Classic Bells > Soapy stuff > Soap pH

Soap pH

My soap has a pH of 8 because my pH test strips tell me so. Why do you disagree?

Inexpensive test strips are not accurate. Inexpensive pH test strips typically show the pH of soap is 2 or 3 units lower than the soap pH really is. In other words, if the strip says the pH is 8, the real pH is probably closer to 10 or 11.

In my experience, the pH test strips that are most accurate are ones produced by Macherey Nagel. They are sold by chemistry lab suppliers and some soaping supply houses. Expect to pay $0.10 to $0.15 per strip in packs of 100.

The test strips, even the good ones, may not be sensitive enough. Most people choose test strips that measure the entire range of pH from 0 to 14. These wide-range strips, even the Macherey Nagel #92110, only tell you the pH to the nearest whole pH unit. (9) That is not very useful for soap making.

Test strips that measure over a smaller pH range are more sensitive and may provide more useful results. For example, Macherey Nagel #92125 strips measure a pH range from 7 to 14 and are sensitive to 0.5 pH unit. (9)

The soap is usually not diluted enough. Even with the best strips or pH meter, the pH will still not be accurate if the soap solution is too concentrated. You will not get accurate pH results by testing:

Lather created by working up suds on a bar of soap

Concentrated KOH (liquid) soap paste

KOH (liquid) soap that has been diluted for normal use

 

How should the pH be measured correctly?

Use the right equipment -- a high quality pH test strip (such as Machery Nagel #92125) or a properly calibrated pH meter. Follow the manufacturer's directions for use.

The pH of soap should be measured at room temperature in a dilute mixture of soap and distilled water. The solution should contain only 1% to 10% pure soap by weight. Pick a concentration within this range and stick with it for the most consistent, comparable results.

I know a lot of soap makers are skeptical about testing soap pH in a 1-10% mixture. I could get into a dry chemistry explanation about why a pH measurement is accurate and repeatable only in a dilute solution, but I realize not everyone gets excited about chemistry.

Instead I want to justify this requirement in a more practical way --

Think about the way soap is used in the real world -- people wash their skin with a small amount of soap dissolved in a relatively large amount of water. For example, I estimate I use 5 grams of soap per shower. Those 5 small grams get diluted in the water that is on my skin and in my washcloth. I want to do a pH test that tells me how the soap will treat my skin when I am using it.

More concentrated soap will tend to have a pH that is appears to be lower than the true pH, so you might think the pH is fine, when perhaps it is not. The best way to get the most useful answers is when you test your soap with a method that simulates real life conditions.

 

I want to use pH test strips to check if my soap is safe or lye heavy. How do I do that?

Measuring pH with test strips, even high quality strips, cannot tell you if your soap is skin safe or lye heavy for two reasons.

The pH of soaps that are all chemically balanced (no excess lye, no excess fat) will vary depending on the fatty acid content in the soaps. One particular soap might be skin safe at pH 10, but a second soap with a different blend of fatty acids might be lye heavy at that same pH. You cannot just measure the pH alone to know if a random batch of soap is skin safe or if the soap contains excess alkali. (10)

The difference between a skin-safe soap and a lye-heavy soap depends on just a few tenths of a pH unit. There are no pH test strips sensitive enough to show these small differences of pH in soap. Even ones with an accuracy of 0.5 pH unit are not suitable.

In his book Scientific Soapmaking, author Kevin Dunn states "...Fully saponified, properly formulated soap will contain 1 ppt [part per thousand] or less of NaOH..." (7)

A soap with a slight lye excess of 1 ppt NaOH will have a pH that is roughly 0.3 pH units higher compared to the same soap that is chemically balanced.

Soap with a large lye excess of 5 ppt NaOH will have a pH that is roughly 0.7 pH units higher. (8)

To see these small differences, you must be able to test pH to an accuracy of 0.1 pH unit or better. Even the best test strips with an accuracy of 0.5 pH unit are not sensitive enough.

 

If the pH test alone cannot tell me if my soap is safe to use, what tests can I use?

The two tests that can definitively tell you if a soap is skin safe or lye heavy are the "zap test" and the total alkalinity test.

