Just Say No to Zap Testing
As responsible soap makers, it is imperative that we make sure that our products are safe to use. This starts with our selection of ingredients, making sure that they are suitable for our intended purposes, and that our recipes and formulations use fragrances, essential oils and other ingredients within skin-safe levels. We also need to keep our equipment and work areas clean to prevent inadvertent contamination, and that we use good manufacturing techniques. Finally, it is critical that we make sure no residual lye is present in the finished soap, by testing to ensure that the product pH is in an acceptable range.
So I was astounded when, as a beginning soap maker, almost every authority promoted zap testing to check the pH level of the finished soap. Mind you, these same references spent pages, sometimes entire chapters of their books with warnings about the dangers of sodium hydroxide and lye solutions. They recommend long pants, long sleeves, rubber gloves, goggles or a face shield, even mixing the solutions outside due to the inherent dangers. And these are all good valid techniques.
And why was I astounded? Because zap testing abandons any pretense of personal safety. For those of you that don’t know, zap testing consists of touching your tongue to a bar of soap to determine if there is still any unreacted lye in it. If your tongue stings after touching the soap, it is considered lye heavy (translation: if you get a chemical burn on your tongue, then it is too alkaline and your soap is unsafe). Needless to say, I was not enamored of this method. Granted when your great-great-grandmother was making soap from wood ash and buffalo grease out on the prairie she probably didn’t have any other method of testing available, but that was long ago and there is no excuse for such reckless methods today.
The other issue with zap testing is it’s accuracy, or lack thereof. I always laugh when I read some zap testing proponent claim that pH test strips aren’t accurate. I guess all those chemical research laboratories around the country are just fooling themselves by using test strips, when they could be just sticking their tongues into test tubes to determine pH. Seriously, do they actually believe they can tell the difference between a pH of 9 and a pH of 10 with their tongue? I bet these geniuses can tell the difference between 110V and 120V by sticking their finger in a wall outlet too… Kids - please don’t try this at home!
These days there are three safe and accurate means of determining the pH level of our finished soaps. Electronic meters can be very accurate, but due to their relatively high cost they may not be particularly well suited to the occasional hobbyist/crafter. Indicator solutions and pH papers, both of which are inexpensive, easy to use and readily available, are perfectly acceptable for most users.
pH indicator solutions are chemical compounds that change color in the presence of hydrogen ions, and thus can be used to determine pH. There are many different indicators available; even universal indicators, which combines a number of different indicators to cover the full range of bases and acids. Generally speaking though, particular indicators will cover specific pH ranges so it is important to select an indicator that covers the pH range we are interested in. Most useful to soap makers is phenolphthalein.
Phenolphthalein (sometimes abbreviated phph) when prepared in an alcohol solution is an excellent indicator to detect alkalis. This solution will change from being colorless to a bright fuchsia color between pH of 8.2 and 10, making it an ideal indicator for our purposes. Simply dissolve a little sliver of your finished soap into some distilled water, add a drop of phph and observe the resultant color. On the low end, at pH of 8 or so, the solution will be just barely pink, while at the high end (about 10), the solutions will immediately turn a vibrant fuchsia color. With a little practice, it is relatively easy to differentiate the color change over the 8-10 range to within ±0.5 (ie, 8 compared to 9 compared to 10).
Phenolphthalein can also be used to determine the total alkaline content of your soap, although this takes a little more effort and some other materials, specifically some 190 proof ethyl alcohol (Everclear or equivalent) and a reference acid, like anhydrous citric acid. To determine the total alkali of the soap, dissolve a small amount of the soap in the ethanol, add a few drops of the phph indicator, and then titrate the solution with the citric acid until the solution turns clear. By using this method, excess NaOH can be determined to about 1 ppt (part per thousand).
Phenolphthalein indicator solutions are also very inexpensive. A two ounce bottle will be enough to test hundreds of batches of soap, and should cost less than $5.00. Here at Farmington Soap Works, we use phph as our primary method of pH testing on every batch of soap.
