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.
Comments: Any acid, including vinegar, will reduce the amount of lye that is available to turn fats into soap unless extra lye is added to account for the lye consumed by the acid.
Many soapers add vinegar to bar (NaOH) soap without adding any extra lye, and they claim their soap looks and acts fine. They assume because they don't see any visible changes to their soap, then nothing is happening. Unfortunately that is not true; the superfat in their soap is increasing in direct proportion to the amount of acid added.
Suppose I use commercial vinegar (5% acetic acid) for all of the water phase in my favorite soap recipe without making any other changes. The acetic acid would consume enough lye to increase the superfat in this recipe by about 7% in addition to the lye discount I built into the recipe.
If enough acid is added to a bar soap recipe without considering the consequences, the result will be a mushy, greasy mess that is definitely not Good Soap! The Soaping 101 video "Cold Process Citrus Soap" is an example of this. Although the acid used in that video is citric acid from lemons, this idea applies to any acid, including vinegar.
The same is true for liquid (KOH) soap. In fact, the consequences are even more obvious and dramatic, because liquid soap is not able to hide a high superfat like bar soap can. As the superfat rises above about 3% in a liquid soap, the fat cannot remain mixed with the soap. Instead, an increasingly thick layer of fat and fatty acids will float on top.