More about the zap test....

The total alkalinity test is done by diluting a weighed amount of soap in alcohol. A calibrated acid solution is added, drop by drop, to this soap solution until the pH reaches a predetermined value, typically a pH of 8.2. The amount of excess alkali (lye) in the soap is calculated based on the amount of acid required to reach that pH. (7)

 

How do I make soap that has a pH of 8 or less?

The normal pH of properly-made soap with no excess lye and no excess fat will always have a pH well above 8 -- typically somewhere between about 9 to about 11.5, depending on the specific kinds of fatty acids in the soap.

Many acids, such as vinegar or citric acid, are stronger acids compared with the fatty acids found in soap. While it is true that adding a stronger acid will reduce the pH of soap, it is also true that a lot of acid has to be added to get a small change in pH.

A lot of acid has to be added to get a small pH change because soap reacts to a stronger acid by decomposing (breaking apart) into fatty acids. Fatty acids in soap behave much like fat does -- they act as superfat and reduce the cleansing effectiveness of the soap.

This chemical breakdown of the soap into fatty acids interferes with the change in pH, so the pH drops more slowly than you might expect. A chemical mixture like soap that resists changes to its pH is called a "buffer."

By the time enough acid is added to reduce the pH of soap by as little as 1 to 1.5 units, chemical analyses show a large portion of the soap will have decomposed into fatty acids. (1) At this point, the "soap is not soap any more" and is no longer an effective cleanser. A "soap" that has been acidified to a pH of 8 or less will be nearly all fatty acids and almost no actual soap.

If you must have a cleanser with a pH of 8 or less, you need to use synthetic detergents -- soap will not meet your needs.

 

How do you know soap cannot have a pH below 8?

The following information is actual, tested pH data for real-life soap made from a single pure fatty acid. (1) Each soap is chemically "balanced" meaning the soap has exactly the right amount of alkali to fully saponify the fat -- there is 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. (2) You can think of the fatty acid content as a type of superfat.

A pH of 7.5 is certainly below the goal of pH 8, but at 7.5, the mixture will be half soap and half superfat and is no longer a functional soap.

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.

The pH of this 50:50 mixture is still well above that "ideal" pH of 8, and the mixture is no longer a functional soap.

Soap made from pure oleic acid has a normal pH of 11.2. Oleic acid is the main fatty acid in olive oil and other high oleic (HO) oils such as sweet almond, HO sunflower, HO canola, avocado, etc.

Oleic soap will decompose into a 50:50 mix of fatty acids and soap if the pH is reduced by only 1.3 units to a final pH of 9.9.

If the pH of oleic soap is lowered to 8 by adding even more acid, the mixture would contain almost no soap.

 

How does this information apply to the pH of soaps we make from a blend of fatty acids?

The data given above for soaps made from pure fatty acids suggests the normal pH of soap made from a blend of fatty acids will lie between 9 and 11.5, possibly a bit higher.

I would expect soap high in coconut oil to have a pH closer to 9-10, due to a high lauric acid content. A soap high in lard, palm, or olive oil should have a pH in the 10-11.5 range due to more oleic and palmitic acid content.​ (3)

The data I have seen for soaps made from a blend of various fats support this conclusion.

In one study, the pH of fifteen commercially-produced soaps ranged from 9.8 to 12.4. (4)

The few accurately-measured pH values I have seen for handcrafted soap also fall in this range. (5,6)

If a stronger acid is added to soap made from a blend of fats, this soap will decompose just like soap made from a pure fatty acid.

If stronger acid is added with the intent to reduce the pH -- even by as little as 1 to 1.5 pH units -- the soap will continue to decompose as explained earlier. The fatty acid content will eventually become so high the mixture will no longer be useful soap. (11)

No soap will remain a functional soap if the pH is reduced to 8 or less, even a pure lauric acid soap.

The only way to formulate a functional cleanser with a pH of 8 or less is to use synthetic detergents.

In the study mentioned above, the pH of fourteen synthetic detergent (non-soap) cleansers was also measured . The pH of these non-soap products ranged from 3.6 to 7.7. (4)

 

Related articles in "Soapy Stuff"

Soap alkalinity

What do acids do in soap?

