What Is the pH of Bath Soap?

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Bar soaps are the most alkaline.
Bar soaps are the most alkaline. (Image: Image by Flickr.com, courtesy of Debbie)

Soapmakers measure the acidity or alkalinity of bath soap by its pH level on a scale of 0 (acidic) to 14 (alkaline). Substances at either end of the scale, such as sulfuric acid and lye, are harmful to skin.

pH Levels

A 2002 International Journal of Dermatology study found pH levels in bath soaps ranging from 3.61 to 12.38. High-pH soaps caused the most irritation.

Significance

Human skin’s pH is a naturally acidic 5. The “acid mantle” that covers us is dissolved by high-pH products.

Liquid soaps have lower pH levels.
Liquid soaps have lower pH levels. (Image: Image by Flickr.com, courtesy of woodley wonderworks)

Types

Liquid soaps generally have lower pH levels than bar soaps. A a 2009 Journal of the European Academy of Dermatology and Venereology study recommends liquid cleansers for bathing infants.

Controversy

Although the word “pH-balanced” abounds in advertising to indicate bath soaps with a chemically lowered pH close to 5, natural product makers counter that it is the purity of the ingredients—not the pH—that makes a bath soap kind to skin.

Testing pH levels
Testing pH levels (Image: Image by Flickr.com, courtesy of Debbie)

Safety

Home soapmaking guides warn hobbyists to test the pH level of their soaps before use. Soapmakers test for safety by using paper or plastic pH test strips, phenolphthalein drops, or even a tongue taste test.

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