Fluorescein dye


Well-known member
In the hopes of complementing Stuart's helpful information above, here's a photo of -ve and +ve cotton wool detectors alongside each other, from an OBA water tracing test. I make them from poly bags with small holes cut in them (to allow water to circulate) and I like to secure the detectors to rocks using inner tube loops. I find these are as flood proof as most other arrangements. The reference numbers are on tabs of insulation tape.

These detectors are photographed under a mains UV lamp but it's possible to buy cheap portable UV torches on Amazon etc. I believe a useful wavelength for OBA detection is around 365 nanometres. An advantage of such a torch is it can be used directly in the field, perhaps removing the need for any detectors (depending on the nature of the test).

Hope this helps (somebody).

Stuart France

Active member
Your last photo in that sequence shows the cotton bud 'Fluorescein' glowing green under UV light. So while fluorescein doesn't fluoresce in the same way OB does, you're suggesting it's still detectable on cotton under UV light?

Not exactly. I dripped some very dilute fluorescein on to the green-looking cotton bud for the purpose of the photo showing the UV effect on it. It's beyond any doubt that OB adsorbs on to cotton, and once its stuck on to the surfaces of the cotton fibres it stays there, but I'm not sure that fluorescein does "stick" in the same way and will not get washed off the cotton by enough passage of running fresh water over it.

As to whether fluorescein shines bright under UV light, yes if does, very well indeed. It also depends on pH. In acid solution it is colourless. The bright green hue starts 'above' pH4 so that suits the alkaline water running off limestone areas. Chemistry people know that pH is a negative log scale for H+ concentration, so of course pH4 (say) is 'below' pH2 in terms of H+ or acidity, not 'above'. Other people, please ignore this.

Stuart France

Active member
As to making an el cheapo UV lamp for fieldwork, all you need is a UV LED and two AA batteries.

I bought a pack of 10 UV LEDs, can't remember where now, but this kind of thing:
That's a 10 pack for about £3 including post.

Mine seem tolerant of being connected directly to 2xAA batteries without any current limiting resistor because the forward voltage of the LED is about the same a couple of new AA alkaline batteries so the current is self-limiting. If it dies on you, then add a small current limiting resistor, e.g. 100 ohms in series with the LED and it will be fine.