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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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