Adding oxalic acid to soap -- good idea?

[DeeAnna]

A person recently contacted me and wanted to know how much extra NaOH was needed to neutralize oxalic acid. Her idea was to add oxalic acid to a soap batch, just like people add citric acid, vinegar, etc.

I didn't answer her question directly because I haven't done the calculations to find the answer she wanted. But I did a bit of research on the uses and toxicity of sodium oxalate, which is the chemical created when NaOH reacts with oxalic acid. I learned sodium oxalate is a "reducing" chemical, and it can be used for removing rust and bleaching color. It is fairly toxic to humans when ingested, just as oxalic acid is.

She told me the oxalic acid would be an ingredient in laundry soap, not bath soap. I found myself wondering if it is wise to include this ingredient in a product meant to be used regularly to wash clothes. (That is the impression I got from what she said, but I'm not absolutely sure about that.)

I suspect oxalate residues left in fabric might be irritating to the skin, just as oxalic acid is irritating and drying. And I'm not at all sure it should ever be used for washing children's and babies' clothes, since everything little kids touch ends up in their mouths.

On the other hand, I could perhaps see sodium oxalate in a stain stick for removing rust spots on fabric, just like oxalic acid (or barkeeper's friend) is used to remove rust stains on sinks, etc.

I'm showing my bias here, but I'm open to being enlightened. Does anyone have any thoughts or experience on this matter?

There's a somewhat different inquiry about the idea of adding oxalic acid to powdered or shredded soap -- see: https://www.soapmakingforum.com/threads/making-a-cleaning-soap-with-oxilac-acid.64686/#post-660238 Other than this, there doesn't seem to be much info available on this topic.

 

[IrishLass]

The only experience I have with oxalic acid is using Bar Keepers friend. I remember the very first time I ever used it several years ago.........and man-alive, was I ever stupid for not wearing gloves. I was cleaning some stubborn stains off my stove with it and a green, 3-M scrubbie, and darn if it didn't suck every last drop of moisture from my hands that no amount of lotion could remedy (at least at first), and maybe even remove a layer of skin off my finger tips along with the stains off my stove. The reason it didn't even occur to me to use gloves back then was because I had grown up watching my grandma use it to clean and polish her Revere-wear pots and pans without ever wearing gloves, and with no worse for wear. My hands and fingertips healed, but boy- lesson learned!

Speaking only for myself, I would never consider using it in laundry soap or any other kind of soap except for a stain stick, perhaps, but maybe not even for that. If it did that^^^ to my skin, it makes me wonder what kind of quick-eroding effects it would have on fabric, not to mention the internal parts of a washing machine and plumbing if used regularly for laundry.


IrishLass

 

[DeeAnna]

Point taken about damage to the washing machine. Thanks for bringing that up, IL. I think oxalate has fairly similar effects as oxalic acid.

I know Bar Keepers Friend is not something I want to use everyday, but it's sure handy for really stubborn cleaning problems. I use oxalic acid for treating my bees to control varroa mites, and I am extremely careful when using it.

 

[SaltedFig]

I've used it in metal restoration work and am familiar with it as a plant component (the lovely lemon of sorrel).

Experiencing how well it dissolves rusty metal, and how acidic it is in water, I would be nervous of any soap that listed it as an ingredient.

A curious idea.

 

[DeeAnna]

I thought it was curious too -- and I'm also nervous about this idea. I am truly unsure whether I should answer this person's question about the weight ratio required to neutralize oxalic acid with NaOH. It may be ethically wiser for me to decline to answer.

 

[lmosca]

I would stay away from oxalic acid, period.

Aside from the dangers of handling it, oxalic acid is quite a strong acid (pKa1 of around 1.3). So if the soap is not stoichiometrically balanced, you'll have a larger SF %, dictated by the amount of oxalic acid.
Its reduction potential is ok, but the oxalate is somewhat kinetically slow in doing that (since it is a 2 electron, proton-assisted process). Its use as a rust-dissolving agent is because it complexes ferric ions (Fe3+) which is soluble.
It might be safe for metal parts though, since it is not an oxidizing acid, and in soap, it will be present as the oxalate salt.
Unfortunately divalent cations, for example, iron-II, calcium, magnesium, form insoluble stuff with oxalate, which is even less soluble that soap-scum. If the reason of making a soap with oxalic acid is just removing rust, I will stick to the considerably safer citric acid or EDTA. Those do not form insoluble stuff.

