Does sodium hydroxide go bad?
September 4, 2004 11:53 AM   Subscribe

ChemistryFilter: Can NaOH lose it's potency? Is there any easy way to test it?
posted by dejah420 to Grab Bag (14 answers total)
 
There is probably a chemist who can supply a better answer, but I'll give it a shot: yes and yes. I use a lot of NaOH at my job, and it is my experience that old NaOH loses its potency. My guess would be that the -OH group bonds with free hydrogen and turns into H2O. You can measure its potency by measuring pH. 'Course I just came off of a 14 hour graveyard shift, so sometimes I make things up.
posted by kamikazegopher at 12:06 PM on September 4, 2004


I tried that...assuming that the pH of fully active NaOH should be 14 (I think)...I used two samples, one which I know is fully potent and the one which I think is problematic. (And I tested the water for the solution before adding the lye so that I had a baseline there, just in case.)

I'm using lab grade litmus with a range up to 14. Both solutions turned the litmus deep indigo blue...which is not a color on my pH chart...so I have no idea what's going on there.

But knowing that the NaOH *can* lose potency is a big help. That may be what's causing my problems. I wasn't sure if it remained chemically stable or could be dilluted by air access. I'm willing to bet that my problems with the last two batches of soap I've made is that somewhere along the supply chain, this batch of NaOH that I just bought was left open. Darn it.
posted by dejah420 at 1:43 PM on September 4, 2004


Are you keeping it in an airtight container? It could be hydrating from the moisture from the air, which would make it heavier and thus less reactive per weight unit.

The 2NaOH + H2 -> 2Na + 2H2O reaction seems unlikely to me, but I'm not a chemist either so what do I know.
posted by fvw at 2:35 PM on September 4, 2004


I keep it in an airtight container, locked in an industrial closet with silica "sea dry" stuff that you normally keep on boats, but there's no telling if my supplier kept it dry. I've just been using the same recipes for soap for ever, and I've never had one fail like these two have failed, and the only thing that I can think of that would cause it would be a NaOH disparity.

But, since the pH test failed, as I don't have a spectrometer handy, I'm going to try making two identical batches...one using the suspect lye and one using lye that I know is fully potent. If the suspect batch turns out squishy and unsaponified, and the potent batch turns out normally...then I'll have (at least for me) proof that it's the lye causing the problems and not something weird in my oils or water.

Then it becomes an issue of figuring out if I can make adjustments so as to use the suspect batch...or if I just need to call the fire marshall and have them come get it. (As NaOH has to be disposed of correctly...I'm not sure it would qualify as a biohazard, but it might when one is dealing with pounds and pounds of it.)

I wonder if there's a safe way to dry it out, as it were, that would make the weight measurements correct. I thought about tossing a silica packet in...but then I remembered that silica tends to store water...and water and lye are thermogenic...and as a nonchemist, I feared that I would just create a sodium hydroxide volcano.
posted by dejah420 at 4:28 PM on September 4, 2004


Remembering chemistry lessons and copper sulfate in specific, heating it should get the hydration away again. I don't think NaOH does anything interesting when heated apart from melting, but definately worth checking in on before you actually attempt it.

However, if it is just water, why not just measure how much water it's absorbed and adjust quantities? Litmus paper is your friend, or you could go by volume if you have fixed-size pellets (or if you're willing to go very approximate, going by volume with granular will work too).

Of course, once you know the lye is bad and you're sure you've stored it properly, send it back to your supplier and demand a refund.
posted by fvw at 5:11 PM on September 4, 2004


The problem would be in defining how much water it's absorbed. I wouldn't begin to know how to judge that. I'm not sure how litmus paper would help...how would that work?

Sending it back to the supplier isn't an option, since I bought it as part of a co-op purchasing thing. Besides, the cost to ship it would probably be 6x the cost of the chemical itself.

And now the nightmare begins of once again trying to find a reliable supply chain.
posted by dejah420 at 6:30 PM on September 4, 2004


is it packaged by weight? if so, you can measue how much water it has absorbed by weighing it. any weight above what it should be is water.
posted by andrew cooke at 6:39 PM on September 4, 2004


Well, except for the fact that I used some of it to make batches, and there's always some level of spillage...and while it was packaged by weight, I didn't see them weigh it and since it was a co-op purchase...which means a bunch of people pooled together to meet a minimum order requirement from a manufacturer, and then split the order when it arrived...the weight could have been off by god only knows how much. And scales all weigh differently.

Granted, if I had an empty container like the one it's in, so I could weigh the container and thus subtract that weight, and add the amounts used in failed and now test batches, and assume that the number I'm looking at could be off by as much as 4-6 ounces...it would still be really sloppy math but it might give me some idea.

Even with that idea, however, that wouldn't tell me how to compensate for the weight when measuring out amounts for recipes.

