NaOH/KOH hygroscopic...

[Sapo]

OK, so we all know how hygroscopic hydroxides are... my question is:

Just how hygroscopic are they? At what point do they stop absorbing atmospheric moisture? More specifically, I make 50% solutions, will the concentration still drop over time, or is that sufficiently dillute?

Thanks for the input!

 

[SaltedFig]

Very.

An experiment you can do yourself, to see just how hygroscopic the crystals are, is to weigh a few ounces of NaOH or KOH in a lye-proof container, and leave it on the scales.

If your scales are sensitive and there is moisture in the air, you can watch the weight increase! After only a few minutes, even the most ordinary scales will register a weight gain. After a slightly longer time, you can see the crystals beginning to dissolve into solution.

Having the hydroxide in solution alleviates part of the problem, but then you have the reaction with carbon dioxide, which forms sodium carbonate (the ash you see on some soaps).

Either way, your hydroxide, in crystal or in solution, will lose strength over time when exposed to air. This happens very quickly with the crystals, but a solution will also be affected if left exposed.

OK, so we all know how hygroscopic hydroxides are... my question is:

Just how hygroscopic are they? At what point do they stop absorbing atmospheric moisture? More specifically, I make 50% solutions, will the concentration still drop over time, or is that sufficiently dillute?

Thanks for the input!

 

[Sapo]

True, forgot about CO2 - This would effectively neutralize the lye over time, regardless of moisture presence.

Very tighly sealed container - this is the part that boggles me.

 

[SaltedFig]

This thread might help (a couple of resident experts on this very topic)

http://www.soapmakingforum.com/showthread.php?t=59316

 

[IrishLass]

I master-batch my lye at a 50% concentration for long-term storage in re-claimed laundry jugs made of HDPE with tight-fitting lids, and so far, my master-batches have never weakened over time- even well over a year. I always weigh my storage jug before and after each soaping session, and my "before" weights for the newest soaping session have always matched my "after" weights from the previous soaping session.


IrishLass

 

[jcandleattic]

ditto for me what IrishLass said. This is exactly what I do where lye solution and masterbatching are concerned.

 

[DeeAnna]

"....Just how hygroscopic are they? At what point do they stop absorbing atmospheric moisture? More specifically, I make 50% solutions, will the concentration still drop over time, or is that sufficiently dillute?..."

I don't have a sure answer is to your question, but I ~think~ from some of the chem data I checked that NaOH will continue to absorb water from the air until it reaches around a 15% to 20% NaOH concentration.* I cannot absolutely confirm that, but that's my best guess-timate at this point.

What I do know for sure is that a 50% NaOH solution is not stable -- it will still absorb water vapor from the air. Storing it in an environment with very low humidity is a good idea. I like KCHaystack's "dry bucket" idea a lot for this reason. I store my dry NaOH and KOH as well as my masterbatched lye solutions in dry buckets.

***

* A way to find out if I'm all wet (or not) would be to weigh some solid NaOH into a suitable open container and weigh the combined starting weight of the NaOH and container. Then put the container in a safe spot in the open air. Periodically weigh the container and its contents until the weight no longer changes. At that point, you're done. That's your combined final NaOH and container weight.

Absorbed water weight = (Final NaOH and container Weight) - (Starting NaOH and container Weight)

Final NaOH concentration % = NaOH starting weight / (NaOH starting weight + Absorbed water weight) X 100

 

[Sapo]

I master-batch my lye at a 50% concentration for long-term storage in re-claimed laundry jugs made of HDPE with tight-fitting lids, and so far, my master-batches have never weakened over time- even well over a year. I always weigh my storage jug before and after each soaping session, and my "before" weights for the newest soaping session have always matched my "after" weights from the previous soaping session.


IrishLass

Good idea with the weighing. Certainly a more elegant indicator than measuring the concentration over and over, which is what I would've done :headbanging:.

* A way to find out if I'm all wet (or not) would be to weigh some solid NaOH into a suitable open container and weigh the combined starting weight of the NaOH and container. Then put the container in a safe spot in the open air. Periodically weigh the container and its contents until the weight no longer changes. At that point, you're done. That's your combined final NaOH and container weight.

Absorbed water weight = (Final NaOH and container Weight) - (Starting NaOH and container Weight)

Final NaOH concentration % = NaOH starting weight / (NaOH starting weight + Absorbed water weight) X 100

On it.

Thanks all!

 

[DeeAnna]

I should also say I was focusing on the idea of the NaOH being in the open air, and not so much with it being stored in a container.

My results with solid NaOH and 50% masterbatched solution is that they do not degrade over time if the storage container is as sturdy as possible and is kept tightly closed as much as possible. I do what Irish Lass and Susie do -- measure changes in weight over time -- and that has given me confidence that this works. The point is to minimize the amount of fresh air (with a fresh dose of carbon dioxide and water vapor) that gets into the container.

The dry bucket takes this idea a step further by surrounding the NaOH storage container with a second container of low humidity air, but it's not absolutely necessary -- it's a belt-and-suspenders solution that appeals to my experience working as a tech in a chemistry lab.