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I started to make soap a few months ago and came up with a few formulas, one of it contains salicylic acid as a exfoliant/anti-acne agent, I've just realized that since salicylic acid is, well, an acid, and sodium hydroxide a base they will react together to form sodium salycilate, which doesn't have the required properties.

I add the salicylic acid and the other agents at (really) thick trace, when the saponification reaction is quite advanced but not complete, actually I couldn't find any data on how far the reaction has completed at this stage (if anyone knows this is always good to know).

Is there any way (besides waiting for reaction completion) to keep the NaOH reacting with triglycerides instead of starting reacting with the salicylic acid?

Is this even necessary?

I mean I know some reactions will be more likely to form a product or another depending on the amount of one of the reactants, like the reaction between salicylic acid and NaOH will give either sodium salicylate or disodium salicylate depending on how much NaOH is available (how is called this phenomenon by the way?), so maybe the unreacted NaOH will be that low at the time I add the salicylic acid it won't react with it at all or so little that there's nothing to be worried about.

What do you think?

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  • $\begingroup$ How do you know that sodium salicylate has not the same anti-acne property ? You say that you "add the salicylic acid and other agents". What are these agents? What is a thick trace ? Now $\ce{NaOH}$ will react immediately with salicylic acid. The saponification rection with oils is much slower. $\endgroup$
    – Maurice
    Commented Feb 2, 2021 at 20:40
  • $\begingroup$ Hi Maurice, I couldn't find any data on a anti-acne property of the sodium salicylate so as far as I know sodium salicylate isn't an anti-acne agent. Perfume, vitamin E and exfoliating walnut powder are the other agents I add The thick trace is when the mixture of oils and NaOH consistency starts to thicken then it hardens in a few minutes/hours depending on the oils used in the recipe, chemically I guess it corresponds to the stage of the reaction where most of the triglycerides have been replaced by fatty acids sodium salts. Thank you $\endgroup$
    – Serybva
    Commented Feb 3, 2021 at 10:29
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    $\begingroup$ What salicylic acid or sodium salicylate enters the human body, they are soon in contact with the plasma whose pH is fixed, and does not change. Both substances are then transformed into the same mixture salicylic acid + salicylate ion corresponding to the pH of the plasma, whatever their origin, acid or salt. $\endgroup$
    – Maurice
    Commented Feb 3, 2021 at 20:50

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Years late to the question,but using potassium hydroxide to make liquid soap would solve this. After you've cooked your soap, dissolve your salicylic acid in vegetable glycerin and add to the water you are using to dilute the soap paste. Don't exceed 2% of the finished solution, and don't use for an all over body soap, small areas like the face only. It is advisable to check the pH of the finished soap to insure it isn't 3.5 or below, as that level of acidity is too much.

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