[Used with Permission]
Comparison of pH Test Strips Used for Testing Soap
by Ann Perius-Parker, copyright 2000 (ann@soapimpressions.com)
First, some background and history:
I've been a soapmaker for nearly 26 years. For more than 20 of those years, I have tested my soaps using paper pH test strips which showed the soap to be very nearly neutral (pH 7.5). Three years ago, a job change gave me access to a laboratory pH meter. Naturally I couldn't resist using it to test my soap. Imagine my surprise when the pH meter gave readings in the range of 9.5-10.5. I knew my soaps were very mild and good, (my skin told me so) and hadn't my soap tested neutral for more than 20 years? I didn't want to believe the meter, and so I tried everything to get a "better" reading. First I made sure the pH meter was calibrated, then I tried increasing the amount of superfatting, (up to 15%). I tried other pH meters in nearby labs. I tested commercial soaps and handcrafted soaps, (not only mine). But still the readings (by now more than 200 of them) hovered around 10. Something was going on here that I didn't understand. Why would there be such a wide variation between the paper strips and the lab meter? Was it the soap, the pH meter, or the pH test strips?
In the mean time I started reading everything about soap pH that I could find. The literature (see references below) suggested that a pH of 9.5 - 10 was normal for soap. While that confirmed the results from the pH meter, it didn't explain the 2-3 point discrepancy. I am grateful to a technical person at Hydrion (a major manufacturer of pH strips) who identified the surfactant nature of soap as being the potential problem. Apparently, surfactants can throw off the indicators of some pH strips by as much as 2-3 units. He was kind enough to send some samples of their products that promised to be more accurate in measuring the pH of soap.
And now a few words about pH:
pH is sometimes referred to as the power of the hydrogen ion in aqueous solution, and refers to the degree of acidity or alkalinity of a substance in water. The scale of 1-14 (with 7 being neural) is logarithmic, meaning each full unit is different by a factor of 10 from the one adjacent to it. For example, a pH of 9 is ten times more alkaline than a pH of 8. So it follows that a pH of 10 is 1000 times more alkaline than an pH of 7 (10 x 10 x 10 = 1000).
It's this logarithmic aspect of pH that makes accurate measurement so important to me. I owe it to my customers, friends and family to produce a safe product, so I test for pH. But, I would prefer my measurements to be accurate, not off by a factor of 10, 100, or 1000.
How I tested these pH strips:
Thus far I have tested six different pH strips, from three different manufacturers. I tested each strip using 3 different soaps, both commercial (#1) and handcrafted (#2 & #3), comparing the readings from the pH meter (the control) to the readings from new strips and my "old faithful" strips, using a 1% soap solution (by weight). Since most of us don't test pH using a 1% soap solution, I also tested each strip by making a slurry with distilled water directly on the bars of soap. I did each set of tests twice, just to be sure.
Products Tested:
Hydrion products tested: (All of these are rolls of paper test strips, accompanied by a color chart.)
The Test Results:
|
Test Material |
|
|
|
|
pH Meter (control) |
|
|
|
|
"Old Faithful" paper strips |
|
|
|
|
Paper Strips |
|
|
|
|
Vivid 1-11 (solution) |
|
|
|
|
Vivid 1-11 (slurry) |
|
|
|
|
Vivid 3-13 (solution) |
|
|
|
|
Vivid 3-13 (slurry) |
|
|
|
|
Spectral 1-14 (solution) |
|
|
|
|
Spectral 1-14 (slurry) |
|
|
|
|
Plastic Strips |
|
|
|
|
EM 7.5-14 (solution) |
|
|
|
|
EM 7.5-14 (slurry) |
|
|
|
|
EM 0-14 (solution) |
|
|
|
|
EM 0-14 (slurry) |
|
|
|
|
MN 7.0-14.0 (solution) |
|
|
|
|
MN 7.0-14.0 (slurry) |
|
|
|
The Vivid 1-11 strips consistently gave readings more like those of the expensive lab pH meter. However, if you were testing a batch that happened to be grossly lye heavy, the limited range of these strips would give a reading no higher than 11.
Both the Vivid 3-13 and Spectral 1-14 have the advantage of reading higher in the alkaline range, but don't quite measure up to thepH meter.
I find the plastic test strips give the best results, providing readings very much in line with laboratory pH meter, while covering the full range one is likely to encounter when testing soap. I especially like the Macherey-Nagel 7.0-14.0 and the EM Reagents 7.5-14, for their ability to read in half unit increments.
Conclusions:
Not all pH strips are created equal when it comes to testing soap. Some are inaccurate by as much as 2-3 units because the surfactant nature of soap can interfere with the indicator dyes used to make the strips. Others can give accurate readings that approximate those of a pH meter. Based on the pH strips I've tested, laboratory grade plastic test strips are more accurate than paper test strips for testing soap.
Recommendations for using test strips:
Some test strips need longer contact with the solution/slurry for an accurate reading. For best results, keep your test strip in the solution/slurry until the color no longer changes, perhaps 30 seconds or one minute.
Store your strips away from direct light which over time, can fade the dye indicators and interfere with accuracy.
References:
Ann Perius-Parker has a business that sells custom and ready-made soap stamps. Also for sale at her site are the kind of pH strips you would use to get a more accurate pH testing on your soap. To visit her site, go to http://www.soapimpressions.com, Soap Impressions.
This page last updated December 6, 2001.
If you still have questions, please read through the information on the Troubleshooting Help page, MOST Frequently Asked
Questions and Modern Procedures. More can also be learned through the Botched Batches and Soapy Success pages. Many common problems have already been addressed on the site and it's difficult for me to keep up with emails these days and get anything else done. If your question involves my looking up information that you can also research, or going over numbers and recipe calculations, I might not respond if in the middle of a project around our home and garden. I apologize for this, since I've enjoyed my correspondence with people and don't like to ignore emails of any kind. Thanks! :-)