Sciencemadness Discussion Board

Deep chemistry of metaphosphates

teodor - 16-3-2025 at 17:04

In the fluorine preparation thread I've already mentioned the reaction of sodium metaphosphate with sodium fluoride. The works of madame Remziye Salih Hisar (just realized she is a woman) looked quite interesting and the chemical reaction in her articles are totally unfamiliar. So I decided to try of some reactions mentioned by her.

4 NaNO3 + 4 NaPO3 = 2 Na4P2O7 + 4 NO2 + O2

So, I suppose it should be N2O5 but because the reaction is performed in the molten state N2O5 is instantly decomposed.

Let me explain the idea.

There is no such compound as NaPO3. There is a family of cyclic forms from (NaPO3)3 to polymers composing of cycles. Mixture of polymers (NaPO3)n with high value of n have the name Graham's salt or sodium hexametaphosphate (they sell it by the name of "Calgon" as a water softener by the way. This story was started with discovery of EDTA and by analogy they found similar properties in metaphosphates. See Oscar Quimby "The chemistry of sodium phosphates"). "Hexa" is misleading here because it is just one of the building units. The good introduction to the composition of this class of compounds is

Attachment: audrieth1948.pdf (6.6MB)
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You can see metaphosphoric acids as an endless attempt of H3PO4 dehydration. P4O10 has some particular 3D form which (HPO3)n never can fold to. But it makes a lot of cycles in plane and in space. When you add water it slowly goes back to H4P2O7.

The interesting thing is that this compound breaks water molecules during hydrolysis. H+ part goes to one part of the broken P-O (-O -> -OH group) and OH- goes to P forming P-OH.

The reaction with NaNO3 works quite similar, -O part instead of H+ takes Na+ and P takes O-Na.

So, basically, when depolymerisation occurs, O-P connections from cycles becomes O- and P which gets oxygen either from water or from the oxyacid salt (NaNO3). So, for this type of reaction water and oxyacids (salts) work similar.

So, it sounds quite unusual (for me), so I did a little experiment.

I put 0.21 g KNO3 and 0.39 g (NaPO3)n into a test tube and heated it in a flame of teclu burner.

The first water vapours appeared. It is probably because (NaPO3)n always absorbs some (even if it looks dry).
The second some cracking noise started probably accomplishing KNO3 melting.
And at the third stage a bubbling reaction happens which filled the tube with brown gas.

So, looks interesting. The question how much it goes to completion. So I will plan some quantitative experiments.

(And this thread I designate to discussion of the fascinating chemistry of metaphosphates).

teodor - 17-3-2025 at 04:32

Today I did a fast check of the potential reactions

KHSO4 + 2NaPO3 = Na2KHP2O7 + SO3

and

K2S2O7 + 2NaPO3 = Na2K2P2O7 + 2SO3

in both cases H2SO4 vapors were detected.

I unable to say quantitative comparison of with the reaction

K2S2O7 = K2SO4 + SO3

(both in terms of yield and the temperature required)
but it looks like adding (NaPO3)n into molten K2S2O7 makes a short release of SO3 and the reaction is ceased after several seconds,
while heating K2S2O7 alone requires more time for completion. Reaction with (NaPO3)n goes, as usual, with active effervescence and K2S2O7 alone is not.

Edit: also in a case of KHSO4 + (NaPO3)n charge the water vapors don't appear as in the case of KHSO4 -> K2S2O7 transition around 300C.

I also checked the potential route to SO2:

Na2SO3 + 2NaPO3 = Na4P2O7 + SO2

but was disappointed. It produces sulfur as vapors and a globe inside the melt and on higher temperature the sulfur globe converts in some black substance (sulfide?). The reaction is not going to completion easily and the black globe continues to bubble. I need to think about possible route how sulfur can appear.
There is some smell of SO2 but I can detect its presence only on a cork stopper.
Update: I am not sure the globe is sulfur. I tried to melt my food grade (quite old) Na2SO3 and I've got a dark red liquid. Could be similar to molten sulfur. I should probably try with purified matherial.
If the reddish-yellow globe is really molten Na2SO3 it could little explain the poor SO2 yield. Probably Na2SO3 is not miscible with (NaPO3)n in the molten state and the reaction doesn't go as smoothly as in other cases.

Caution: the molten (NaPO3)n forms a glass mass on cooling which breaks a test tube. But in my experiments I always used excess of the methaphosphate and didn't allow the reaction to go to completion. I would suggest to try the excess of other reagent as a possible way to protect glass.

Update:
My speculation about the reaction mechanism doesn't explain all the the experimental results.

KI + (NaPO3)n -> I2 . Vapours, sublimation of I2 crystalls on the test tube walls

KCl + (NaPO3)n -> Cl2 (?) . The melting temperature is out of range that I use in my test tube experiments, but the mix softens, the smell looks like dilute HCl ("medical smell") but no smoke. The indicator paper shows strong acid.

Na2S + (NaPO3)n -> H2S. Can't explain that except by the present of water in the reagents.
Update: stupid me. I used CAS 27610-45-3 which is Na2S * xH2O
After dehydration the result is quite different. Some yellow-to orange solid with no/low volatility. I assume a polysulfide mix.



