicator dyes. Now, that you have alcohol + water mixture and dyes, the pH concept becomes murky. In short, for the universal pH indicator solution, we cannot state its pH.

Yes, pH concept can be extended to organic solvents and their aqueous mixtures but the question of measuring it is a headache!


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