Should Sodium metabisulfite be added last thing to prevent it from reacting with Oxygen in air?

Should you add it as one of the last things to prevent it from reacting with Oxygen in air and giving off smelly SO2,
especially if the manufacturing process needs vigorous mixing or emulsification that may draw extra air in?

Comments

  • Up! I also want to know the answer.

    @Gunther, I am using sodium metabisulfite for my hydroquinone cream. I wonder if it's useful for other products since I have more than I need for that formula.
  • PharmaPharma Member, Pharmacist
    Personally, I would add it late or together with/right before the compounds to protect such as hydroquinone.
    Given the rather widespread use of sulfite in foods, the comparatively high prevalence of sulfite hypersensitivity (SH) seems obvious (no concrete numbers available and probably SH was a hype like today's coeliac disease or lactose intolerance). Like with other similar preservatives (similar regarding frequent use = frequent allergies), it's now less often used (or has to be declared on the label if used >10 ppm) than back before ~1980. SH occurs mostly after ingestion of sulfite treated foods and drinks, is more prevalent in asthmatic people (up to 10%!), and the most common symptom is urticaria (skin rashes -> obviously a scare factor for cosmetics) whilst the most problematic reactions are respiratory symptoms. There are even a few cases of reported deaths as a consequence of ingestion of sulfite-treated food. Apart from that, sulfite antioxidants are highly efficient and versatile :) . One of the main advantages they have is colour preservation or discolouration prevention and another is that sulfites also act as preservative. However, they shouldn't be used as sole antioxidant (radical formation with air, something that I didn't know till very recently).
  • @Pharma what additional antioxidants do you suggest?
    Are BHT and propyl gallate fine when combined with Sodium metabisulfite?
  • PharmaPharma Member, Pharmacist
    @Gunther I honestly don't know. As a guess (knowing the numbers would be better, obviously), I think BHT should be fine. I know that, as an example, a combination with ascorbic acid is used to stabilise hydroquinone creams.
    I had a long day at work (11 hours, as long as usual, but felt like 20) and my brain is fried right now... so no deeper insights possible at the moment ;( .
  • MarkBroussardMarkBroussard Member, Professional Chemist
    @Gunther:

    Are you trying to use Na Metabisulfite because your formula contains dyes and you're trying to stabilize the color?
    Chemist/Microbiologist formulating in the Organic & Naturals arena under ECOCert/Natural Products Assn/Whole Foods/National Organic Program guidelines focused skincare & haircare products. 

    See website for details www.desertinbloomcosmeticslab.com
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