Home Cosmetic Science Talk Formulating Sodium Hexametaphosphate Reactions in Lye Soap

  • Sodium Hexametaphosphate Reactions in Lye Soap

    Posted by jiroband on April 14, 2020 at 11:57 pm

    Does anyone have any advice/warnings on adding Sodium hexametaphosphate (NaPO3)to a cold process soap recipe? Cold process soap uses Lye (NaOH, aka Sodium hydroxide), and I was wondering if I could add Sodium hexametaphosphate as a lather enhancer/water softener, or would there be a bad reaction with the Lye? Any other possible dangers?

    Cst4Ms4Tmps4 replied 4 months, 1 week ago 4 Members · 5 Replies
  • 5 Replies
  • belassi

    April 15, 2020 at 4:32 am

    “Sodium Hexametaphosphate was a severe skin irritant in rabbits, whereas a 0.2% solution was only mildly irritating. A similar pattern was seen with ocular toxicity.”

  • jiroband

    April 15, 2020 at 4:03 pm

    Thank you, Belassi! 

    I saw that information as well, but it is my understanding that when researchers do these experiments, they tend to bandage (whatever the chemical is) directly to the skin and leave it there for hours. In a soap, it would be in contact with the skin for seconds before it is rinsed off. You can find this ingredient in DIY online stores for making a variety of things, including soaking bath salts for a “luxurious bathing experience.”

    I also saw that it is a common ingredient in whitening toothpaste. It is even available in “food grade” and used commercially for: “cheese powders and dips, breakfast cereals, beer and bottled beverages, imitation cheese, frozen dessert, packaged eggs and artificial maple syrup.”

    I would still like to know if it would cause a dangerous reaction with lye.

  • lmosca

    April 15, 2020 at 4:35 pm

    polymetaphosphate starts to hydrolyze at pH above 10. You get metaphosphate that is not a water softener. (It’s appalling how little information you can find in literature about this pretty much ubiquitous stuff).

    Being a very ionic species, you would have a decrease in lathering (like you would with salt), otherwise, depending on how much you are putting in your bar, you might create a brittle bar that breaks apart. I suppose for water softening, however, you don’t need much.

    For lathering, you can always change your oil composition to a higher capric/palmitic fraction, add sodium lactate, increase ricinoleic fraction up to 5%, or reduce your superfatting. 

    For water softening, you can use sodium citrate (if you are worried about being naturally derived), or tetrasodium EDTA (best option), which also prevents rancidity in the bar in conjunction with BHT. 

    • Cst4Ms4Tmps4

      July 22, 2023 at 6:17 pm

      I cannot create new post since the revamping of this site. I do not know why. Maybe only paid members can post, and non-paid members can read only. Do tell me how to post, if non-paid members can also post. This post is considered very old, no one would see it and respond to it. (The old system of this site has a check box asking people if they have anything to contribute to the old post as soon as a post becomes old. Probably only a few months old is already too old.)

      I agree with you that there is so little information about sodium hexametaphosphate when in fact it is a common and inexpensive chemical.

      You mentioned about it hydrolyzes at pH above 10 attracts me. Seems like you know something about it and I read something about it. I thought it is a cool and smart stuff. Information that I found says it is awesome and sentient. Suddenly, an article says that it is not really used in toothpaste as certain level of moisture/water causes sodium hexametaphosphate to hydrolyze. It does not say much afterwards whether it is a good or bad thing. I presume it is a bad thing because it will have long replaced pyrophosphate in all toothpaste brands otherwise. It also does not talk about pH, only water content in toothpaste that starts that hydrolyzation.

      Another confusing thing is pH. No mention of pH-metal ions interactions since information about it is so basic (limited to what it is rather than how it works and how to best utilize it). However, there is an article stating chelated calcium ions are released in stomach acid, returning calcium to the body. I presume sodium hexametaphosphate is useless as a chelating agent in low pH, but I do not know how low is “low” that (the upper limit) it starts to be more and more useless. (It is all about calcium because sodium hexametaphosphate I found the most is in tartar and plaque articles. Zero information about other metal ions)


      1. What happens when sodium hexametaphosphate hydrolyzes? Is it a bad thing? It is rendered useless? It becomes weak in chelating stuff?

      2. I mentioned ‘smart’. It is the impression that I get. Sodium hexametaphosphate selectively binds to “teeth type calcium” on teeth creating a protective layer protecting teeth, while selectively removes excess calcium ions and interfering “plaque/tartar type calcium”. If it is not smart then I do not know what is! But, is it true?

      3. What happens to sodium metaphosphate in different pH? pH is perhaps the most important factor that decides it works or fails with certain metal ions.

      4. EDTA seems to be common and sort of inexpensive (actually twice the price of sodium hexametaphosphate in where I live), and above all, kind of remain effective in a wide range of pH, why it is not used in toothpaste? Because it is not as smart as sodium metaphosphate knowing exactly how to enhance “good calcium compounds” and to disrupt “bad calcium compounds”, and knowing exactly when to release calcium ions?

  • jiroband

    April 15, 2020 at 9:35 pm

    Great - thank you so much, Imosca!

Log in to reply.