Sodium Hydroxide and Potassium Hydroxide Used in Soapmaking

How Lye and Potash Affect a Soap's Consistency

Young woman pouring liquid lavender soap into moulds in handmade soap workshop
 RUSS ROHDE/Getty Images

Making soap is not difficult once you get the compounds you need, which are relatively inexpensive, and carefully put them together in a controlled environment. The type of soap you make, bar versus liquid, is determined by the choice of compound you use for your salt, sodium hydroxide (NaOH), which is also known as lye, or potassium hydroxide (KOH), also known as potash.

To confuse matters somewhat, both sodium hydroxide and potassium hydroxide are colloquially called, "lye." Lye made with potassium hydroxide will be called out as potassium hydroxide lye.

The Difference Between the Two Salt Compounds

When it comes to soapmaking, there is many the story that starts with, "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?" 

There are a few things that might have gone wrong, such as making a big mismeasurement in the amount of lye or oils (soap is 50 percent superfat).

But, usually, the culprit is the white, flaky powder that was used to make the soap was potassium hydroxide instead of sodium hydroxide. It is easy to mistake the two.

  • Sodium hydroxide is used to makes bar soap. It forms a solid, opaque bar soap.
  • Potassium hydroxide makes liquid soap, which is flowing, clear, or translucent.

How Soap Is Made

Soap is technically a salt product that is made by combining an alkali with fats or fatty acids. The alkali is the lye. The fats (or fatty acids) are the oils.

Sodium hydroxide results in a salt soap that crystallizes to become opaque. A soap made with potassium hydroxide does not crystallize in the same fashion, so it does not become solid or opaque. Although there are ingredients and recipes that can make liquid soap cloudy. No matter the salt compound used, both types of soaps lather, clean, and function like soaps.

The major difference is the consistency when completed.

Classic Recipes for Soap

Old fashioned or "pioneer" soaps were made from a lye that was made from wood ashes. The result was primarily a soft, gooey soap. Wood ashes have a tendency to produce mostly potassium hydroxide. To compensate for the consistency issue, some old recipes say, "Add a handful of salt until the soap thickens." By adding some sodium to the mix, it firms up the texture.

Luckily, today you can get both sodium hydroxide and potassium hydroxide in pure versions with consistent strength from chemical vendors. Unlike your ancestors' unpredictable lye soap, you can predict that soaps made today, whether liquid or bar, made with sodium or potassium hydroxide, are designed to be gentle on your skin.​

Another Chemical Reaction

The firming up (or not) of a soap is a visible chemical reaction between the salt compounds and the fats. Another interesting chemical reaction that soap makers notice when 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, but a potassium hydroxide lye solution gets so hot that it almost boils.

The flakes bubble and rattle in the bottom of the mixing container.

Safety Matters

Due to the corrosiveness of the chemical compounds, it is important, as with any lye solution, to be sure that you are wearing proper safety gear such as gloves, goggles, and long sleeves. For the sake of safety, it is important to concentrate entirely on the soapmaking process, distraction-free from children, pets, spouses, and phones.