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 soap making, 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 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 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 at the bottom of the mixing container.
