Phenolphthalein, pH and Soap

First, terminology: "Phenol" has become a generic 'nickname' term for a class of chemicals with a certain type of chemical structure. The original, actual "Phenol" is derived from coal tar and is a 'root stock' for lots of chemical products. A dilute solution of Phenol is also known as "Carbolic" and is/was used as a disinfectant. So, don't say "phenol" when you mean "phenolphthalein".

Phenolphthalein is a "pH indicator", meaning that it tests the quantitative presence of positive hydrogen ions in a solution, or how acidic/basic a solution is. The pH scale runs from 1 to 14, with 7.0 being neutral (neither acidic nor basic). Like the earthquake Richter scale, the pH scale is logarithmic, meaning that a pH reading of 10.0 would be a hundred times more basic than a reading of 9.0 and a thousand times more basic than a pH reading of 8.0, and so on.

There are many different substances that serve as pH indicators. For soap making, we want phenolphthalein, specifically, because it's range of measurable color change due to pH level is 8.2 to 10, which nicely includes soap's natural pH of about 9.4 to 10.0. (Soap's natural pH depends some on what oil(s) the soap is based on and really can range about a half a pH point. I tested this with a bunch of one-oil liquid soap bases, using a pH meter).

Note that  soap makers refer to soaps that are at their natural pH level as being 'neutralized'. 'Neutral', however, is actually a pH level of 7.0, which is impossible for lye-plus-oils soaps. It's also impossible, though, to get soapers to use any term but 'neutralization' for the correction of a soap to its natural pH as we've all called it this for many decades, so why buck the flow? But do realize the difference between how soapers use the word and what it really means or you might have your teenage chemistry student sneer at you someday.

Be wary of finding other pH indicator solutions and confusing them with the phenolphthalein we need to use. "Phenol Red", found at pool supply stores is NOT phenolphthalein. It is, instead, "phenolsulfonphthalein" or "PSP", and it's measurable pH range is 7.0 - 8.0 pH. This makes sense, as you wouldn't want to dive into water that is 9.5 to 10 pH, which would be like washing your eyeballs with soap! Don't look for "phenol" at pool stores - You'll very likely get the wrong stuff and the guy behind the counter may not know the difference.

I've found some references to phenolphthalein at wine and beer makers' supplies websites, but I honestly don't know why, as the range of pH's in wine making is 2.8 to 4.4 pH and in beer making, 4.6 to 6.2 pH. I admit, though, that I know little of wine making (beyond making 'balloon wine' with grape juice in college) and even less about beer making. Wine making is on my list of things to learn someday, but so far, I've only become adept at drinking it.

At Summer Bee Meadow, we sell phenolphthalein as "Phenol P" as everyone knows the word "phenol" and we want to differentiate our phenolphthalein from Phenol Red, for instance. Besides, who the heck can pronounce "phenolphthalein", anyway?.

So why do we soapmakers need to care about our soaps' pH level anyway? Get some in your eye and you'll know! Higher pH soaps are harsher. You can test this by touching some newly made soap to the tip of your tongue, which is actually remarkably sensitive to pH level and makes a pretty good 'pH meter'. If it 'bites' sharply, it's pH is too high (This is how 'old time' soapmakers tested their soaps).

So - How does one properly use phenolphthalein to test our soaps? Keep in mind that phenolphthalein measures pH by displaying a color in solution and that phenolphthalein is soluble in alcohol, but not soluble in water.The proper way to use phenolphthalein is to dissolve some liquid or solid soap in a roughly 50:50 mix of distilled water and alcohol (ethyl or isopropyl rubbing alcohol is ok) in a clear glass. When the solution is as clear as it will get, add a drop or two of phenolphthalein solution and stir it. Then view it well-lit in front of a white background. This way, one can see the slightest little bit of pink color (which is ok) and all its graduations up to a deep red (definitely not ok). Interestingly, the measured value of a strong base in solution (like sodium hydroxide) is, in practical terms, not affected by changes in the concentration of a solution. This means that a soap solution's measured pH does not measurably change if extra water is added.

Note: Do not simply add drops of phenolphthalein directly to all of your just-finished soap and see if it turns red. While phenolphthalein was for decades the active ingredient in Ex-Lax and other laxatives, it has since been withdrawn from human-contact or ingestion uses as it was found to increase the incidence of cancer in lab rats and is also absorbed through the skin. Not good. Don't be afraid of it, but use it on a separated soap sample and dump that when you're done with it. Obviously, keep your dropper bottle of phenolphthalein away from children and pranksters!

Phenolphthalein is available in one or two ounce bottles (or two one-ounce bottles) at Summer Bee Meadow (as "Phenol P") and at many other soapmaking supplies stores online. You should look for "1% phenolphthalein in 50:50 (w/w) solution of water and alcohol" or similar wording on the label.

