Quick Question re: EDTA & Trace

Does adding it to a formula change the trace rate at all? In other words, does slow down, accelerate, or do nothing to trace?

I don't see a reason why it would have any effect at all, as it does nothing but remove di-valent metal ions from solution. Essentially converts tap water to distilled water.

It has essentially no effect at all on mono-valent metal ions (sodium and potassium), and none at all on hydroxyl ions.

I've only made a few batches of cold processs soap, and haven't noticed any real effect, they behaved as expected for me.

Actually, tetrasodium EDTA does react with NaOH in the high concentrations we use for soaping. Yes, I know EDTA can chelate multivalent metallic ions, but that's obviously not all the chemistry it can do. I haven't studied the reasons why; all I can tell you is I know the mixture of EDTA and NaOH in water quickly thickens and turns opaque white.

But when this thickening is done, its done -- it doesn't keep thickening as if it's soap batter coming to trace -- so I've learned to pretty much ignore it. For the record, I always use EDTA and I usually get 15 to 30 minutes of open working time with my soap.

I can see a definite effect on the time to trace from the fragrances I use and from the amount of stick blending, but EDTA doesn't seem to be a culprit in altering time to trace.

Ok, what does the addition of EDTA do for soap making?

There is lots of discussion about it here. Use this search phrase in google:

edta site:soapmakingforum.com

Also: https://classicbells.com/soap/EDTA.html

Granted, I've not been making soap with it, but I've NEVER seen EDTA react to NaOH in any way except to dissolve. I've added the free acid to NaOH solutions, I've dispersed the acid (and di-sodium salts, with are also not particularly water soluble) in water and added NaOH solutions, I've dissolved tetrasodium EDTA in water and added lye for making soap, and added the tetrasodium EDTA to the lye after I dissolved it -- never seen anything but clear liquid.

Very interesting that you get a white, thick solution (or dispersion). I don't know of any reason to expect such a reaction between EDTA and lye, the sodium salt of EDTA should be infinitely soluble in water, and it's not affected by high pH -- low pH can displace metal ions and force the sodium ions off and cause it to precipitate, but that won't happen in soap.

Very strange.

Anyway, EDTA is a metal chelator -- it ionically binds multivalent cations and holds them out of solution in a water soluble form. This means that calcium and magnesium ions in water will not form insoluble fatty acid salts (they attach to two, not one fatty acid, and the resulting molecule is very insoluble in water -- to whit, soap scum). So it helps prevent soap scum and makes the soap a little more water soluble. And since oxidation of fatty acids and fats is greatly accelerated by the presence of multivalent metal ions (calcium, magnesium, iron) it will help prevent DOS or general rancidity. It's NOT an anti-oxidant, it just removes metal ions from solution.

Makes sense. Haven't heard anyone complain about acceleration in my research before asking, so guess I'll treat it like it doesn't do anything.

".... I've not been making soap with it, but I've NEVER seen EDTA react to NaOH in any way except to dissolve..."

I understand your point perfectly. But I HAVE mixed EDTA and concentrated NaOH solution (meaning the typical concentrations used for making soap) and I can guarantee that there is a chemical reaction. Clearly your perception and the reality are two different things.

I also seen the same thickening/opacity in my NaOH solution when adding liquid EDTA. It's not unmanageable for me though.

What were the conditions under which this reaction between EDTA and lye took place (concentration and temperature of lye, amount of EDTA solution)? I'd like to see if I can replicate it. Was there any glycerine in the lye?

I'm concerned that if there is indeed some sort of chemical reaction leading to the formation of a polymer that there will be much less EDTA in the soap than intended. I did a quick search on the internet today, and found nothing much about this, nor anything else concerning reactivity of EDTA.

I'm an analytical chemist, not a polymer chemist, but I don't see anything about EDTA and lye that would cause a true chemical reaction. Acid groups aren't going to condense into a polymer in just strong lye, I suspect there would have to be something to condense them with, on their own there just isn't anything reactive enough to get actual re-arrangement.

I'll try a 33% lye solution and make up some 39% tetrasodium EDTA and see what happens -- I use 0.5% EDTA as percent of oils, if you are using something different let me know.

I want to make soap for Mom in the next couple days (too wet to work outside, had heavy thunderstorms this afternoon) anyway, good time to do experiments.

I normally add 0.5% tetrasodium EDTA powder (about 85% purity) based on the paste weight (fat + NaOH + water), not the fat weight. Others typically use 0.5% based on the fat weight. I use anywhere from 30% to 40% NaOH concentration for soaping.

You can mix tetrasodium EDTA to slightly above 50% w/w in distilled water at room temp -- you aren't limited to a max of 39%. That's just how it's sold for convenient use in industry.

I don't have access to science journals anymore, so I can't do a proper literature search. I haven't found any concrete info from the sources I can access -- just one obscure comment that "changes might occur above ph 11" and nothing more. That raised more questions in my mind than it answered.

