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I've always been told soap is basic in pH. A few years back in a high school chemistry class, we measured the pH of various substances with universal pH indicator. We measured some liquid hand soap and it came out as acidic, which puzzled even the teacher. Unfortunately I have no idea what brand it was and I still don't know why it came out as acidic.

I learned yesterday that the fundamental reaction that produces soap happens when a base reacts with an oil chain which breaks down the glycerol backbone. Depending on which exact reaction path is being used to make the soap, some base may be added to ensure no oils remain unreacted. After the reaction is complete, some acid can be added to neutralize the base. Could it be likely that extra acid was added to the soap to neutralize the remaining base and the solution was left acidic? What other possible ideas could explain why the soap was acidic?

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  • $\begingroup$ Soaps are made to give off slightly acidic solution, one can hear about "pH 5.5 soap" even from commercials, so "I'm surprised you were surprised" ;) $\endgroup$
    – Mithoron
    Mar 1, 2018 at 22:59

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It has been claimed that skin has a pH of about 5.5. Of course, this must be the sweat coming out of your skin, since skin isn't a liquid. So many body washes add citric acid to their formulas to be milder to tender skin.

For washing after poison ivy or car grease, pH 11 is much more effective.

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