I think basically, I'm just hosed as far as this bunch of lye is concerned. :)
posted by dejah420 at 6:49 PM on September 4, 2004


Yeah, as you've discovered NaOH solutions get less basic with time. It's almost impossible to use analytically---normally if you need to titrate an acid, you make the solution alkaline by dumping in known mass of dry base and retitrate with a simple acid like HCl.

The problem is carbon dioxide. It dissolves readily in water and is mildly acidic:
CO2g + H2Ol ---> H+aq + [HCO3]-aq
This reacts with the dissolved hydroxide:
Na+aq + OH-aq + H+aq + [HCO3]-aq ---> Na+aq + H2O + [HCO3]-aq
I've left the sodium and bicarbonate spectator ions in the second reaction so you can see what's happening, but they have no effect on the reaction. Any (strong, unbuffered) base is not stable in air.

It's very difficult to dry NaOH, it's really hydroscopic. It forms a gel with water which can be hard to break down. You could try baking it in a really hot oven, but be sure to seal it from the atmosphere as it cools. Keep it tightly wrapped with dry silica gel if you need high purity.

Who did you order from that made you get a large quantity? Fisher Scientific or Sigma-Aldrich will both ship very small quantities (250g or so) if that's all you need. Sodium hydroxide is really cheap.

What's your application? With a bit more info, other solutions might be better.
posted by bonehead at 8:33 PM on September 4, 2004


BTW, baking NaOH is fairly safe. It's the same stuff they use to make oven cleaner.
posted by bonehead at 8:39 PM on September 4, 2004


I'm using it to make soap...and thus, I actually go through a fair amount of it...easily 5+ pounds a month now...and I'm hoping that once I get my supply chain nailed down and can launch the business outside of the local spas and salons and clients I already have, that I'll need to double or triple production. (hope springs eternal when the ink on the business plan is still wet, no?) As an example, a 2# test batch of soap generally requires about 96g of NaOH.

That said, I'm going to check with both those suppliers and see what smallish large amounts they'll ship. There's a chemical company over in fort worth that makes NaOH, but for some reason, they won't ship anything smaller than a pallet of it, they'll sell a 50# bag if you go get it, but I'm just not willing to break out my passport and make a 4 hour round trip, so I have to find another solution.

I'd rather toss what I have and replace it that try to dry it in the ovens that I use for food though...as you've mentioned, it's an exceedingly inexpensive chemical. (Unless, gods forbid you find yourself in the position of needing to buy it at the consumer level...that's an insane price for lye.) What I am going to try, because I already have the components handy is packing it into 1# ziplocks, put that ziplock in a ziplock full of silica, close the outer bag and hope that the silca pulls some dampness out.
posted by dejah420 at 12:56 PM on September 5, 2004


I can remember years ago calculating the concentration of a sodium hydroxide solution by measuring its specific gravity with a hydrometer. If you to adjusted your recipes to use a lye solution instead of dry lye, you could do the same.

Hydrometers are simple (essentially a float with a scale) and easy to use. They're widely used in various endeavors like brewing beer, making maple syrup, preparing ceramic glazes, testing car batteries, etc. Homebrewing shops, auto and lab supply places carry hydrometers. Unfortunately a quck Googling on "lye hydrometer soap recipe" turned up a bunch of Anarchist Cookbook references.
posted by TimeFactor at 2:12 PM on September 5, 2004


You may want to look at picking up a desiccator. They're a bit of an investment, but they're not hugely expensive. A quick google turns up this and this.

An other option might be one of those vacuum bag systems. That would work quite well too, especially if you dropped some silica gel in with it.

By the way, silica gel isn't magic---it stops absorbing water after a while. To regenerate it, heat it in the oven at 250 F or so for a couple of hours. This is utterly safe---I've done this every day of the week for more than a decade. Silica gel is a fancy name for highly pure sand.
posted by bonehead at 5:46 PM on September 5, 2004


TimeFactor, it's an interesting idea...but I'm not sure I want to recalculate recipes it's taken me a while to perfect just so I can use this one bad batch of NaOH. :) Basically, I'm an art school of doom kinda person, not really a mad scientist. (Well, I would be a mad scientist, but the home owner's association won't let me climb on the roof during storms and scream "Give my soap....LIFE!" anymore. )


Bonehead, funny you should mention that...I was researching building a big dessicator cabinet today. Something big enough to store the chemicals, but also big enough to use as a fast-cure cabinet for soaps and bath bombs as well.

(Now...this thread is bound to be picked up by Carnivore, or whatever they're using this week...we've got mention of the Anarchist's Cookbook, talk about chemistry, mention of chemicals which are becoming restricted and difficult to find outside of commercial applications, and now the word bomb...as in bath bombs, those big balls one tosses into the tub to create a giant bathtub alkaseltzer...)
posted by dejah420 at 9:12 PM on September 5, 2004


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