BTW calcium oxalate is the major culprit of kidney stones and raphides (https://en.wikipedia.org/wiki/Raphide).

Cheers,
L.

 

[DeeAnna]

Most recent chapter in this story -- This person is selling a "laundry butter" that she says is great stuff, but she wants to make it even better.

She is wanting to add oxalic acid to this product for stain fighting. I have been gently but firmly discouraging this and I suggested she look into enzymes as an effective, safer alternative.

She also was thinking of adding sodium percarbonate (oxygen bleach) to the water-based mix. I had to explain that percarbonate breaks down in water, so there is little point to include it in a water-based laundry mix. Either add it to a dry mix or add it to the washing machine load as a separate additive.

She also asked for my advice on how to make the soap rinse out of fabric more cleanly. I briefly explained about the relationship between hard water, soap, and soap scum and ways to control the scum.

I still haven't got an answer about the 60:40 blend of KOH:NaOH she says she is using. She is concerned because the soap is coming out oily. I asked if the lye blend is based on a stoichiometric (molecular) ratio or is a weight-based ratio. I'm guessing weight.

<...sigh...>

 

[earlene]

Sounds like she needs a chemist on staff.

 

[lmosca]

She also was thinking of adding sodium percarbonate (oxygen bleach) to the water-based mix. I had to explain that percarbonate breaks down in water, so there is little point to include it in a water-based laundry mix. Either add it to a dry mix or add it to the washing machine load as a separate additive.

Hopefully, she will not use an airtight container! I've had my share of ruptured, old peroxy compounds in alkaline solutions.
I recently stumbled on a paper that claimed that peroxides, in the presence of carbonates (like some laundry detergents) decompose 10000x faster than in plain sodium hydroxide.
That, and hopefully she won't oxidize all the double bonds present in her formulation. Oh, organic hydroperoxides are fun compounds!

Most recent chapter in this story -- This person is selling a "laundry butter" that she says is great stuff, but she wants to make it even better.

I still haven't got an answer about the 60:40 blend of KOH:NaOH she says she is using. She is concerned because the soap is coming out oily. I asked if the lye blend is based on a stoichiometric (molecular) ratio or is a weight-based ratio. I'm guessing the weight.

<...sigh...>

Stoichiometry is a harsh beast. I've taught it to a couple hundred bio-majors. I never tried it again.

 

[SaltedFig]

It seems like there's some basic's to iron out first before any "interesting" additives are considered.
Getting the oiliness sorted out will make the single, most dramatic improvement to her laundry soap. (Oily?!)

Do you suspect that she has used the ounces/grams weight of the NaOH and used the same weight in KOH?

Actually, that question says that she hasn't given you the recipe yet.

If you're being asked to make recommendations on how to use oxalic acid in her soap recipe, she needs to supply you with the full recipe (including all additives). You cannot be expected to make such serious formulation recommendations blind.

And, I had a peek around, and this is interesting. Oxalic acid might not be the best chelator of choice ...

From: https://www.greatplainslaboratory.c...ntrol-is-a-major-new-factor-in-autism-therapy
Oxalates may also function as chelating agents and may chelate many toxic metals such as mercury and lead. Unlike other chelating agents, oxalates trap heavy metals in the tissues.

"Oxalates trap heavy metals in the tissues".
This picture gives a pretty good idea why this is so:

Most recent chapter in this story -- This person is selling a "laundry butter" that she says is great stuff, but she wants to make it even better.

She is wanting to add oxalic acid to this product for stain fighting. I have been gently but firmly discouraging this and I suggested she look into enzymes as an effective, safer alternative.

She also was thinking of adding sodium percarbonate (oxygen bleach) to the water-based mix. I had to explain that percarbonate breaks down in water, so there is little point to include it in a water-based laundry mix. Either add it to a dry mix or add it to the washing machine load as a separate additive.

