I'm interested in a pH indicator that will strongly fluoresce under a typical black light that anyone could buy. I want to use an acid base reaction to get an idea of how fast two liquids mix in a new type of mixing device. Therefore the pH indicator has to have a very rapid reaction rate with either acid or base so that mixing is the rate limiting step.

I remember someone using coumaric acid decades ago for something similar. He showed me some beautiful photographs of how the fluorescence went from bright green to colorless when the pH went from basic to acidic. However, I don't remember the particular isotope and can no longer contact this person. In my planned tests, I was thinking of using a 20% excess of either acid or base (depending upon the chosen indicator) so that after the liquids mixed and reacted, the resulting solution would be either strongly basic or strongly acidic. Of course this depends on the indicator. But something like the coumaric acid idea would work well in my case where I want to take photographs. Ideally, I would like the chosen indicator to be environmentally safe.


  • $\begingroup$ google.com/… $\endgroup$
    – Ed V
    Apr 3, 2020 at 21:05
  • $\begingroup$ E.g., thermofisher.com/de/de/home/references/… mentions some of these as suitable for a determination in living cells. $\endgroup$
    – Buttonwood
    Apr 3, 2020 at 21:07
  • $\begingroup$ I've collected fluorescent rocks. Typical lighting washes out any fluorescence so you really need to look at UV fluorescence in the dark. Also glass doesn't transmit UV well. // How about adding a strong base, say NaOH, to an acidic solution with phenolphthalein? The indicator goes from colorless to pink. $\endgroup$
    – MaxW
    Apr 3, 2020 at 21:46
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    $\begingroup$ Going the other way, you could use quinine (from ordinary tonic water or more concentrated solution) as the fluorophore and quench the fluorescence with chloride ion (from ordinary salt water). For the light source, use a blacklight or 405 nm BluRay laser diode or UV LED, etc. $\endgroup$
    – Ed V
    Apr 3, 2020 at 23:27
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    $\begingroup$ @MathewMahindaratne Ah, your linked answer is your usual excellence! I have bookmarked it for the next time! As for the BODIPY, that link was what popped up in a quick search of “fluorescent pH indicators”. Personally, I would try the salt water and tonic water: safe, inexpensive, and no nasty dye mess if a leak happens. I had a CHEE colleague who studied fluid mixing, using ethylene glycol and rhodamine 6G laser dye. Cool under blacklight illumination, but it attracted lots of ants and dye ended up all over. $\endgroup$
    – Ed V
    Apr 4, 2020 at 13:07


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