What is acetic acid?
Acetic acid is created when acetobacter bacteria ferment sugars or alcohols into acetic acid. Vinegar and kombucha are two common foods that contain acetic acid.
The acetic acid content of kombucha varies a lot. Most kombucha sold commercially as a pleasantly tart beverage most likely has less than 1% acetic acid by weight. (1) The acid content in homemade kombucha can range from mildly tart to so sour and strong it is undrinkable. Since kombucha varies so much, it is hard to give reliable advice about how to use it in soap.
Commercial household vinegar is typically 5% acetic acid by weight. Although stronger acetic acid solutions can be purchased, they are more hazardous to use and harder to find.
I am only going to discuss how to use 5% commercial vinegar in this article.
What does vinegar do in soap?
Acetic acid and sodium hydroxide (NaOH) react to make sodium acetate. Sodium acetate is a type of salt (as chemists use the word "salt").
Sodium acetate adds physical hardness to soap similar to sodium lactate, a salt created by the reaction of sodium hydroxide with lactic acid (the acid found in yogurt and other fermented dairy).
What does vinegar not do in soap?
Some people think the pH of their soap has to be reduced to near neutral (pH of 7) or even acidic (pH less than 7). They think adding an acid such as vinegar will accomplish this goal. If any soap is acidified to a pH of 8 or less, it will be almost entirely fatty acids and is no longer a functional soap. For more information, see these articles -- Soap alkalinity and What do acids do in soap? and Soap pH and Kitchen chemistry: Adding acids and alkaline salts to soap.
When people wash their hair with soap, a common recommendation is to rinse the hair afterward with a mixture of water plus vinegar or citric acid to reduce roughness. Some people think adding vinegar to their soap will eliminate the need for this acid rinse. Vinegar added to soap does not remain an acid, however. You will still need to do an acidic rinse if you wash your hair with soap.
Can I use apple cider vinegar (ACV)?
Sure. ACV is an acetic acid solution just like distilled white vinegar. ACV will darken the soap somewhat, so keep that in mind when making design plans. Distilled white vinegar works just as well and will not change the color of the finished soap.
How much vinegar can be used?
You can use regular commercial vinegar for up to 100% of the water in the recipe.
How much vinegar is typically used?
Many soap makers use vinegar for about half of the water in the recipe.
Should I reduce the amount of water used?
Definitely. The vinegar should be included as part of the total water in the recipe.
Simple method -- Subtract the vinegar weight from the total water weight.
Extra water to add = Total water weight - Vinegar weight
For example, if your recipe calls for a total of 238 grams of water, and you decide to use 100 grams of vinegar, then --
Extra water to add = 238 - 100 = 138 grams.
Exact method -- Calculate the water content in the vinegar. Subtract that weight from the total water weight.
Water in 5% vinegar = Vinegar weight X 95 / 100
Extra water to add = Total water - Water in 5% vinegar
For example, if your recipe calls for a total of 238 grams of water, and you decide to use 100 grams of vinegar, then --
Water in 5% vinegar = 100 x 95 / 100 = 95 grams
Extra water to add = 238 - 95 = 143 grams
How much lye does vinegar neutralize?
Commercial vinegar sold for cooking and household use is 5% acetic acid. About 1 fluid ounce (2 tablespoons, 1 ounce by weight, or 28 grams) of this commercial vinegar contains 1.5 g acetic acid.
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.
How to calculate the extra lye needed to neutralize the vinegar?
When using vinegar in your recipe, add the appropriate extra weight of lye needed to react with the acid. If you do not add any extra lye, the acid will increase the superfat in your soap.
Sodium hydroxide, NaOH
Decide how much vinegar to use. If you aren't sure, I suggest using commercial vinegar for half of the water in your recipe.
For example, if a recipe calls for a total of 200 grams of water, use 100 grams of vinegar and 100 grams of water.
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
Potassium hydroxide, KOH
Decide how much vinegar to use. If you aren't sure, I suggest using vinegar for half of the water in your recipe.
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 vinegar 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.
Caution! Your finished soap may need to be cut earlier than usual when you use vinegar. Check your soap 8 to 12 hours after making it, and cut it then if it is ready. If you wait too long, the soap may be too hard and brittle to cut cleanly.
The best time to cut soap is when it feels firm like refrigerator-cold colby or mild cheddar cheese. When pressed with a fingertip, the soap should yield slightly to the touch but will not easily dent. The soap is too soft to cut if it is easily dented like cream cheese and probably too hard to cut if it feels rock-hard like parmesan cheese.
If you cut early, wear gloves to protect your skin from any active lye.
Caution! If you make a "masterbatch" lye solution that will be stored for longer than a day, do not use vinegar. Use only water to make the masterbatch. If you use vinegar, the mixture will thicken into an unpourable gel a day or so after it is made. (2)
If the vinegar-lye mixture does gel, it can still be used if you are willing to deal with the goopy mixture. But it will be safer and easier to make only enough vinegar-lye solution for the current day's soap making.
More discussion about vinegar
https://www.soapmakingforum.com/threads/replacing-water-with-vinegar.73429/
https://www.soapmakingforum.com/threads/acv-in-soap.72480/#post-730115
https://www.soapmakingforum.com/threads/math-question-vinegar-and-superfat.69945/#post-698080
References
(1) Hannah Krum. pH and Kombucha: Kombucha Defense Mechanisms. Kombucha Kamp website. "...Kombucha checks in at a much more palatable ~1% acetic acid, often much lower in commercial versions...." Source: https://www.kombuchakamp.com/ph-kombucha-alkaline-acid-balance
(2) Carolyn Z. Soapmaking Forum user "cmzaha". Lye gel thread started 2 January 2020. Source: https://www.soapmakingforum.com/threads/lye-gel.77652/
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