Soapmakers test the pH of their soap to make sure it’s safe and gentle for the skin. pH is a scale that measures how acidic or alkaline a substance is, ranging from 0 to 14. A pH of 7 is neutral—like pure water. Numbers below 7 are more acidic, and numbers above 7 are more alkaline. If the pH is too high (too alkaline) the soap can be harsh or irritating.
Testing helps ensure the soap is mild, effective, and skin-friendly. That’s where the “zap test” comes into play (and a few other methods discussed below). It’s a simple, yet effective way to check if your soap has completed saponifying and that all the lye has reacted with oils to create soap.
Why Perform a Zap Test?
Most soap makers use the Zap Test when they see a concerning what spot on their soap, or if they notice crumbling of their soap bars. Both of these can indicate a lye heavy soap.
The zap test is a quick, traditional way to check if saponification is complete—that is, all the lye has reacted with the oils to make soap. This helps prevent any harsh reactions on the skin from residual lye.
Quick reality check on soap pH: soapers don’t just get confused about how to test pH — we also get confused about what pH means for true soap. If you’re here because someone told you handmade soap “isn’t suitable” for face or hair because of pH, here’s a myth-busting breakdown that explains the real deal (and what actually matters in a gentle bar): The Truth About pH in Handmade Soap…. and now back to how to test the pH! 😊
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How to Perform a Zap Test
Before we get to the “right” way to do a Zap Test, let me first de-bunk a common myth… DO NOT LICK YOUR SOAP! That’s not a zap test. That’s just licking your soap. At best, it is kind of yucky. At worst, you could be licking a spot of lye and burn your tongue!
Here’s a step-by-step guide to performing a zap test (followed by a highly comical video short for a visual explanation):
- Wet Your Finger: Slightly dampen your index finger with water.
- Touch the Soap: Gently touch the surface of the soap with your wet finger.
- Test: Touch your finger to your tongue quickly. If you feel a slight zap, similar to touching your tongue to a battery, the soap still contains unreacted lye and needs more curing time. If there’s no zap and it tastes like soap, it’s safe to use.
Warning: the following Zap Test demonstration video will make you laugh! Do not watch while drinking a beverage. You might spit it out.
The Importance of pH Testing
Ok, back to being a bit more serious….
While the zap test is useful, pH testing gives you a more scientific understanding of your soap’s acidity or alkalinity. Using pH test strips or phenolphthalein can provide insight into the soap’s readiness, regardless of appearance or “suspected” lye heaviness.
The pH scale measures how acidic or basic a substance is, ranging from 0 to 14. Natural soap typically has a pH between 8 and 10. Anything lower on the pH scale and it’s not soap, it’s probably a detergent. Anything higher on the pH scale would be suspect for being lye heavy.
How to Use pH Test Strips to Test the pH of Handmade Soap:
- Prepare a Soap Solution: Shave off a small amount of soap and dissolve it in distilled water to create a clear solution.
- Dip the Test Strip: Submerge the pH test strip in the solution for the time specified by the manufacturer (usually a few seconds).
- Read the Result: Remove the strip and compare the color change to the chart provided with the test strips. A pH between 8 and 10 indicates a gentle soap suitable for most skin types.
Testing to Test the pH of Handmade Soap with Phenolphthalein:
- Make a Soap Solution: Dissolve a bit of soap in distilled water.
- Add Phenolphthalein: Introduce a drop of phenolphthalein to the solution.
- Check for Color Change: If the solution turns pink, the soap is still too basic. No color change suggests a neutral pH, indicating the soap is likely safe for use.
Remember, the zap test is a quick and easy way to check your soap, but it should not replace other quality control measures such as proper formulation and cure time. Always handle soap with care, especially when testing for lye content.
Happy soap making!
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