Uncovered lye solution; does it degrade?

[AliOop]

Hi everyone, I searched the threads on this forum and didn't find the exact answer.

I am researching to respond to a specific FB post about uncovered lye solution. Aside from the obvious safety issue (who leaves lye water uncovered?? and why??), and water evaporation, can anyone provide a scientific answer as to whether the lye solution degrades when left uncovered for a couple of days?

Perhaps I was confused with the fact that dry lye crystals degrade and absorb moisture when exposed to air, but I was pretty sure I read that the liquid solution also degrades if not stored in an airtight container. I was thinking that I read this in Scientific Soapmaking, but have loaned my copy to someone and thus cannot check it. Thanks in advance for any help you can provide!

 

[amd]

I wouldn't think the lye in the solution would degrade. I would however have the same concern regarding safety, as well as evaporation. When I pre-mix my lye solution I will often leave it uncovered until it has cooled a bit, reweigh it and add any lost water (here's where knowing your container weights comes in uber handy), and then cap it.

 

[shunt2011]

I agree with amd. I mix but I generally cover my cooling lye lightly with a lid or paper towels until cooled.

 

[AliOop]

Thank you! Another person on the original question thread on FB said this: "When exposed to air, water and aqueous solutions take on CO2, generating carbonic acid. This is the reason pure distilled water actually has a pH of around 4 unless it is kept in an airtight container. Carbonic acid will neutralize some of the free OH- ions in solution, leaving you with CO3(2-) and HCO3(-) ions in place of your OH-. So, over time, your lye solution will slowly weaken if left exposed to air. However, I wouldn't stress about 3 days as long as you aren't selling the product."

That is very similar to the information I recall reading, but I guess I used the word "degrade" instead of "weaken." Not a scientist so not familiar with the terms of art that would have been more appropriate to describe this.

 

[DeeAnna]

"...can anyone provide a scientific answer as to whether the lye solution degrades when left uncovered for a couple of days?..."

Yes, NaOH solution will degrade if left exposed to the open atmosphere for two reasons.

NaOH reacts with carbon dioxide in the air to form sodium carbonate (Na2CO3, washing soda). This is the white scum that people sometimes see on top of lye solution. If enough time passes, all of the NaOH exposed to open air will eventually convert to Na2CO3.

Concentrated NaOH solution as well as the solid chemical are strongly hygroscopic, meaning NaOH will absorb water vapor from the air. This water absorption and dilution will continue until the NaOH concentration in the solution is in equilibrium with the water vapor content in the air. (Or until all of the NaOH has reacted to Na2CO3.) I don't remember what that concentration is off the top of my head, but what I do remember is it's a lot lower than the usual NaOH solutions we use for making soap.

If people are skeptical, just take a spoonful of dry NaOH and leave it in a safe place in the open air. See what happens. If you don't care to try this yourself, I have a short video of what happens --

​

Extra credit questions for after you watch the video or try it yourself -- Do you believe the absorption of water should stop after NaOH is in a liquid solution? If your answer is "yes," then why does the amount of liquid in the spoon continue to increase?​

NaOH, whether dry or in solution, should always be stored in an air tight container for safety, to limit exposure to CO2, and to minimize absorption of water vapor.

 

[AliOop]

Oh DeeAnna, I was hoping you'd join the conversation! During my research, I found one of your YT videos and almost commented there to ask. But I see how active and informative you are on this forum, and I'm so appreciative of your help. Do you mind if I copy and paste your answer over to that FB post, either with or without source attribution?

ETA: NM, the original poster deleted the FB thread. I think she felt embarrassed to admit that she'd left her solution uncovered since so many responded about the safety issues, and a couple of us got into a debate on the science issue.

 

[DeeAnna]

"...When exposed to air, water and aqueous solutions take on CO2, generating carbonic acid..."

I agree that's true for dilute aqueous (water-based) solutions and pure water, but for the concentrated NaOH solutions we use, there's no chance any carbonic acid will form.

There are many discussion groups on FaceBook and elsewhere that I won't belong to and a few others that lurk in and never say a peep. The culture of some groups is just plain mean and rude. In others there's an entrenched "don't confuse me with the facts" mindset. In either case, I don't usually say much.

