The Difference Between Sodium Hydroxide and Potassium Hydroxide

Carefully Add the Lye to the Water
Carefully add the lye to the water. David Fisher

What's the Difference Between Sodium Hydroxide (or NaOH) & Potassium Hydroxide (or KOH)?

I once had a soap maker email me saying, "I know I've measured everything right, but my soap just won't harden! I've let it sit for two days now and it's still this messy, liquidy goo! What went wrong?" 

Well, there are several errors that could have caused this such as making a big mismeasure in your lye or oils (think of a soap with a 50% superfat)  But what had gone wrong in this instance is that she had used potassium hydroxide instead of sodium hydroxide in her recipe.

So what's the difference? They're both white, flaky powders that make soap.  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 that we use. The fats (or 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 solid or opaque. (Although there are ingredients and situations that make liquid soap cloudy.) They both lather and clean and work like soap works. They just have a different consistency when completed.

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.

One additional noticeable difference that often surprises soap makers making liquid soap for the first time is that potassium hydroxide heats up significantly when you add it to the water to make the lye solution. Sodium hydroxide lye solutions get hot - most definitely - but a potassium hydroxide lye solution gets so hot that it almost boils. The flakes kind of bubble and rattle in the bottom of the pitcher.

As with any lye solution, be sure that you are wearing proper safety gear - gloves and goggles and long sleeves - and that there are no children, pets, spouses, etc. to distract you when you're making your lye solution.