[Edited on 17-3-2025 by teodor]

teodor - 19-3-2025 at 00:58

By the analogy of

2 NaO-NO2 + 2 NaPO3 = Na4[O3P-O-PO3] + O2N-O-NO2

the reaction

2 NaO-CH3CO + 2 NaPO3 -> Na4[O3P-O-PO3] + CH3OC-O-CCH3O

has its right to happen. But I think in the range of temperatures which the molten salts mixture can get the connection of organic molecules could be less favorable than the more straight way of dehydration:

NaO-CH3CO + 2 NaPO3 -> Na3H[O3P-O-PO3] + CH2=C=O

( It doesn't require connection of 2 remnants).

There is a way of producing Ac2O by dropping AcOH on a heated mixture of (NaPO3)n , NaOAc and diatomes, it was mentioned in the acetic anhydride preparation thread already. The need of acetic acid probably is explained by the fact that metaphosphate - acetate gives mainly ketene which reacts with the acid to produce the desired compound.

Reflux of the solution of acetate in acetic acid in presence of metaphosphate theoretically can give similar results (probably much safer and slower) but 2 practical issues must be solved:
1. Bumping of the acid/salt mixture.
2. Detection of possible unreacted ketene escape.



[Edited on 19-3-2025 by teodor]

teodor - 21-3-2025 at 02:30

After a few days of experiments I can confirm that Graham's salt, CAS 10361-03-2 is extremely powerful drying agent but a peculiar one. For example, it requires drying before usage and the way it gets to equilibrium is hardly can be compared with other chemicals.
I need several days to systematise the experimental results, after that I plan to publish the first part of them as "getting 100% acetic acid" story.

woelen - 21-3-2025 at 06:40

What I read from your posts is that heating (actually fairly strongly) is needed for drying, and that is a very important difference with many other drying agents (e.g. P4O10, CaCl2, NaOH, CaO). It makes the use of this drying agent for the average home chemist much more difficult, especially because it also attacks glass quite strongly at the temperatures you mention.

teodor - 21-3-2025 at 09:44

Quote: Originally posted by woelen  
What I read from your posts is that heating (actually fairly strongly) is needed for drying, and that is a very important difference with many other drying agents (e.g. P4O10, CaCl2, NaOH, CaO). It makes the use of this drying agent for the average home chemist much more difficult, especially because it also attacks glass quite strongly at the temperatures you mention.


My last experiments I did in the range 114-125C in the boiling anhydrous acetic acid media. At this range it can act with differet speed depending on the compound. It much more eager for sidium ions than for hydrogen ions. So it cannot dehydrate acetic acid alone but it can dehydrate e.g. sodium acetate solution in acetic acid (fast). The boiling point suggest I was probably able to get 20% acetic anhydride solution as the result of sodium acetate dehydration at temperature lower than 125C. I continue quantitative part of experiments and if there is at least your interset I'll keep you informed.
As for aqueous acetic acid it can dehydrate it to 100% where equilibrium between hydrogen pyrophosphate and the acid doesnt cause further dehydration. This is during several hours and not minutes as in the case of NaOAc.
I think it would be possible the same way to dehydrate some salts if they are soluble in AcOH and not decomposing by the action of metaphosphate. (And this IF is a very big one. Most of compounds are decomposing by metaphosphate fusion. The question is - at given temperature).
But ones need a reflux setup for such kind of experiments.

The compound in the acid media acts in solid state (Graham's salt is not soluble in the acid), so I don't think it can attack glass.
The product of the reaction (e.g. Na4P2O7) could be soluble. I can check what is the dry residue after I will evaporate acid. But it is not the first priority. I try to find the most optimal way to dry the compound itself because adsorbed water percent is quite high and part of its power is wasted to self-dehydration. Which is probably not happening below 100C. That's why I said it is the peculiar drying agent, it can coexist with water at room temperature.
Boiling in AcOH dehydrates it but then you need much excess for the primary reaction.

20250318_171824.jpg - 3.2MB

If you ever thinking how I can get the amount of water adsorbed - easily, by the acid boiling point drop.

And yes, woelen, you got the point of my latest experiments quite right - the temperature and (re)usage of glass. I can confirm - even suspension at 112-125C works great.


Update: the problem of the high water contents in metaphosphates is mentioned e.g. in Inorganic syntheses, vol 2 or 3, see Na3PO3F synthesis. For that purpose Audrith recommends trimetaphosphate as starting matherial, and more of that, freshly prepared.
Usage of hexamethaphosphate instead would simplify many reactions if we would be able easily dehydrate it. Reflux in AcOH definitely works, but I feel should be also another way.

[Edited on 21-3-2025 by teodor]

clearly_not_atara - 21-3-2025 at 14:38

Quote:
The boiling point suggest I was probably able to get 20% acetic anhydride solution as the result of sodium acetate dehydration at temperature lower than 125C.

Did you take the bp after distillation or before?

teodor - 21-3-2025 at 15:11

Quote: Originally posted by clearly_not_atara  
Quote:
The boiling point suggest I was probably able to get 20% acetic anhydride solution as the result of sodium acetate dehydration at temperature lower than 125C.

