Ph

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The usual reason why people want to test soap pH is to learn if the soap has excess lye. Just testing the pH doesn't give you that answer, however, because the pH of a skin-safe soap will vary anywhere between 9 and 11 depending on the fatty acid content. To verify the soap is skin safe, you either need to do a zap test or do a titration test for free alkalinity.

But here is my advice about pH strips --

Inexpensive pH test strips typically show the pH of soap is 2 or 3 units lower than the soap pH really is. In other words, if the strip says the pH is 8, the real pH is probably closer to 10 or 11

In my experience, the only pH test strips that seem to be reasonably accurate in soap are Machery Nagel products. LabRat Supplies is another reputable source of quality pH test strips, but I am not familiar with their products, so I do not have a specific recommendation about which strips to use for soap. Expect to pay $0.10 to $0.15 per strip in packs of 100

Most people opt for test strips that measure pH from 0 to 14, but you will get more useful information if the strips test a narrower pH range. Machery Nagel #92125 strips measure a pH range of 7 to 14, which is a better choice.

Even with the best strips or pH meter, the pH will still not be accurate if the soap solution is too concentrated. You will not get accurate pH results by testing...

Lather created by working up suds on a bar of soap​
Concentrated KOH (liquid) soap paste​
KOH (liquid) soap that has been diluted for normal use​

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