pH indicator that will fluoresce under UV light

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.


Thanks.