So ... I don't mind if anyone rephrases what I say in your own words to make it sound like the info is coming from you personally. On the other hand, I'm not comfortable having my name and direct quotes used in arguments or debates in which I am not personally involved. Makes me feel like there's a target stuck to my back and I have no way to defend myself.

 

[AliOop]

That totally makes sense - thank you! The post ended up being deleted, so it's moot... for now.

I agree about the "lalala-not-listening" mentality that is prevalent on FB - so many low-information soapers. Not to say that I'm never wrong, but I for one appreciate being corrected and am happy to publicly recant if I've been mistaken about something.

And I definitely never want to quote + name someone without her permission, that is for sure. Although I've been active on some of the FB soap pages in order to pay forward the help that I've been given, it has become wearying because most people won't be civil when disagreeing about things. Shame.

 

[AliOop]

Ok, I ended up posting a new thread bc the person who was debating me wanted a response, and I am not into DM'ing with people I don't know. DeeAnna, I posted a rephrased and unidentified version of your response (along with the two others I received from other sources). Your technical terms were much more accurate, but I was purposely trying not to quote you so exactly, so that meant using some far less precise wording. Please let me know if my rephrasing works for you:

"NaOH, whether dry or in solution, should always be stored in an air-tight container for safety, to limit exposure to CO2, and to minimize absorption of water vapor. Otherwise, it reacts with carbon dioxide in the air to form sodium carbonate (aka washing soda or soda ash - also the white scum that can be seen sometimes on top of the lye solution). After enough time, all NaOH exposed to open air will eventually convert to washing soda.

"Concentrated NaOH solutions will also absorb water vapor from the air. This water absorption and dilution will continue until the solution reaches equilibrium with the humidity in the air -- or until all of the NaOH has reacted into washing soda. This will result in a much weaker solution than what we use for making soap."

 

[DeeAnna]

I'm going to simplify* and rephrase some chemistry information into terms most of us are more comfortable with. This is my detailed geek explanation behind the general advice I gave in my first post in this thread.

Let's say I make a saturated NaOH solution (roughly a 50% concentration) and cool this solution to normal room temperature. I then put some of that 50% NaOH solution in an open dish and let it sit in the open room air. What I will find is this solution will absorb water vapor from the air if the relative humidity in the air is 10% or higher. **

In other words, the open air has to be as dry or drier than the Mojave Desert before a 50% NaOH solution can't pull water out of the air.

"...Average daytime relative humidity [of the Mojave Desert] ranges from 10 percent to 30 percent...." https://sciencing.com/humidity-mojave-desert-19526.html​

This combination of less than 10% relative humidity (RH) in the air and 50% NaOH solution in the dish, will create what chemists call an "equilibrium." That means the water in the air and the water in the NaOH solution are in balance and are not changing.

***

Most homes are usually more humid than 10% RH, even in winter. For example, the air in my house is about 45% RH today. If I put some of that same 50% NaOH solution in an open dish in my house, the NaOH solution would absorb water from the air and become less concentrated.

Eventually when the NaOH solution had absorbed enough water, it would become diluted enough that it will stop absorbing water. At that point, this diluted solution would be in equilibrium with the higher relative humidity in my house. The water in the air and the water in the NaOH solution would be in balance and would not be changing.

I could measure the NaOH concentration of this more dilute solution using a simple chemistry test (for the chem geeks -- I'd titrate the alkali solution with a calibrated acid solution). This test would tell me what NaOH solution concentration is in balance (in equilibrium) with the relative humidity of the air in my house today. If I do the test correctly, it should tell me the NaOH solution concentration is about 30%. **

***

The following table shows the NaOH concentrations that are in equilibrium with various amounts of relative humidity --

NaOH concentration, weight % ... Relative humidity, % (RH) **
21% NaOH is in equilibrium with 70% RH
25% NaOH ... 60% RH
28% NaOH ... 50% RH
32% NaOH ... 40% RH
35% NaOH ... 30% RH
40% NaOH ... 20% RH
48% NaOH ... 10% RH​

To give one example of what this means --

Many people use a 28% lye concentration (aka "full water") to make soap. This "full water" NaOH solution will absorb water from the air any time the relative humidity is higher than 50%. A 28% lye solution will not absorb water vapor if the air is drier than that (less than 50% RH).​

***

* A BIG simplification in this explanation is that I'm totally ignoring the reaction of NaOH with CO2 (carbon dioxide). At the same time the NaOH solution is pulling water out of the air, it is also reacting with CO2 to make Na2CO3 (sodium carbonate, washing soda). This chemical reaction will also lower the concentration of the NaOH solution. I decided to keep my explanation simple for the sake of clarity.

