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What's the Difference Between Sodium Hydroxide/NaOH & Potassium Hydroxide/KOH?


Question: What's the Difference Between Sodium Hydroxide/NaOH & Potassium Hydroxide/KOH?
I once had a soap maker email me saying, "I know I've measured everything right, but my soap just won't harden! It's still this messy, liquidy goo! What went wrong?" What had gone wrong is that she had used potassium hydroxide instead of sodium hydroxide in her recipe.

So what's the difference?

Answer: Quite simply:
  • sodium hydroxide (often called just "lye") makes bar soap - solid, opaque bar soap
  • potassium hydroxide (often called "potash") makes liquid soap - flowing, clear or translucent liquid soap
Soap is technically a "salt" that is made by combining an alkali with fats or fatty acids. The alkali is the lye we use, the fats/fatty acids are the oils. Sodium hydroxide results in a salt (soap) that is crystallized enough to be opaque. The soap made with potassium hydroxide doesn't crystallize in the same fashion, and hence, is not as opaque. (Although there are ingredients and situations that make liquid soap cloudy.)

Old fashioned or "pioneer" soaps were made from lye that was made with wood ashes - and were primarily a soft, gooey, soft soap. Wood ashes produce mostly potassium hydroxide. I've seen old instructions/recipes that said "add in a handful of salt until the soap thickens" - that appears to be just adding some sodium to the mix to make it firmer.

Luckily, today we can get both sodium hydroxide and potassium hydroxide in pure versions with consistent strength from chemical vendors. Unlike Grandma's unpredictable "lye soap" - we can know that our soaps, whether liquid or bar, made with sodium or potassium hydroxide, are going to be gentle on our skin.

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