Testing Your Soap PH Level

(How to be Absolutely, 100% Sure That Your Soap is Safe to Use.)

Testing the pH of your soap is an easy way to be sure that your soap is safe to use, and doesn't contain any extra lye. The pH scale shows the strength of an acid or base. Soap with a high pH (above 10) is likely to be too harsh, or lye-heavy, for use.

The pH of homemade soap is an important indicator that the soap is safe to use, but not for the reason most people think! Many people believe that a soap with a lower pH is more "mild", but without expensive lab equipment, soap testing is just not accurate enough to give us the precise pH. Instead, we use testing to check that our soaps are within the normal pH of soap, from 7 to 10. This assures us that there is no free lye remaining in the soap (lye has a pH of 14) and that the soap is safe to use.

This is critical. Free lye in soap is just as caustic and painful as the lye water we used to make the soap, and nobody wants to discover that their soap is "lye heavy" in the middle of a shower!

The pH scale is organized so that 7 (the pH of water) is "neutral". Values less than 7 indicate acids (the lower the number, the more acidic the item) and values higher than 7 indicate bases (the higher than number, the more basic the item.)

As a quick point of reference:

pH 0 - hydrochloric acid
pH 2 - lemon juice
pH 7 - water
pH 9 - baking soda
ph 10 - milk of magnesia
ph 14 - lye!

You want your soap to have a pH between 7-10. This is the range that is considered safe to use.

Above 10 indicates that your soap is harsh, and may have "free lye" (lye that didn't react with your oils, and can burn you if you use the soap!)

You might end up with free lye if you made an error measuring your ingredients (for example, if you used too much lye, or not enough oil), or if you used a recipe that did not calculate the amount of lye correctly.

Soap Testing: Three Ways to Check Your Soap's pH

  • 1. The Traditional "Tongue Test"
    Soap makers will just barely tap the tip of their tongue to a fresh bar of soap. An electric zing, as if touching your tongue to a battery, indicates lye-heavy soap that is not safe to use. (The only problem is, who wants to risk lye burns on their tongue? Not me!)


  • 2. pH Test Strips
    Soap makers who value their tongues have turned to pH testing strips. The strip can detect the pH of a drop of water placed on fresh soap. Unfortunately, because of the way soap interacts with water, the readings are not highly accurate. The test strips can make your soap appear to have a lower pH than it really does.


  • 3. Phenolphthalein
    This leaves us with one last method: phenolphthalein (pronounced as "FEE-nol-THAL-een"). Phenolphthalein is a liquid that can be applied, one drop at a time, to soap. It turns pink when it touches a base. The darker the shade of pink, the higher the pH reading. You won't get an exact pH value, but you will be able to tell, quickly and easily, how far along the pH scale your soap is.

    To use phenolphthalein, wait until your soap has completely cured (6-8 weeks for cold-process soap). Cut a bar of soap, and place a drop or two of phenolphthalein on the cut edge (so that you are testing the "inside" of the bar). Wait for the color of the drops to stabilize, and see what you have.

    If it's clear or light pink, your soap is safe to use. If it's darker, proceed with caution! The darkest shades indicate that the soap is dangerously heavy in lye, and should not be used, though you may be able to save it using rebatching, if you're feeling brave.