** R. H. Stokes and Robinson R. A. Standard Solutions for Humidity Control at 25 C. Ind. Eng. Chem. 1949, Vol 41, Issue 9. https://pubs.acs.org/doi/abs/10.1021/ie50477a041

This has been edited and updated for accuracy and clarity.

 

[DeeAnna]

@AliOop My response to your question about rephrasing my stuff -- sounds good!

 

[AliOop]

Thank you @DeeAnna - that was very clear, even the post-script at the end about CO2. I've got a mental framework for the concepts now. Fortunately you are good at math details, because going from concept to details is usually where I mess up!

Interestingly enough, the person who originally got into the debate with me is not responding at all to the new post, despite the tag. Of course, she may be on the other side of the world, or at work. But it's interesting that the confrontation stopped immediately (complete radio silence now) once I posted the various scientific replies I received.

ETA: Ok so that person has now responded, and politely as well. I appreciate all who helped that end well!

 

[DeeAnna]

I finally found the info I was looking for, and I've updated my post #10 . Please re-read post #10 if you have a serious interest in this discussion.

 

[AliOop]

I finally found the info I was looking for, and I've updated my post #10 . Please re-read post #10 if you have a serious interest in this discussion.

Got it - thanks! Some of that science is over my head, but if I read slowly I am following the basic train of thought. I think

Thank you again for helping out on this one. I really, really like to know the what, but also the "why" and "how" ... and sometimes "when" and "where."

 

[DeeAnna]

For all intents and purposes, all those facts and figures in Post 10 are just a TL; DR geeky way of saying the same advice I gave in my first post.

The short 'n sweet version --

Any lye solution for making soap will absorb water from the air unless the humidity is Mojave Desert dry.

 

[AliOop]

For all intents and purposes, all those facts and figures in Post 10 are just a TL; DR geeky way of saying the same advice I gave in my first post.

The short 'n sweet version --

Any lye solution for making soap will absorb water from the air unless the humidity is Mojave Desert dry.

My original FB post on this issue was similarly short but the response was, "I doubt your science." Umm, okay.... apparently some people really NEED the TL;DR version or your intellect is suspect!

 

[DeeAnna]

Even a TL; DR answer doesn't work all that well if you're trying to convince a science-intolerant skeptic.

A person can spend a huge amount of time researching the math and science and writing up a well reasoned, clear explanation, but a skeptic can and often will dismiss and devalue all that hard work in an instant. Many skeptics are not interested in making an effort to learn and grow, they're more interested in getting an easy attention fix by creating drama.

The skeptic avoids the work of understanding the science and tries to create more drama by making a brief devaluing comment like what you heard -- "I doubt your science" or even responding with crickets (silence). And if someone digs really deep and gives a rebuttal complete with theory and detailed info, the skeptic typically dismisses that with an airy "It's over my head" comment.

Anymore, when I decide to research someone's question, my main goal is to find answers to satisfy my own curiosity. I could easily keep that knowledge to myself, but I also like to teach. That is why I have another personal goal of sharing what I've learned with other curious people. If I was on a crusade to convert science-intolerant skeptics, I'd have gone crazy a long time ago.

 

[AliOop]

@DeeAnna that is pretty much what happened here... except that I am the one who actually responded to her last comment with "that's over my head." It truly was, but more importantly, she wasn't actually responding to the issue of whether the uncovered solution weakens over time. Instead, she picked apart some of the science in one of the other answers. I took that as a tacit admission that your science (and something similar shared by another soaper) was correct, and figured that was as good as it would get.

On the plus side, she did thank me for indulging her in the whole discussion. Thus, the whole thing ended much better than I hoped, given how it started. I don't think we would have gotten there without your help, so again, THANK YOU.

 

[DeeAnna]

I'm glad it worked out okay in the end. That is the best outcome you could hope for.

 

[AliOop]

Yes. I try hard to respond politely when people start off rough or rude, because I assume they didn't intend that; perhaps they were rushing or multi-tasking, or something hit a nerve and their emotions flared up a bit.

But as you noted, some folks just love drama. One can usually figure out quickly the category into which they fall, by their subsequent responses.