Soap is created when the fatty acids in soaping fats react with an alkali (also called a base), such as sodium hydroxide (NaOH) or potassium hydroxide (KOH).
The chemical reaction between any acid and any alkali is called an acid-base neutralization. The specific acid-base reaction that makes soap is called saponification.
The offspring of an acid-base neutralization is a salt. Table salt (sodium chloride, NaCl) is one kind of salt; there are many other salts. Soap is the salt created when fatty acids react (saponify) with an alkali such as NaOH or KOH.
Sometimes soapers add other acids to the soap pot in addition to fatty acids. When they do this, non-soap salts are created along with soap. Depending on which kind of non-soap salt is made, it can make bar soap easier to pour and unmold, make liquid soap easier to dilute, increase the hardness of bar soap, protect against rancidity (DOS, dreaded orange spots), reduce the amount of unpleasant soap scum, and/or add label appeal.
Acids commonly added to soap include lemon juice (citric acid), vinegar (acetic acid), and yogurt (lactic acid). Adding these acids complicate the soap making process, because they react faster and more easily with NaOH or KOH than fatty acids do.
Why do other acids react faster than fatty acids?
There are strong and weak acids; likewise there are strong and weak bases. Stronger acids and bases are "bullies." They take what they want first in an acid-base neutralization. Weaker acids and bases react with the leftovers.
Baking soda is a weak base, and sodium hydroxide is a strong base. Vinegar (acetic acid) is a weak acid and battery acid (sulfuric acid) is a strong acid. Fatty acids are weaker than any of the other acids mentioned so far, so they always lose when they compete against these acidic "bullies."
That means if you add a stronger acid to your soap, the stronger acid will react with all of the alkali it wants. The weaker fatty acids will react with whatever alkali is left over.
If there is not enough NaOH or KOH to properly react with all of the acids in the soap pot, that means the fatty acids will not fully saponify. The soap will have more free fat and/or fatty acids than expected, and the superfat will be higher than you planned.
How can this extra superfat problem be avoided?
If you want to add extra acids to your soap, you should also add enough alkali to react with all of the acids in the soap pot.
Any good soap recipe calculator will calculate the proper amount of alkali for making the soap itself. To know the extra alkali needed to react with an added acid, you will have to calculate this by hand or use the SoapmakingFriend.com calc, which can do this extra math for you. The articles about acids linked above explain how to do these calculations by hand.
My soap looks fine, so why bother with this extra math?
I am aware that quite a few soapers add vinegar (or other acid) to bar (NaOH) soap without adding extra alkali. They claim their soap is perfectly fine, because there are no visible changes to the soap.
Just because a soaper cannot see any changes in her soap does not mean there are none. The superfat in the soap has increased in direct proportion to the amount of acid added. This extra superfat can make bar soap softer, reduce lather, cause plumbing problems, and increase the chance of rancidity (DOS).
An example -- If I use commercial vinegar (5% acetic acid) for all of the water in my favorite soap recipe and do not add extra NaOH, the added acetic acid would increase the superfat by an extra 7%, in addition to the lye discount already built into my recipe.
If a lot of extra 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 extreme example of this. Although the acid used in that video is citric acid from lemons, this idea applies to any acid, including vinegar.
What if I make soap first and then add the stronger acid?
Sometimes soapers add an acid after saponification is over, on the theory that the added acid will remain intact or will only lower the pH or will otherwise do what the soaper wants it to do because the soap is already made. This does not work.
The added stronger acid will still have a bullying nature when mixed with finished soap. It will bump the weaker fatty acid out of its place on the soap molecule and take its place. The altered molecule will then be the salt of the stronger acid. The extra fatty acids will add to the superfat.
In other words, adding a stronger acid to soap after the soap is made is roughly the same as adding the acid at the start of the soap making process.
My soap has a pH of 8 because my pH test strips tell me so. Why do you disagree?
Most soap makers use inexpensive test strips that are not accurate for complicated chemical mixtures like soap. Cheap strips might work great in water or other simple chemical systems, but not soap. In addition, most soap makers use pH test strips in concentrated soap solutions and that makes the results even less accurate.
Inexpensive pH test strips typically show the pH of soap is 2 or 3 units lower than the pH really is. In other words, if the strip says the pH is 8, the real pH is probably closer to 10 or 11.
