Discussions like this can easily come down to a battle of almost religious conviction. “I heard that if you add vinegar to soap, it uses up all the lye and you are left with an oily mess.” “Well, my grandma handed down her recipe, and my customers all swear by it.” “Basic chemistry proves that if you add lye to vinegar, it turns the soap back into oil.” “I add a teaspoon of vinegar to my recipe, and I don't get soap scum.” The last thing I want to do is pontificate on the matter, but I think I can clear up some misunderstandings and offer a demonstration that you can try for yourself.

Vinegar is an acidic solution of acetic acid in water. Traditionally, it is made from alcoholic beverages like wine or cider. Bacteria eat the alcohol in these beverages, and excrete acetic acid. Commercial vinegar is typically 5% acetic acid in water, both of which are colorless. Any color that a particular vinegar has is due to compounds present in the beverage it was made from. Percentage-wise, these are dwarfed by the acetic acid content, so for the purposes of this article I will consider vinegar to be simply a 5% solution of acetic acid in water.

By definition, soap is an alkali salt of a fatty acid. Sodium laurate, for example, is the sodium salt of lauric acid, and each contains a straight chain of 12 carbon atoms. This chain, or “tail,” constitutes the “fatty” part of the fatty acid. Devoid of charged atoms like oxygen, the tail is referred to as hydrophobic (water-fearing) or lipophilic (fat-loving). At the “head” of a fatty acid are two oxygen atoms. These negatively-charged atoms are attracted to water molecules. Thus, we refer to the head as hydrophilic. This combination of a hydrophilic head and hydrophobic tail allow soap to do what it does: bring oil and water together.

The other fatty acids share the same basic structure with lauric acid, but differ in the length and shape of their tails. Myristic, palmitic, and stearic acids contain straight chains of 14, 16, and 18 carbon atoms, respecively. Oleic, linoleic and linolenic acids contain bent chains of 18 carbon atoms. The length and shape of their tails affect the properties of the fatty acids and their soaps, but the most important attribute is that the shorter molecules are more soluble in water than the long ones. Conversely, the longer molecules are more soluble in oil than the short ones.

Acetic acid has the same basic structure as the fatty acids, but its tail is only 2 carbon atoms long. Compared to the fatty acids, it is very soluble in water and not very soluble in oil, so much so that we do not refer to it as a “fatty” acid at all. It's just an acid. The sodium salt of acetic acid is sodium acetate. While it has the same basic structure as a soap molecule, its tail is so short, and its attraction for oil so weak, that we do not refer to sodium acetate as soap.

Unlike fats and oils, acetic acid reacts almost instantly with sodium hydroxide to form sodium acetate. If you don't account for this, you will have less sodium hydroxide than you thought. How much less? Each gram of vinegar consumes 0.033 grams of sodium hydroxide. Each ounce of vinegar consumes 0.033 ounces of sodium hydroxide. Each pound of vinegar... you get the picture. However much vinegar you use, you need to provide 0.033 times that weight in extra sodium hydroxide to account for the acetic acid.

While this might not seem like much, leaving it out leads to an unintended lye discount. A typical soap formula might include 1000 grams of oil, 144 grams of sodium hydroxide, and 288 grams of water. If we substitute vinegar for water, the vinegar consumes 9.5 grams of sodium hydroxide, leaving 134.5 grams. That's a lye discount of 6.6% on top of whatever discount you thought you were using. The ratio is the same, no matter what units of weight you prefer.

So if you want to try using vinegar, you have several approaches you can take. If you simply substitute it for water and do not account for it, you will wind up with more super-fat than you expected. Depending on the lye discount you normally use, this may be acceptable. If you add extra sodium hydroxide (0.033 times the weight of the vinegar), you can use your regular lye discount. The easiest path, though, is to use a 0% lye discount in the lye calculator, and count on the vinegar to provide what amounts to a 6.6% discount.

One claim I've seen made about vinegar is that it lowers the pH of soap. When used to make lye for cold or hot process soap, this is not the case because the acetic acid reacts more quickly than the oil. Adding vinegar at trace matters very little, since most of the oil remains unsaponified at trace. But if you add vinegar to finished soap, either after the HP cook, or during curing, or prior to use, you can convert some of the soap to fatty acid, thus lowering the pH. This happens during curing even without the use of vinegar. Carbon dioxide from the air is mildly acidic, and naturally neutralizes any excess sodium hydroxide and converts a small portion of the soap to fatty acid. This is why we perceive cured soap as milder than uncured soap.

If you want to see this taken to an extreme, try washing your hands with soap and vinegar. You will see first “hand” why the acids are called “fatty.”