So what are the downsides of phph? Mostly, you will probably have to order it. While it is readily available online, I have not been able
to locate a local supplier that will sell in small quantities.
You should also be aware that there are some reports that suggest that phenolphthalein in crystalline form, when ingested in large quantities, may be carcinogenic. Before you start to panic however, you should also know that phenolphthalein has been used medicinally in over the counter laxatives for over a century in the US, and is still used for this purpose overseas. Furthermore, only about one-third of the studies* conclude that phenolphthalein may be carcinogenic – the other two thirds conclude that it is not. You should also know that the dosages used to see carcinogenic results in mice, when translated to humans ingesting phenolphthalein indicator solution, would be equivalent to drinking a little over 3 gallons of phph per day. My guess is that if you actually tried this, the ethanol used to make the solution would kill you long before you could possibly develop any tumors.
* Source: Carcinogenic Potency Project, http://potency.berkley.edu
Another option is to use pH papers. Litmus paper was the original pH paper. First used about 700 years ago, litmus paper is a porous paper that is impregnated with particular lichens. There are two basic types, red and blue, that will change to the opposite color in the presence of acids or bases.
These days, we have pH test strips. These are small thin plastic strips with a sensitized area at one end. The advantage of pH test strips over litmus paper is that the test strips act more like universal indicators – that is, they can tell you how acidic or alkaline a solution is, not just that it is acidic or alkaline. To use them, prepare a small solution of soap dissolved in distilled water, and place the sensitized end of the test strip into the solution. The sensitized area will change color, and by comparing to the chart that comes with the strips, the ph of the solution can be determined.
The test strips are a little more expensive than phph, but are still affordable. Even better, you will probably be able to find some locally – various versions of ph test strips are used to test aquarium water pH, soil pH, and also body chemistry. Generally prices run from $5-$10 for 100 test strips. Check at pet stores, greenhouses and pharmacies for local availability, and online at many lab supply and soap maker supply websites.
The one thing you will have to watch for is the particular pH range that the test strip will cover. There are full range sets (1-14), acid sets (0-7), base sets (7-14), and many others in between. For soap making, the pH range we are interested in is generally between 7 and 11, so make sure when you purchase pH strips they cover this range. I think that many of the problems people have with test strips is that either they are colorblind, or they purchased strips in the wrong range. For example, if your test strips cover the acidic scale (0-7), then all of your soap samples will have an apparent pH of 7, no matter what the actual pH is. We use the test strips periodically, when we wish to independently confirm the indicator solution accuracy.
Finally we have pH meters. These are electronic devices which use a special pH probe, which can measure the hydrogen ions in a solution and convert it to a voltage that can be displayed as pH on a digital voltmeter. Cost of these range from $20-30 on up to thousands of dollars for laboratory grade instruments. Accuracy is higher than either phenolphthalein indicator or pH test strips, provided that proper calibration is done, and using an electronic pH meter is great for those that cannot differentiate colors well. At some point we will buy one of these to do our testing, and I’m sure I will enjoy it as much as I enjoy having an infrared pyrometer to measure temperatures!
One downside to pH meters is the calibration requirement. Because the glass pH probe changes over time, it is important to calibrate the pH meter at least once per day, if not before each measurement. Calibration is usually done by inserting the probe into two known buffer solutions, one on the high end of the expected range, and one at the low end. Some meters will also require the user to enter the solution temperature, as this can affect the pH reading – other meters can sense the temperature at the time of measurement and compensate automatically.
So is there any time when zap testing is appropriate? In the interest of full disclosure, I have actually zap tested a bar of soap once… I was fairly confident that the pH was ok, in that I had calculated SAP values, added a significant discount, and didn’t see anything suspicious in the soap itself (no streaks, pockets with liquid in them, sweating, etc), and so I took a gamble and lucked out – no “zap”. But once was enough, and I can’t recommend that anyone else try it either. Test strips and phph are so easy, so inexpensive and so readily available that there really is just no need to zap test.