Science Buddies: "Ask an Expert"

Hello,
I am trying to test effect of water-insoluble chemical (oxybenzone, a major chemical in sunscreens) on dinoflagellates.
I'm thinking of testing 1%, 0.1%, and 0.01% concentrations by differentiating the grams of the chemical added, but since they are insoluble and thus would not make solutions, I'm not sure if I can still label them by percentages.

Also, can insoluble chemicals alone, without other chemicals to help them dissolve, have an effect on the sea organisms like dinoflagellates when added into the seawater?

Hi,

This sounds like a very excellent science project.

You have a good question. Oxybenzone has a very low solubility in water, estimated at .69 mg/L Please note that the molecular structure of this molecule includes one OH (hydroxyl) group, so even though the rest of the molecule is extremely hydrophobic (carbons and hydrogens only), the polar hydroxyl group will allow a slight solubility in water.

https://pubchem.ncbi.nlm.nih.gov/compou ... Solubility

For your science project, I recommend making a saturated solution of the oxybenzone in water by adding an excess of the molecule to water and shaking or stirring it at the temperature you will be using for an hour or longer. Then remove that top layer of the water to eliminate any oxybenzone that has not dissolved. Next, dilute the saturated oxybenzone 1:2 in plan water and make another dilution of 1 part saturated plus3 parts water. This will allow you to test pure water, saturated oxybenzone, 50% and 25%. You could do additional dilutions if you want to.

I hope this helps. Please let me know if you have any other questions.

Donna Hardy

That sounds like a really good idea for a project! Some research suggests using zinc-based sunscreens as they are less likely to cause damage to coral reefs and their algal symbionts.

You might consider using zinc oxide as well, as a sort of experimental control. It can help show whether zinc sunscreens are safer than the oxybenzone-based sunscreens.

Keep in mind that the concentration of oxybenone, even in popular beaches, is likely to be low. So I suggest making the saturated solution as Donna suggests, and then testing a series of dilutions even down to 1% or lower.

Good luck!