That's what I've been using, dry powder dissolved in the water before making lye, or once added to the lye since I forgot it before. Just goes into solution, no excitement.

It is possible for there to be a carboxylic acid condensation if there is something in the solution to form an ester linkage -- glycerol is one possibility -- but I don't think the conditions are energetic enough for polymerization unless the other compound is highly reactive (imide or amide for instance). I suspect we can rule out things like phosgene (I work in a Lexan plant -- polycarbonate is a condensation of bis phenol A and phosgene), and I cannot imagine what else there would be in a typical soapmaking environment.

I did find an article describing short polymers produced from freshly made imides and EDTA, but I don't know how such things would appear in the lye for making soap unless it's something in the EDTA powder.

A mystery for sure.

I will get fired up here in a minute and try a fluid hot process soap (small batch) and use 33% lye and add my dry EDTA to it and see what happens when it's quite hot.

Just finished a quick hot process soap. 33% lye, EDTA dissolved easily with no signs of murkiness or thickness.

Wonder what's going on here... I have all the mysteries I can deal with keeping things working at my job!

I think I will have to get some sodium lactate (or make some, I have 88% lactic acid around somewhere for brewing) to get really fluid HP soap, but it stayed quite mixable long enough that I think I can try some tester bars with individual scents rather than CP.

I also use tetrasodium EDTA in my soap (in 39% solution form), and it reacts the same way for me as it reacts for DeeAnna if I mix it with my 50% lye solution. After I saw the reaction during those first few times of adding it to my lye solution, I decided to add it to my oils instead. For what its worth, during those times I added it to my lye solution, I had already added 3% of sodium lactate to it in the form of a 60% solution.


IrishLass

I will try with 50% lye next time I make soap (probably this weekend). It may just be the EDTA precipitating due to the high sodium concentration (salting out, eh?). Hopefully NOT polymerization....

I add mine to the oils at the same time I add the sugar/water.

Here's evidence of what we're talking about, Psfred.

I made 4 mixtures of NaOH in distilled water -- 50% w/w, 37.5%, 25%, and 12.5%. To each of these, I added 2 grams of freshly made tetrasodium EDTA solution, 50% w/w in distilled water. (edit: EDTA was purchased in powder form from Lotioncrafter.) Mixtures were swirled to mix.

50% NaOH + EDTA -> Mixture initially had a translucent curdled look, but quickly changed to a dense white precipitate. Slightly thickened.
37.5% -> Slight amount of white precipitate. No apparent thickening.
25% -> No apparent change. Solution remained water clear.
12.5% -> Ditto

Somewhere between a 25% NaOH concentration and a 37.5% NaOH concentration is the tipping point between the EDTA obviously reacting with the alkali vs. not apparently reacting.

I generally do what the others do -- mix the EDTA and other additives into the fats and then add the lye solution. I don't know whether that really accomplishes much besides hiding the formation of the precipitate, but it makes me feel better.

Edit: After thinking about it for a bit, most of us tend to soap with a final NaOH concentration of 33% or less, even if we use a 50% masterbatch, so I imagine adding the EDTA to the fats, etc. probably does do some good -- keeps the overall NaOH concentration lower.

Edit: I'll add distilled water to the two more concentrated solutions to bring them down to, say, 33% NaOH and see if the precipitate clears.

First photo: All four mixtures of NaOH, distilled water, EDTA. L to R: 50% NaOH, 37.5%, 25%, 12.5%
Second photo: 50% NaOH. Opaque white precipitate
Third photo: 37.5% NaOH. Slight white precipitate.
Fourth photo: 25% NaOH. Water clear. (The 12.5% mixture looks the same)

I'm very interested to see if that EDTA re-dissolves with added water (as I intended to do just that). Room temp caustic just isn't strong enough to cause a polymerization of a fairly inert compound like EDTA, it would have to rupture the double bond or something. Decarboxylation would cause the release of CO2, and as far as I know (which isn't all that much, I'm definitely NOT a polymer chemist) carboxyl groups won't react with each other in any other way.

The precipitate partially reverses, but not completely. (edit: I drew this conclusion too soon -- see my next post made about 15 minutes after this one.)

I diluted the 50% NaOH solution first to 33% and then to 25%. There was an immediate big change in the amount of precipitate when the concentration dropped from 50% to 33% but only a slight reduction in the precipitate when I diluted from 33% to 25%.

I also diluted the 37.5% mixture to 33% and 25% with similar results.

I did not observe evolution of gas in any of the mixtures.

In each of the two photos below:
Left: Original 50% mixture after this solution was diluted to 25% NaOH concentration.
Right: Original 25% NaOH solution (right).

And now to do a last check and rinse all this down the drain. I'll report back if there's any change, otherwise I'm signing off for tonight. Time to read a good book and chill!

edit: Wups. Spoke a wee bit too soon about the precipitate not clearing. All mixtures are now water-clear.