She also asked for my advice on how to make the soap rinse out of fabric more cleanly. I briefly explained about the relationship between hard water, soap, and soap scum and ways to control the scum.

I still haven't got an answer about the 60:40 blend of KOH:NaOH she says she is using. She is concerned because the soap is coming out oily. I asked if the lye blend is based on a stoichiometric (molecular) ratio or is a weight-based ratio. I'm guessing weight.

<...sigh...>

 

[DeeAnna]

It seems like there's some basic's to iron out first before any "interesting" additives are considered....

Yes, I think this person has a lot to learn about soap making. If she were asking these questions here on SMF, I'm sure she'd be getting the usual "you shouldn't be selling" warnings from all and sundry.

...Do you suspect that she has used the ounces/grams weight of the NaOH and used the same weight in KOH?

Yes, that's a strong probability in my mind. Or she got the total weight of NaOH needed to saponify the recipe, then just divided up that weight between KOH and NaOH. That would explain the oiliness.

...You cannot be expected to make such serious formulation recommendations blind.

If I thought using oxalic acid in soap was okay, I agree I'd need the recipe to know the full story. Since I cannot recommend using oxalic acid in soap due to my concerns about safety, it's moot whether I know the recipe or not, so I haven't pressed her for more info about her recipe and method.

This incident has been an eye opener to me. I like providing geeky answers to soapy questions, and I enjoy teaching what I know. And I started sharing information with others in an effort to dispell some of the crazy myths and misinformation about soap making. This situation has made me more aware, however, of how my advice and information can be misused or misinterpreted.

 

[DeeAnna]

Hopefully, she will not use an airtight container!

Yes, I agree. Some of the laundry butters and mixes out in internet blogger land are scary. Or utterly ineffective. Or both.

I recently stumbled on a paper that claimed that peroxides, in the presence of carbonates (like some laundry detergents) decompose 10000x faster than in plain sodium hydroxide....

That supports my understanding that oxygen bleach should be added as a separate ingredient to the wash load rather than including it in a water-based laundry soap "butter". I was aware that the bleaching effect of percarbonate (coming from hydrogen peroxide) degrades quickly in water, but I wasn't aware that carbonates accelerated the degradation.

Your info makes me wonder if percarbonate should not be included in a dry laundry soap blend either. Most of the dry blends contain sodium carbonate (washing soda). I know there's not much water in a dry blend, but still ...

 

[lmosca]

Your info makes me wonder if percarbonate should not be included in a dry laundry soap blend either. Most of the dry blends contain sodium carbonate (washing soda). I know there's not much water in a dry blend, but still ...

Hi DeeAnna,
I would say you are quite safe for that, as far as the mixture is kept somewhat anhydrous.
This paper in JPhysChem A reports the following (J. Phys. Chem. A 2013, 117, 1880−1889)

Sodium percarbonate (SPC), with the formula Na2CO3·1.5H2O2, is the adduct of hydrogen peroxide and sodium carbonate1 and is a major component of oxygen bleach utilized industrially and domestically.2Granular SPC is utilized in conventional oxygen bleach,3 in which the surface layer of the granule such as Na2CO3 has an important role, i.e., to protect the internal SPC crystals from decomposition by the effects of atmospheric water vapor.

The decomposition of the superficial layer of an SPC crystal is crucial for protecting the inside from further decomposition. I believe this is due to the fact that anhydrous Na2CO3 tend to form tightly bound decahydrate, which traps all the water. Let's call it a sort of passivation!
I wouldn't say this is going to last forever, but it might grant a 1-2 years lifetime of a properly stored product at low temperatures and reasonable humidity.

One could always test for peroxides. Which is a fun experience to do. The iodine test is the quick one usually, without going for expensive stuff like enzymatic strips.
1 g of substance is dissolved in 100 mL of water and acidified to a pH between 3-5 (acetic acid or sulfuric acid). Then, an aliquot of the solution (5mL) is added to 1mL of a 5% solution of potassium iodide (KI) in water (stored in the dark). A yellow to dark brown discoloration (after 30 seconds, but not further, since after 5 minutes it might be a false positive test) is indicative of the presence of peroxides. One could always prepare a semiquantitative reference scale by weighing out a known amount of percarbonate (or perborate, will work the same), and build a calibration curve. The cool stuff is that if you prepare the solution of KI with water that has been boiled and decanted with some starch (corn or rice) a positive test appears as a deep rich sapphire blue coloration! Chemistry is wonderful!

