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Sodium Metabisulfite an antioxidant?
I’ve seen sodium metabisulfite in at least one skincare formulation so far and I’m curious if it fulfils a role in protecting the formulation against oxidation. Does it offer anything that alpha-tocopherol does not?
edited April 2020
Correct, it is an antioxidant. It's great for removing air from the formulation, something tocopherol does not. Ascorbic acid does it too though not as efficient. Like ascorbic acid, metabisulfite is water soluble and also a good partner for tocopherol (both regenerate 'spent' tocopherol). It is a quite strong antioxidant and hence, does react with more ingredients (such as natural pigments) than other antioxidants but does not protect as long because it gets used up fairly quickly by the ever present oxygen if not used in an airless dispenser. It's also a possible allergen (if memory serves me right, ~1% of the population is allergic to sulfites in food, tendency decreasing due to less abundant use, though no precise numbers are known).
Member, Professional Chemist
What Pharma said holds true. (Thanks, Pharma) I've long thought sodium metabilsulfite an underrated ingredient in personal care, especially given its water-solubility and low cost. I've used it when formulating with highly oxidation-sensitive materials such as resveratrol, anthocyanins.
Hi guys, while two experts are here, may I ask you to advise some source (cosmetic formulation book, article etc) where I can find more on antioxidants? I find information here and there but I can’t find a comprehensive guidance on which one to use with which ingredients. I see BHT used for retinol, ferulic acid for LAA and sodium metabisulfite for hydroquinone but not being a chemist I don’t know whether there’s a logic behind it or it’s trial and error.
I haven't found a book on antioxidants which would even remotely satisfy me ;( .
However, there is logic to it (apart from a lot of trial and error as well): redox potential which can be listed in an electropotential series (see tables & figures in
topic-unrelated publication). What this means is, that in order to protect substance A from oxidation, substance B has to be a stronger antioxidant than A. In case of BHT/retinol: BHT is a highly sterically hindered phenol which results in a more stable radical whereas retinol can be regenerated but also tends to react further and thereby would degrade irreversibly. Although both are phenols with similar redox potentials, BHT is the fall guy due to stability. But this is just an exception to the rule of redox potential.
Also, it sometimes pays off to choose couples either for one or for two electron reduction/oxidation but this may be more for lab settings. After all, mother nature goes with redox potential and spatial separation (lipophilic and hydrophilic antioxidants and different organelles for different redox steps). The latter is not readily applicable in cosmetics and doesn't bring any benefits because a cosmetic product isn't alive and constantly regenerating.
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