The only test strips that I know of that are reasonably accurate are Machery Nagel strips, and even they are accurate only if used properly. You cannot accurately measure pH in a concentrated soap solution.
Working up some lather on a bar of soap and rubbing the strip in the lather will not give reliable, accurate results.
Sticking the test strip on a dab of liquid soap paste will not give reliable, accurate results.
Liquid soap diluted for use is usually still too concentrated for measuring pH, so this solution must be further diluted or you will not get accurate results.
The pH of soap should be measured at room temperature in a mixture of soap mixed with distilled water. The solution should contain only 1% to 10% pure soap by weight. Pick a concentration and stick with it for the most consistent, comparable results.
I think the high pH of soap is bad for skin. How do I lower the pH to 8 or even 7?
The short answer -- If you add enough acid to lower the pH below about 9, the soap will not be functional soap anymore. It will be a mixture of fatty acids and soap.
The pH of properly made soap ranges from about 9.5 to about 11.5. If you must have a cleanser with a lower pH, you should be using synthetic detergents rather than soap.
The long answer -- Here are some actual, tested numbers to illustrate my point based on soaps made from a single pure fatty acid. (1) Each soap is chemically "neutral" meaning it has no excess alkali and no excess fat.
Soap made from pure lauric acid has a normal pH of 10.1. Lauric acid is a major fatty acid in coconut oil.
If acid is added to lower the pH by 2.6 pH units to a final pH of 7.5, the soap will have broken down into a mixture of 50% fatty acids and 50% soap. Using terms we often use as soap makers, this decomposed soap would have a whopping 50% superfat.
Soap made from pure palmitic acid has a normal pH of 10.7. Palmitic acid is one of the main fatty acids in lard, palm, and the butters.
If enough acid is added to lower the pH 1.9 units to a final pH of 8.8, this soap will have decomposed into 50% fatty acids and 50% soap.
Soap made from pure oleic acid has a normal pH of 11.2. Oleic acid is the main fatty acid in olive oil.
If enough acid is added to drop the pH 1.3 units to a final pH of 9.9, the soap will have decomposed into a 50:50 mix of fatty acids and soap.
If I continue to add even more acid to get these soaps to an "ideal" pH of 7 or 8, the mixture will become mostly fatty acids and very little soap.
Pure soap made from any other single fatty acid will have a characteristic normal pH and will respond similarly to added acid. (2)
Soap made from a fatty acid with a lower molecular weight (butyric, capric, caprylic, lauric, myristic, etc) will tend to have a lower normal pH.
Soap made from a fatty acid with a higher molecular weight (palmitic, stearic, oleic, etc.) will tend to have a higher normal pH.
These examples are unusual in that each soap is made from a single fatty acid, but we can use this information to better understand the soap we do make. Most soap makers make soap from fats, which are made of many fatty acids.
The pH of a soap made with a fatty acid mix will likely be somewhere between 9.5 and 11.5, the range of pH defined by the soaps made from pure fatty acids. Exactly what that normal pH is will depend on the main fatty acids in that particular soap.
You can estimate a high coconut oil soap will have a lower pH -- closer to 10 -- due to more lauric acid content.
And you can guess that a high lard or palm or olive oil soap will probably have a higher pH -- closer to 11 -- due to more oleic and stearic acid content.
No matter what the normal pH of your soap is, if you drop the pH by 1.5 to 2 pH units below that normal by adding acid, the result will be a 50:50 mixture of fatty acids and soap.
Does liquid soap have the same problem with added acids?
Yes, it does. The consequences of adding extra acid to liquid (KOH) soap are even more obvious and dramatic, because liquid soap cannot hide a high superfat like bar soap can.
As the superfat rises above about 3% in a liquid soap, the extra fat and fatty acids cannot remain mixed with the soap. Instead, an increasingly thick layer of fat and fatty acids will separate out and float on the diluted soap.
Acids added to liquid soap include the ones already mentioned as well as boric acid and borax. (Borax is technically a salt, but it functions as an acid when added to soap.) Any of these acids will increase the superfat of liquid soap and cause unwanted separation.
(1) Kevin M. Dunn. Scientific Soapmaking. Clavicula Press. 2010. pp 227-230.
(2) An increasing number of double carbon bonds in a fatty acid will also tend to reduce the normal pH of the soap. See reference (1) for more information.
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