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I am trying to find visual ways to demonstrate water contaminants. For example, chlorine reacts to N,N-diethyl-p-phenyldiamine (DPD), forming a magenta color. The chemical doesn't need to be accurate, as this is more of pass-fail test of the water. What reacts with fluoride to provide similar results?

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Fluoride has a high affinity for $Fe^{3+}$, and $Fe^{3+}$ forms deeply coloured complexes with phenols. The decomposition of the latter by fluoride ion is the basis for colorimetric determination of fluoride.

Ligands used in this kind of assay include 5-phenylsalicylaldehyde and resacetophenone. For occasional testing of well water I'd recommend any analytical lab. I'd be very surprised if Merck didn't offer any fluoride test strips, at their price of course, although whether they go down to the 5 ppm range is a different question. For those that really like to spend money, there's fluoride-sensitive electrodes.

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