Soap will always have an alkaline pH ranging from about 9.5 to about 11.5, even if the soap does not have any excess lye. Why is that? Here is my attempt to explain a complicated subject.
Alkaline, acidic, and neutral pH
The pH value is a measurement of the hydrogen ions (H+) in a liquid. Hydroxide ions (OH-) are also present in this liquid, but the OH- and H+ ions are not always evenly matched. Only the extra H+ ions that do not have a matching OH- ion are counted when measuring pH.
A neutral pH has a value of 7. When the pH is 7, every H+ ion is balanced with a matching OH- ion. Drinking water is an example of a solution that is usually pH neutral.
A pH is acidic if the pH value ranges from zero up to 7. An acidic pH means there are more H+ ions than OH- ions.
A basic or alkaline pH ranges from 14 down to 7. A basic or alkaline pH means there are more OH- ions than H+ ions.
Strong acids and alkalis versus Weak acids and alkalis
The mere fact that a chemical is alkaline, acidic, or neutral is only part of the story, however. Alkalis can be strong or weak. Acids can also be strong or weak.
Strong acids and alkalis want to break apart (dissociate) easily. A strong acid dissociates (breaks apart) completely in water to release all of its H+ ions. Its pH will be close to zero. A strong base will dissociate completely when mixed with water to release all of its OH- ions. The pH of a strong alkali will be close to 14.
An example of a strong acid is hydrochloric acid (HCl) with a pH near 1.
The alkalis we normally use to make soap -- sodium hydroxide (NaOH) and potassium hydroxide (KOH) -- are both strong bases. These alkalis have a pH around 13 when diluted with water to the concentrations used for soap making.
Weak acids and alkalis are not so eager to break apart. Weak acids -- ones with acidic pH values closer to 7 -- do not entirely dissociate when mixed with water. Weak alkalis, ones with alkaline pH values closer to 7, also only partly dissociate when mixed with water.
The fatty acids found in soap are weak acids. The lauric acid found in coconut oil has a pKa value of 5.3. (1,2)
Other weak acids include vinegar (pH 2.5-3), yogurt (pH 4-4.5), beer (pH 4-6), tomato juice (pH 4-4.5), egg yolk (pH 6-7), coffee (pH 5), and honey (pH 4.5). (3)
Bicarbonate of soda (baking soda, H2CO3) is an example of a weak base. It has a pH of about 8.4 when it is freshly dissolved in water.
Other weak bases include human blood (pH 7.3-7.5), egg white (pH 7.5-9.5), and milk of magnesia (pH 10.5). (3)
What happens when acids and bases are mixed?
If you mix chemically equal parts of a strong base and a strong acid in a solution of water, these chemicals completely dissociate and create exactly as much hydroxide (OH-) ions as there are hydrogen (H+) ions. Since each H+ ion is directly neutralized by an OH- ion, the ending pH will be a neutral pH of 7.
This idea of "acids canceling out bases" is what most non-chemists have in mind when they think about pH. But this is not the way it works if a strong acid is mixed with a weak base or vice versa.
If you mix chemically equal parts of a strong acid and a weak base, the pH of the resulting solution will be acidic (pH below 7).
The mixture is chemically equal, so why does this happen?
The strong acid completely dissociates, which means it releases all of its H+ ions. The weak base only partly dissociates, so only some of the OH- ions are free to directly neutralize some of the acid's H+ ions. The rest of the OH- ions remain bound to their parent molecule.
Even though there are equal parts of H+ and OH- ions in the mixture, not all of the OH- ions are free to directly neutralize the acidic H+ ions.
If you mix chemically equal parts of a strong base and a weak acid, a similar thing happens.
The pH of the resulting soap solution will be alkaline (pH above 7) even though the mixture is chemically balanced. The strong base completely dissociates so all of its hydroxide (OH-) ions are freed.
The weak acid only partly breaks apart, so not all of its hydrogen ions (H+) are free to directly neutralize the alkaline OH- ions.
What chemicals are created when an acid and a base neutralize each other?
The offspring of an acid-base neutralization is a "salt" as chemists define this word. Water (H2O) is also created in an acid-base neutralization.
The salt that most of us know best is table salt, sodium chloride, NaCl. It is the salt created when the strong base sodium hydroxide, NaOH, and the strong acid hydrochloric acid, HCl, neutralize each other.
Soap is also a salt mixture. It is created when we react s strong alkali such as sodium hydroxide, NaOH, with weak fatty acids. Most batches of soap contain many kinds of fatty acid salts.
Soap -- the marriage of a strong alkali and weak acid
Handcrafted soap makers usually create soap by mixing fat with a strong alkali. The most common alkali used for soap making is sodium hydroxide (NaOH), but potassium hydroxide (KOH) and other alkalis are also sometimes used.
The fat molecule contain four main building blocks -- one glycerin and three fatty acids. The NaOH breaks the fat apart and then the NaOH combines with each fatty acid to create a soap molecule.
Since NaOH is a strong alkali and fatty acids are weak acids, soap is always alkaline, even if the total number of fatty acid molecules is perfectly balanced with the total number of alkali molecules.There are always more free OH- ions in a mixture of soap and water than there are free H+ ions.
This means the pH of a properly made soap is always alkaline, somewhere between about 9.5 to about 11.5 depending on the fatty acids in the soap. See also Soap pH.
Related articles in "Soapy Stuff"
Kitchen chemistry: Adding acids and alkaline salts to soap
References and Notes
(1) pKa is a specialized type of pH measurement
(2) Kevin M. Dunn. Scientific Soapmaking. Clavicula Press. 2010. pp 227-230.
(3) This pH data is available from many sources, including https://www.quora.com/What-are-some-common-household-acids-and-bases
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