Low pH enough to inhibit mold growth?

ZinkZink Member
in Formulating
Wondering if you could increase the glycolic acid content of a formula to inhibit mold growth, also can you go below pH 3 if the strength of the acid is still low? E.g. low concentration, but with no buffers used.

Comments

  • ZinkZink Member
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  • PerryPerry Administrator, Professional Chemist
    Mold growth can happen at pH as low as 3.5 so if you go lower, that should inhibit mold growth.

    However, the pKa of Glycolic acid is 3.8 so I don't think you will be able to get a pH much lower than that without some buffers. I'm not positive though as my knowledge of acid/base chemistry is a bit rusty.
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  • PharmaPharma Member, Pharmacist
    A 1 M solution of pure glycolic acid has, due to the lower pKa, a pH of (estimating/guessing here) less than 2 because acetic acid has about 2.5.
    pKa only matters if there is a buffer present or a salt used but not in case of a pure acid. Given that you intend using diluted glycolic acid, the pH might slightly differ. You easily get down to a pH where nothing (at least nothing you commonly encounter in human inhabitable surroundings) can grow but then again, your skin might not be very happy... unless you use a strong enough dilution to get something which can be 'buffered away' by whatever is on your skin (many constituents of NMF buffer nicely).
  • PerryPerry Administrator, Professional Chemist
    @Pharma - yeah, I couldn't find a pH for "pure glycolic acid" in any of my sources.  hmmm.

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  • ZinkZink Member
    In my experience as long as you use a low percentage of acid, low pH is not a problem, like Pharma says, it's buffered away by the skin.

    Even though the acid is a solution easily loses protons, there isn't that many of them.
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