If you're fairly new to soap making, like me, you might be wondering if you could interchange potassium and sodium hydroxide. The answer is depends. It depends on what you are looking for in your soap and if you are prepared to deal with outcome.
First you need to know a little about soap making history to help you in your decision making processes. Back when soap making started there were probably as many opinions about how it should be made as there were families. One of the most common was to make soap was with ashes from a fire that were boiled. Boiling the ashes of a fire, particularly hardwood, would produce pot ash or potassium hydroxide. The potassium hydroxide was the active ingredient that would help saponify the oils and fats (saponify, or saponification, is a more recent term meaning to turn into soap.). A lot of these older recipes used different techniques to tell when you had reached saponification, but there was no numerical standard.
More recently scientist have continued to do what they do best, experiment, and have produced saponification tables for different oils and fats. These tables show how much lye, or sodium hydroxide, is needed to saponify the specified oils and fats. Some tables are in grams and some in ounces. To make this process even easier, so you don't have to do it by hand, some websites provide a "lye calculator" that adds up the amount of lye you need to saponify the oils you choose. It is truly much more simple today than it was in the days of the pioneers. Still, there are some questions that aren't easily answered.
If you are like me and have some background in chemistry, but not in soap, you might think that this process is similar to polymerization and that the only part that really matters is the hydroxide. The truth is that saponification is dependent on the hydroxide but also acts as a salt. Therefore, you will get different results depending on which hydroxide you use. Typically you will get a harder soap from sodium hydroxide than from potassium hydroxide. Older recipes that used pot ash would typically have you add salt, sodium chloride, to harden the soap. One other difference is that you tend to get more lather from the potassium hydroxide than the sodium hydroxide.
I want to make a shaving soap and do it all myself. I have read several different articles and a lot of them seem to mention that there is potassium in shaving soap because it helps give a better lather, but none of them seem to mention the softer soap. I tried using just olive oil and used 100% sodium hydroxide, 50% sodium hydroxide 50% potassium hydroxide, and 100% potassium hydroxide in three different batches. The first batch was a little soft but hardened up after some time curing, while the second batch was like mush and the third just stayed a liquid. It turns out that olive oil produces a soft low lather soap anyway, which made it less than ideal for my test, so it was hard to get it to lather. However, even with the low amount of lather I noticed that the 50/50 mixture produced a better lather than the 100% sodium hydroxide.
So if you want a bar of soap you should probably stick to sodium hydroxide, but if you want a liquid soap us potassium hydroxide. However, if you are looking for lathering characteristics look first at the properties of the oils, because that will make your biggest difference, and then pick your hydroxide. I personally will be experimenting more with a blend of the two and trying different oils to produce the best lather for shaving. I hoped this helps.