Kitchen chemistry: Adding acids and alkaline salts to soap

 

References and Notes

(1) Kevin M. Dunn. Scientific Soapmaking. Clavicula Press. 2010. pp 227-230.

(2) The pH of a mixture of 50% soap and 50% fatty acids (or any 50:50 mixture of undissociated salt and dissociated ions) has a specific name -- the pKa value. These percentages are on a molecular basis, not a weight basis. For the fatty acids normally found in soap, however, the weight percentages are roughly the same as the molecular percentages.

(3) Soap made from an unsaturated fatty acid tends to have a slightly lower normal pH than soap made from a saturated fatty acid with the same carbon backbone. See (1) and compare the pH of soaps made from polyunsaturated linoleic or linolenic acid, monounsaturated oleic acid, and saturated stearic acid.

These four fatty acids share the same backbone of 18 carbon atoms, but the pH of soap made from each pure fatty acid is slightly different. The pH of these soaps going from the highest pH to the lowest is as follows -- stearic soap > oleic soap > linoleic soap > linolenic soap.

(4) Baranda, L, et al. Correlation between pH and irritant effect of cleansers marketed for dry skin. International Journal of Dermatology. 2002, vol 41, pg 494–499.

(5) Faith. pH Testing and Lowering the pH of Liquid Soap. Alaiyna B Bath and Body. .http://alaiynab.blogspot.com/2015/05/ph-testing-of-liquid-soap-and-lowering.html

(6) Kenna. How to pH Test Handmade Soap. Modern Soapmaking. http://www.modernsoapmaking.com/how-to-ph-test-handmade-soap/

(7) Kevin M. Dunn. Scientific Soapmaking. Clavicula Press. 2010. pp 245-249. The measurement of "ppt" in Dunn's book is his abbreviation for parts per thousand by weight.

(8) A solution of NaOH in water that is 1 ppt (part per thousand by weight) has an NaOH molar concentration of 0.45 mM (millimoles). A 0.45 mM solution of NaOH in water has a calculated pH of 10.65 based on this pH calculator -- https://www.omnicalculator.com/chemistry/ph

Here are calculated pH values for other solutions of NaOH and water --

1 ppt (0.45 mM) NaOH in water -- pH 10.65
2 ppt (0.9 mM) -- pH 10.95 -- the change in pH from 1 ppt to 2 ppt is 0.3 pH unit
3 ppt (1.35 mM) -- pH 11.13 -- pH change from 2 to 3 ppt is 0.18 pH unit
4 ppt (1.8 mM) -- pH 11.25 -- pH change from 3 to 4 ppt is 0.12 pH unit
5 ppt (2.25 mM) -- pH 11.35 -- pH change from 4 to 5 ppt is 0.1 pH unit

The total difference between slightly lye heavy (1 ppt) and very lye heavy (5 ppt) is only 0.7 pH unit. A pH test strip that is accurate to 0.5 pH unit is not sensitive enough to show differences this small.

(9) Rapid pH testing products. Machery Nagel. https://www.mn-net.com/us/rapid-tests/ph-tests/

(10) There is one specific situation where the pH alone can be a useful test for lye heaviness, and that is if you make a lot of soap using the same recipe. You would need to make multiple batches of this recipe using varying amounts of superfat, including negative superfat (excess lye), test the pH and the total alkalinity (see next section below) of each batch, and correlate the changes in pH to the total alkalinity.

This correlation between pH and alkalinity would be accurate only for that recipe. If you alter the recipe, the blend of fatty acids will change, and you will have to repeat this correlation process for the new recipe.

(11) Some soap makers intentionally superfat their soap with fatty acids rather than fat. Even though an intentional fatty acid superfat does not reduce the pH by much, a soap that is superfatted with fatty acids will be somewhat milder to the skin than the same soap that is superfatted with intact fats.

To superfat with fatty acids, soap is made with a zero superfat or a slight lye excess to ensure no intact fat remains. A small measured amount of a stronger acid is added to this finished soap. Some of the soap decomposes into fatty acids in response to the added acid.

Fatty acid superfatting is best done when making soap with a hot process method rather than a cold process method.