Home Cosmetic Science Talk Formulating Advanced Questions pKa of combined acids

  • pKa of combined acids

    Posted by ChristopherA on March 24, 2024 at 9:13 pm

    Could I kindly request advice on how to determine the pKa of a combination of acids. For example, if Trichloroacetic acid (pKa 0.51) is combined with Lactic Acid (pKa 3.86), how can I determine the resulting pKa of the solution? Is this even possible? Many thanks!

    fareloz replied 1 day, 9 hours ago 3 Members · 6 Replies
  • 6 Replies
  • fareloz

    March 25, 2024 at 8:49 am

    Disclaimer: not a chemist here

    pKa of an acid is pH value when half of the acid is still an acid and half is “neutralized” to conjugated base. For example if we take lactic acid, it’s pKa is 3.86. So if we take pure lactic acid solution and adjust it’s pH with NaOH to value 3.86 that will mean we have half of original lactic acid and half converted to Sodium Lactate.

    Now when we talk about pKa of combined solution I don’t completely understand what this should mean and moreover what application it could have. Thinking logically we can extend pKa definition and say that pKa of combined acids solution is the pH value when half of combined solution is “neutralized”.

    The problem here is it doesn’t mean that each specific acid is also half-neutralized under this value. In case of a mix of very strong acid and some weak acid it means strong acid will be neutralized almost completely with part of the weak acid.

    Anyway, if we agree on definition above. I think pKa of combined solution depends on pKa of each acid adjusted by it’s concentration. I don’t think there is a trivial formula especially if the acids can interact affecting their individual pKa values.

    Still I’m wondering how you want to use this measure…

    • ChristopherA

      April 14, 2024 at 5:31 am

      Thank you so much for your detailed explanation and I do apologise for the delay in replying. What I am trying to determine is if/how the pKa can be determine when combining these acids. The specific reason is in determining the specification of chemical peeling solutions. For example, combining 10% Lactic Acid (pKa 3.86) and 10% Trichloroacetic acid (pKa 0.7) in an aqueous chemical peeling solution that is a free acid, with no neutralisation and no pH adjustment. Is there a way to determine the resulting final pKa from combining these two acids? From reading your reply, is the pKa now somewhere between 3.86 and 0.7?

      Sincere thanks for your consideration again. I hope the above provides more information.

      • fareloz

        April 15, 2024 at 4:53 am

        I still don’t see the actual application of the value you are trying to get. Maybe you just want a characteristic which is just meaningless and no need to bother with. What you are trying to achieve with this number, what are you going to use it for?

        • ChristopherA

          April 21, 2024 at 5:01 am

          The purpose is to characterise the solution, which is not entirely meaningless. Determining the specifics of chemical peeling reagents are important in my case, even if you feel otherwise. I simply asked if and how it is possible to determine.

          • fareloz

            April 21, 2024 at 7:18 am

            You still don’t provide the use except of this characteristic. Is it just for labeling?

  • PhilGeis

    April 15, 2024 at 5:42 am

    Formulation has been reported to impact “effective” pKa of benzoic acid*, a phenomenon I’ve seen as efficacy in ~neutral pH shampoos.


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