Classic Bells > Soapy stuff > Acetic acid

Acetic acid (Vinegar) in soap

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.

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"). It adds hardness to soap similar to sodium lactate, a salt created by the reaction of lactic acid and sodium hydroxide.

What does vinegar 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.

Can I use apple cider vinegar (ACV)?

Sure you can, but 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 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?

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 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.

Caution! If you make a "masterbatch" lye solution that will be stored for longer than a day, use only water to make the masterbatch mixture. If you use vinegar, the mixture will thicken into an unpourable gel a day or so after it is made. 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. (2)

Caution! Your finished soap may be ready to cut sooner than usual when you use vinegar. Check your soap 8 to 12 hours after making it, and cut it then if it is ready. Wear gloves to protect your skin from any active lye. If you wait too long, the soap may be too hard and brittle to cut cleanly.

 

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/