Soap pH Explained: Why Real Soap Is Alkaline

You may have seen "pH balanced" on cleansers and wondered whether natural soap is bad for your skin. Here is the honest picture.

Real soap is always alkaline

Soap is made by reacting oils with lye, and the result is naturally alkaline. That is not a sign of leftover lye; it is simply the chemistry of soap. A study published in the Indian Journal of Dermatology that tested common bathing soaps found most were alkaline, generally around pH 9 to 10.

Skin is slightly acidic

The surface of healthy skin is slightly acidic. This "acid mantle" is part of how skin protects itself. Washing with an alkaline cleanser temporarily raises skin pH, and skin then returns to normal.

Does it matter?

• For many people, washing with real soap is perfectly fine.
• People with dry, sensitive or eczema-prone skin may find alkaline soap more drying.
• On hair, alkaline soap can leave a rough or coated feel, which is why some people follow castile soap with an acidic rinse.

What "pH balanced" usually means

Cleansers labeled pH balanced are almost always syndets (synthetic detergents), not true soap. They can be closer to skin's natural pH. Dove, for example, describes its Beauty Bar as a mild cleansing bar rather than soap.

The pH scale in one minute

pH measures how acidic or alkaline something is, on a scale from 0 to 14. Seven is neutral (pure water). Lower numbers are acidic, like lemon juice or vinegar, and higher numbers are alkaline, like baking soda or soap. Each step is ten times stronger than the one before, so pH 10 is ten times more alkaline than pH 9.

Soap vs syndet vs body wash

• True soap (bars and liquid castile): alkaline, typically around pH 9 to 10. Made from oils and lye.
• Syndet bars ('beauty bars', 'cleansing bars'): made with synthetic surfactants and often formulated closer to skin's pH.
• Body washes and face washes: usually detergent-based and can be formulated at a wide range of pH levels.
• Combination bars: some bars blend true soap with milder synthetic cleansers to soften the effect.

Labels rarely print the pH, so the ingredient list is your best clue. Ingredients such as sodium olivate, sodium cocoate or 'saponified oils' point to true soap. Ingredients such as sodium cocoyl isethionate point to a syndet.

Can you lower the pH of soap?

Not really, without changing what it is. If you add enough acid to bring true soap down to skin's pH, the soap molecules break apart into free fatty acids and stop cleaning well. That is also why mixing castile soap with vinegar makes an oily mess. Products that need a low pH use detergents instead of soap.

Testing soap pH at home

Hobby soap makers sometimes use pH strips or a few drops of phenolphthalein to check that a finished bar is not lye-heavy. These tests are rough. A well-made bar will still read alkaline, because soap itself is alkaline. A very high reading, or a bar that 'zaps' the tongue as some makers test, can signal excess lye. If in doubt, do not use the bar on skin.

Who might prefer a pH-balanced cleanser

• People with eczema or very dry skin, especially in winter.
• People washing the face, where skin is often more reactive.
• Anyone whose skin feels tight or itchy after using soap.
• People with hard water, where soap also leaves scum. See hard water and soap scum.

Practical tips

• If your skin feels tight after washing, try a gentler bar or a syndet.
• Keep showers short and lukewarm.
• Moisturize right after washing.
• See a dermatologist for ongoing irritation.

Related: shampoo bars vs liquid shampoo.