As part of my IB Chemistry curriculum, we are required to conduct our own experiment researching our own question. I'm in the preliminary research phase and wanted to conduct an experiment looking at the effectiveness of zinc oxide (the active ingredient in many physical sunscreens) in absorbing UV light at different pHs ranging from 4 to 7.

I've found some procedures similar to the experiment I want to conduct, but all of them seem to dissolve a commercial sunscreen in ethanol and run it through a spectrophotometer. I want to isolate the active ingredient itself, and as such, I wanted to make a solution of zinc oxide -- though the molecule is not soluble in water, might there be another way to make a solution (perhaps dissolving in 2-propanol?). Then, I would ideally change the pH of the solutions using a buffer.

Does this seem viable, or is it likely I will need to purchase a zinc oxide suspension or perhaps look at a different chemical? Thank you for your help.

  • $\begingroup$ Pure water won't dissolve $\ce{ZnO2}$, nor the typical organic solvents used in the lab (EtOH, EtOAc, ether, etc.). Similar with $\ce{TiO2}$ in toothpaste, $\ce{ZnO2}$ in sun creme will be provided as a suspension of small particles in an emulsion. Thus searching for a dispersion at a chemical supplier (e.g., sigmaaldrich.com/catalog/…) may be a way to go. Seek for similar experiments, e.g. in J. Chem. Educ., too. $\endgroup$ – Buttonwood Oct 8 at 12:59
  • $\begingroup$ Note it is ZnO, not ZnO2, use of the later in sunscreens would not be a very good idea, see en.wikipedia.org/wiki/Zinc_peroxide $\endgroup$ – Ian Bush Oct 8 at 13:05
  • $\begingroup$ en.wikipedia.org/wiki/Zinc_oxide#Laboratory_synthesis $\endgroup$ – Nilay Ghosh Oct 8 at 13:12
  • $\begingroup$ @IanBush Your interjection is correct, I meant zinc oxide. The suggest to look into the 34 articles in Journal of Chemical Education with both concepts of «ZnO» and «UV» present (pubs.acs.org/action/…) shouldn't invalidate this. $\endgroup$ – Buttonwood Oct 8 at 13:19
  • $\begingroup$ @Poutnik Your are correct. $\endgroup$ – Buttonwood Oct 8 at 13:20

Zinc oxide is not soluble. It does dissolve at very acidic or alkaline pH, but it is a chemical reaction, not true dissolution.

The role of zinc oxide ( or titanium dioxide ) is mainly to cause UV light dispersion, effectively prolonging a lot the ray paths to allow sufficient absorption. Dissolving the sunscreen formula would create a thin suspension with very different UV properties, which would have tendency for sedimentation.

I see a sense rather in measurement of passing UV through a thin film of a cream with zinc oxide, prepared by some controlled way, e.g. controlling its mass per area.

You have to do some sunscreen solubility tests first.

The used formula would be an emulsion either "oil in water", either "water in oil". The solvent should dissolve well both. I would perhaps dissolve sunscreen in acetone, performing filtration/centrifugation or sedimentation, whatever applicable, to get rid of eventual $\ce{ZnO}$ or $\ce{TiO2}$ particles.

Then I would evaporate the aceton and add defined $\ce{ZnO}$ amount.

Or, you can try your own sunscreen formula.

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  • $\begingroup$ Thanks for your help! Since my experiment is based around sunscreen, do you think simply dissolving (or maybe a better word is just mixing?) sunscreen with an active ingredient of zinc oxide into EtOH would work, or is it better to avoid a commercial sunscreen in a chemical investigation such as this? Otherwise, I probably will look at a zinc oxide dispersion online. $\endgroup$ – ibchemstudent Oct 8 at 13:34
  • $\begingroup$ @ibchemstudent See the A update. $\endgroup$ – Poutnik Oct 8 at 13:43

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