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Matthew's picture

I realize this thread is

I realize this thread is rather old, but for anyone who stumbles upon it and is wondering, the Phenolphthalein found in winemaking supply stores is the same as that used by us soapmakers (I'm not directing people to purchase their Phenolpthalein elsewhere.  For those like me who also make wine, you might already have some stashed away!).  As was mentioned, the pH of wine and beer is considerable less than that of soap, but Phenolphthalein is not used to test the pH of wine and beer; it is used to test the titratable acidity:

A small quantity of distilled water is given a few drops of Phenolphthalein.  It is then treated with a small amount of standardized NaOH solution until the pink end point is reached (pH of 8.2).  A measured amount of wine or beer is added (this doesn't work with red wine) which will drop the pH and the pink color fades.  Drop by drop, the same standardized NaOH solution is added until the pink end point is reached again.  If one knows how much NaOH was used, the acidity (grams/ liter) of the wine or beer sample can be calculated.

For wine and beer makers, 8.2 is "close enough" to neutral to work with.

This has nothing to do with soap making, but it might ease someone’s curiosity.


Administrator's picture

Thanks for the interesting

Thanks for the interesting info on winemaking. We have a winemaking supplies shop nearby that I've browsed a few times and find it fascinating, although i'm not sure I could handle more than one addicting hobby at a time )

Summer Bee Meadow

Wings's picture

Huh theres something new to

Huh theres something new to learn ever day! Thanks for the info and I guess I'll be making an order in the next day or so provided my paychecck is good. Long story. Keeping my fingers crossed :) Glad this forum got started and hope more people join in. One can never learn enough :)



Success is not final, failure is not fatal: it is the courage to continue that counts.
- Winston Churchill

Administrator's picture

Hi Becky The most proper way

Hi Becky

The most proper way to test solid soaps would be to dissolve a bit of it in a clear glass in some hot water with a bit of alcohol added (rubbing alcohol would be ok) & let it settle until the solution is as clear as it can be.

Then add a drop of Phenol P and swirl it gently to avoid bubbles and hold it up in decent lighting in front of a white background to see if there is any pink or red showing. The alcohol helps because phenolpthalein is actually insoluble in water, but is soluble in alcohol or in a water-alcohol mixture.

Clear is good. The very slightest of pink in front of a white background is ok. Anything redder than that is not ok.

Some cloudiness for solid soaps is inevitable because solid soap is by its nature not fully soluble. Excess cloudiness is a function of either incomplete saponification or excess superfatting. If the test solution is cloudy but not pink in hue, you can assume that you're looking at not-so-soluble solid soap and/or unsaponified superfat oils. So long as it's not pink, it's ok. Cloudy and pink needs extra hot-curing time (a problem sometimes in small batches of cold-process soap) or extra cook time if it's hot-process soap.

For solid soaps, as long as there is at least a little bit of superfatting and all was measured correctly, the pH will be ok when saponification is complete. If the batch is fully processed but still tests pink, the formula or the measurings of ingredients were faulty.

Extra heating can ensure full saponification, but if the ingredients were not formulated or measured right, no amount of heating will cure an actual 'excess lye' situation.

The way around possible unfinished saponification in cold-process soaps would be to
make larger batches and insulate the mold well after pouring (no peeking!) or to cover the soap with Saran Wrap (the original type) so there's no evaporation and put the whole batch-in-mold into a well-controlled low temperature oven at 140-160 degrees for 3 to 4 hours. This needs very careful control because a larger batch at a higher oven temp risks boiling over due to its own added heat of saponification. Unsaponified soapmaking oils could cause a fire if they drip on an oven burner, so caution is very important with this method.

[This all is about solid soaps - liquid soaps are a different situation because they don't tolerate superfatting well (if at all) and proper neutralization is an added concern that solid soapers take care of in the initial superfatted formulation.]


Summer Bee Meadow

GreeneRoseSoaps's picture

Hi Steve, I have a question

Hi Steve,

I have a question about the Phenol P.   After my Cold Process soap has cured for at least 4 weeks, I shave a bit off and put a few drops of Phenol P on the soap bit and wait for a few minutes.  I haven't seen the chemicle turn pink by using this method.  I also make Crock Pot Hot Process soap using the "tounge test" and when I don't get "zapped" I use a few drops of Phenol P on a bit of the soap.  If it turns pink, I keep cooking the soap until the Phenol P remains clear.  Is this an acceptable practice?  Will it ensure that my soap PH is safe?  My husband and I use my soaps every day and so far, my skin couldn't be better.

Thank you for starting this forum.  I also belong to several others and am pleased with how helpful everyone is.  I am new to soap making since November 2009 and spend many hours on line looking at handmade soap and reviewing recipes.  This has become an addicting hobby!!!