I am planning to synthesize thymolphthalein, a pH indicator. I am wondering if phosphoric acid can be used to substitute sulfuric acid in the preparation. I know it can catalyze Friedel-Crafts reactions, which are a part of the synthesis. Any advice would be appreciated.

Thymolphthalein synthesis

  • $\begingroup$ It would be appreciated if you could attach an image of the normal synthesis involving sulphuric acid. Would help those who are not aware of the compound. Thank you :) $\endgroup$
    – TRC
    Jul 25, 2021 at 14:44
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    $\begingroup$ If you use polyphosphoric acid (PPA) I think it will work. PTSA will also probably work $\endgroup$
    – Waylander
    Jul 25, 2021 at 15:05
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    $\begingroup$ What is your reasoning for, what is your reasoning against the substitution of the strong Bronsted acid $\ce{H2SO4}$ by phosphoric acid (i.e., review of the literature)? By the way, the waxy polyphosphoric acid may be considered, too (an elder review, a current application). $\endgroup$
    – Buttonwood
    Jul 25, 2021 at 15:06
  • $\begingroup$ I do chemistry in a workspace with wooden benches, and sulfuric acid would eat through them quickly. Thanks for suggesting those, I can make or get both of those. $\endgroup$
    – user102173
    Jul 25, 2021 at 15:19
  • $\begingroup$ But the question is: would straight phsophoric acid work? $\endgroup$
    – user102173
    Jul 25, 2021 at 16:13

1 Answer 1


I used polyphosphoric acid and it worked.


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