Acetic acid (Vinegar) in soap
What does it do in soap? Acetic acid (the acid in vinegar) and sodium hydroxide (NaOH) make sodium acetate. Sodium acetate is a type of salt (as chemists use the word "salt"), and it appears to harden soap similar to sodium lactate, a salt created by the reaction of lactic acid and sodium hydroxide.
What does it not do in soap? There are a lot of myths about vinegar in soap. Vinegar does not do much to reduce the pH of lye-based soap, instead it causes other chemical changes in the soap. Using vinegar to make a soap for washing hair does not eliminate the need to rinse the hair afterward with a separate vinegar or citric acid rinse.
How much should I use? Use regular commercial vinegar for up to 100% of the water in soap. Regular commercial vinegar is 5% acetic acid, so about 1 fluid ounce (2 tablespoons, 1 ounce by weight, or 28 grams) of commercial vinegar contains 1.5 g acetic acid.
How much lye does it neutralize? 1 oz by weight (28 g) of commercial 5% vinegar neutralizes 1 g NaOH. 1 oz by weight (28 g) of commercial 5% vinegar neutralizes 1.4 g KOH. For every 1 ounce (28 g) of commercial vinegar in your recipe, add the appropriate extra weight of lye needed to react with the vinegar. If you do not add extra lye, the vinegar, like any other acid, will increase the superfat in your soap.
NaOH for vinegar, grams = Vinegar, grams X 1 / 28
Total NaOH, grams = NaOH for vinegar, grams + NaOH for saponification, grams
Replace the 1 in the first equation with 1.4 to find KOH for vinegar. Add the KOH needed for the vinegar to the KOH needed for saponification to find the total KOH for the recipe.
Making a dual-lye recipe? For recipes that use both NaOH and KOH as well as acetic acid (vinegar), please see my tips here....
How should I add it to my soap? Measure the weight of vinegar needed for your recipe and mix it with any additional water you might be using. Stir the lye slowly into this mixture. Proceed with your recipe as usual.