We currently screen all our waste chemicals for peroxides this way, before going to quantitative peroxides strips for the positive ones.

Cheers,
L.

 

[DeeAnna]

Thanks for elaborating -- I appreciate the extra details. I'm always curious about the chemistry of soap making.

 

[SaltedFig]

Your info makes me wonder if percarbonate should not be included in a dry laundry soap blend either. Most of the dry blends contain sodium carbonate (washing soda). I know there's not much water in a dry blend, but still ...

I like using the percabonate reaction for cleaning, and I use a washing soda/percarbonate mix - it's very effective. I have noticed that it takes hot water to activate well, with cold water it sort of dawdles along. Not super-sciency, but I would think that if you are storing the powder long term (or selling it) it might eventually become an issue, but not so much if you are making some for personal use (unless you are a prepper, I suppose, but then that does add extra layers of long-term storage issues anyway).

On a side note, I had a look at my favourite Japanese brand of washing powder. It doesn't contain percarbonates with the washing soda, they do use enzymes.

The cool stuff is that if you prepare the solution of KI with water that has been boiled and decanted with some starch (corn or rice) a positive test appears as a deep rich sapphire blue coloration! Chemistry is wonderful!

Does it eventually revert to brown?

 

[lmosca]

Does it eventually revert to brown?

It slowly does go back to brownish or colorless, depending on concentration. It is a very slow process, though, and it's mainly due to the re-reduction of iodine to iodide, or loss of the iodine by evaporation/sublimation.
If you want to know why it is blue, the link: https://en.wikipedia.org/wiki/Iodine_test has a nice picture that explains why. It will be far too wordy to explain here using text.
One of the reasons for boiling the starch and decanting the powder away, is so you get rid of amylopectin (branched starch), which is insoluble in water, and keeping only the helical amylose (soluble).

The reaction between potassium iodide (or any other iodide, but potassium is one of the most common commercial compounds), with peroxide(s) is the following:

2I- + H2O2 + 2H+ --> I2 + 2H2O

I- (iodide) comes from dissociation of KI,
H2O2 (hydrogen peroxide) comes from the hydrolysis of percarbonate or perborates in acidic environment
H+ (proton) comes from the acidic environment

Since you are in excess of KI (I-) the iodine molecule forms a complex with it:

I2 + I- --> I3-

this is called triiodide anion, and, alone, it is a rich brown. Then, when this molecule is wrapped up in amylose molecules, magic happens! Magic = some really nerdy exciting stuff happens between the starch and this triiodide.

;-)

 

[DeeAnna]

"...Magic = some really nerdy exciting stuff happens..."

I'm with ya! Love it.

 

[SaltedFig]

Thank you for the detailed info!

Isn't amylose the insoluble one? I always get that muddled, because being an -ose I automatically think it's soluble, but it is marked as insoluble [in the gut] and to be avoided [in the diet] by some folk.

 

[lmosca]

Isn't amylose the insoluble one? I always get that muddled, because being an -ose I automatically think it's soluble, but it is marked as insoluble [in the gut] and to be avoided [in the diet] by some folk.

Dang! Good thing you caught me. I have your same problem or swapping them out.

Yes, the amylopectin is soluble, the amylose is not.

That is in cold water. Boiling the water has the effect of destroying the crystalline structure of the starch in three steps: 1) swelling and hydration, 2) melting of the helical structure of starch (amylose-amylose, amylose-amylopectin, amylopectin-amylopectin), the inter-chain (weaker) bonds are broken and more water seeps in, 3) amylose leaching. The last step is the amylose dissolving.
This whole thing is called starch gelatinization and while the starch is dissolved, it takes a loooooong time to revert back to crystalline structure, expelling water and crashing out of the solution.

L.