Classic Bells > Soapy stuff > NaOH or KOH purity check

NaOH or KOH purity check

Soap makers often assume the purity of the alkali (NaOH or KOH) in their soaping stash is whatever the distributor says it is. Even if the purity is high when the alkali is newly purchased, the purity can drop as time passes, sometimes by quite a bit.

NaOH or KOH does not "go bad" simply because of time, however. What makes either one go bad is exposure to water and carbon dioxide (CO2) in the air. If you can keep an alkali properly protected, it can last for centuries. If you don't protect it from water and CO2 (or other things it wants to react with), it will lose a lot of its purity within mere hours or days.

When NaOH or KOH absorbs water from the air, it gains water weight. One gram of NaOH that is clumpy from absorbing water is not one gram of pure NaOH. Some of that weight will be useless water.

When NaOH or KOH also absorbs CO2 from the air, NaOH will react with the CO2 and become sodium carbonate (washing soda) and KOH reacts with CO2 to become potassium carbonate. The carbonates are alkali chemicals but they don't easily react with fats to make soap. One gram of NaOH that's contaminated with sodium carbonate is not one gram of pure NaOH.

If you base your recipe on using 100% pure NaOH or KOH (and many online soap recipe calculators make this assumption!), but you soap with NaOH or KOH that has a lower purity, the fats will not fully saponify. You can expect a range of results from "the soap looks fine but if I could measure the superfat, I would find it is is overly high" to "the soap will not come to trace -- it has stayed soupy for hours!"

Just because NaOH or KOH is less than 100% pure does not mean it should not be used to make soap. But how can we tell what the purity really is? The best way is to actually test the purity; otherwise we're basically guessing. Once we have a number for the purity, that information can be used to adjust the alkali weight so enough actual alkali is used to make good soap.

Based on information given in a presentation by Kevin Dunn, author of Scientific Soapmaking, here is a method to quickly and easily test the purity of your NaOH or KOH:

Video tutorial of this purity check

Step-by-step instructions and the math behind the formulas


Adjusting your recipe for NaOH purity

Most online soap recipe calculators assume the NaOH purity is 100%, but that is seldom true, even for NaOH shipped fresh from the manufacturer. The calculators' assumption of 100% purity builds in a "hidden" superfat in addition to the superfat you type when you create a recipe.

You can adjust your recipes to compensate for NaOH purity being less than 100%. The easiest way to do this is to reduce the superfat (also known as the lye discount) by the difference between the assumed 100% purity and your actual NaOH purity.

I know this is hard to understand when put into words, so here are some examples --

My NaOH is 97% pure -- that's 3 percentage points below 100%.
I want my actual superfat to be 5%.
I'd set the lye discount in the soap calc to 5% - 3% = 2%.

My NaOH is 75% pure -- that's 25 percentage points below 100%.
I want my actual superfat to be 8%.
I'd set the lye discount in the soap calc to 8% - 25% = -17%.
Yes, the number is negative -- you'd enter a minus 17 for the superfat setting.

My NaOH is 95% pure -- that's 5 percentage points below 100%.
I want my actual superfat to be 5%.
I'd set the lye discount in the soap calc to 5% - 5% = 0%


NaOH's remarkable ability to absorb water

NaOH and KOH are powerful desiccants, meaning they absorb water easily and quickly from the air. Here is a video using NaOH that makes this point:

See how fast NaOH absorbs water from the open air....

In the video, I use the word "deliquescence." Deliquescence is the name for the unusual ability of some chemicals, such as NaOH, to absorb so much water that the chemical turns from a solid into a liquid.


Dry bucket for storing NaOH or KOH

So what can we do to keep NaOH and KOH as pure as possible? A "dry bucket" as shown in this video is a homemade storage method that works well to protect your NaOH or KOH.

To summarize the video, smaller air-tight containers of NaOH and KOH are put inside a sturdy 5- or 7-gallon plastic bucket. A commercial desiccant (water absorbent chemical) is added to the large outer bucket to remove moisture from the air around the smaller alkali containers. The bucket is kept tightly covered with its original snap-on lid or a screw-on "gamma" lid.

A dry bucket has other important benefits. It will safely contain spills if an NaOH or KOH storage container leaks. It will also discourage children and pets from getting into these chemicals.

Caution: You will want to add a proven child-resistant closure for the best safety with children.

IMPORTANT: Do NOT put desiccant in direct contact with NaOH or KOH! The desiccant must go in the larger dry bucket OUTSIDE the alkali containers. It will NOT function properly if put directly in with the alkali.

Why won't the desiccant work this way? NaOH or KOH is a powerful desiccant in its own right -- MUCH more powerful than the safer chemicals commercially sold as desiccants, such as silica gel, calcium sulfate, calcium chloride, etc.

If put together in the same space, the NaOH will absorb any water out of the commercial desiccant as well as absorb any water vapor in the space. The "official" desiccant will never be functional as long as it is in the same space as NaOH or KOH.

So what's the point of having desiccant in the outer bucket? The smaller containers are the first and best defense to keep the alkali inside from absorbing moisture. No container is absolutely perfect, however. The desiccant dehumidifies the air in the larger bucket to add an extra layer of protection.

More discussion about the dry bucket storage method by SMF member KCHaystack....