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. How much vinegar is typically used? Many soap makers substitute vinegar for half to all of the water in the recipe.
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 about 1 g NaOH. 1 oz by weight (28 g) of commercial 5% vinegar neutralizes about 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.
For sodium hydroxide, NaOH --
NaOH for vinegar, grams = Vinegar, grams X 1 / 28 = 0.0357 X Vinegar, grams
Total NaOH, grams = NaOH for vinegar, grams + NaOH for saponification, grams
For potassium hydroxide, KOH --
KOH for vinegar, grams = Vinegar, grams X 1.4 / 28 = 0.05 X Vinegar, grams
Total KOH, grams = KOH for vinegar, grams + KOH for saponification, grams
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 the vinegar. The mixture will thicken as the lye reacts with the acetic acid, but this is normal. Use the lye-vinegar mixture to make your soap as usual.
Because vinegar adds hardness to soap, you may need to cut your soap sooner than normal. Check your soap 8 to 12 hours after you put it in the mold and cut it then if it is ready.