Did you take the bp after distillation or before?


This is preliminary results and I have much more questions than that one, and answering that one will require me to answer others and I am not ready to go so far becase my experiments are still going. There are peculiarities which I need to understand, the high boiling point could be reached only by some particular order of manipulations. The same components in the reaction miture but mixed in different order and with different reaction history will not give anything more than 118.2C (doesn't matter how much salt is dissolved). This 118.2C is so stable as an upper limit in most of experiments that I assume it is the real boiling point of AcOH for my termometer at 770 mmHg air pressure.
Getting more than this requires much attention and explanation of exact conditions of the reaction. After I will be sure about this I will answer your question.

teodor - 22-3-2025 at 05:23

So, according to my preliminary results and thinking the most serious problem is the equilibriums

2 NaPO3 + H2O <-> Na2H2P2O7
NaPO3 + H2O <-> NaH2PO4

For drying of crude hexametaphosphate I can imagine several ways:

1. By heat. Drawbacks: could change the chemical properties.
2. Dessicator over H2SO4. I think anything more powerful is more expensive or did I miss something? I don't want to dry a drying agent with more expensive drying agent, it is a nonsense. Drawbacks: none.
3. Washing with AcOH. Drawbacks: only for reactions with AcO-, other reactions would require drying from AcOH.
4. Reflux with AcOH. Drawbacks: equilibriums mentioned at the beginning.
5. Azeotropic distillation (benzene, alcohol, hexane etc). Drawbacks: possible dehydration reaction with some compounds, the solvent residue should not interfere with further experiments.

I would check all those ways, but having limited time today I started my experiment with the option 2.

20250322_134426.jpg - 2.7MB

The weight of the empty "pralinevormpje" is 0.529g and together with (NaPO3)n (bought from labshop.nl) is 60.645g.

Let's check the mass change in few days.

[Edited on 22-3-2025 by teodor]

LesserLordKusanali - 26-3-2025 at 00:55

Somewhat related, have you done any experimenting on reducing metaphosphates? the metaphosphate I have quite a bit of is sodium hexametaphosphate. I'm going to attempt a low temp reduction with liquid lithium and zinc chloride under mineral oil but I was curious if you had stumbled upon anything since you like metaphosphates so much :D

teodor - 26-3-2025 at 03:59

Quote: Originally posted by LesserLordKusanali  
Somewhat related, have you done any experimenting on reducing metaphosphates? the metaphosphate I have quite a bit of is sodium hexametaphosphate. I'm going to attempt a low temp reduction with liquid lithium and zinc chloride under mineral oil but I was curious if you had stumbled upon anything since you like metaphosphates so much :D


Reducing to P4? Well, I plan to try with other phosphate but for that I need to study a different part of chemistry, the chemistry of clays. Because clay retorts have the best reputation for reducing phosphates I think, starting from the end of 17 century. Old people made P4 easily even from their own pee. It is a shame we lost those noble skills. I mean pottery and glassblowling which every chemist of the old centuries was required to have.

davidfetter - 26-3-2025 at 15:01

Quote: Originally posted by teodor  
Quote: Originally posted by LesserLordKusanali  
Somewhat related, have you done any experimenting on reducing metaphosphates? the metaphosphate I have quite a bit of is sodium hexametaphosphate. I'm going to attempt a low temp reduction with liquid lithium and zinc chloride under mineral oil but I was curious if you had stumbled upon anything since you like metaphosphates so much :D


Reducing to P4? Well, I plan to try with other phosphate but for that I need to study a different part of chemistry, the chemistry of clays. Because clay retorts have the best reputation for reducing phosphates I think, starting from the end of 17 century. Old people made P4 easily even from their own pee. It is a shame we lost those noble skills. I mean pottery and glassblowling which every chemist of the old centuries was required to have.


Do we have any idea which clays, or anything else about this?

I've done a little scientific glassblowing, nothing fancy or novel, but ceramics are completely new to me in this context. There are glass shops in big-enough chemistry departments/divisions, but I hadn't heard of specialty ceramics other than those used for the military, and those tend not to be on site.

clearly_not_atara - 26-3-2025 at 17:10

Apparently the reaction with NaH gives phosphite salts which can be isolated by using barium. Not sure exactly how the Ba phosphite is extracted but it's interesting.
https://pubs.acs.org/doi/full/10.1021/acscentsci.1c01381

Hypophosphite was also observed, but was not considered a primary synthetic target.

[Edited on 27-3-2025 by clearly_not_atara]

teodor - 31-3-2025 at 08:34

Quote: Originally posted by davidfetter  
Quote: Originally posted by teodor  

Reducing to P4? Well, I plan to try with other phosphate but for that I need to study a different part of chemistry, the chemistry of clays. Because clay retorts have the best reputation for reducing phosphates I think, starting from the end of 17 century. Old people made P4 easily even from their own pee. It is a shame we lost those noble skills. I mean pottery and glassblowling which every chemist of the old centuries was required to have.



I've done a little scientific glassblowing, nothing fancy or novel, but ceramics are completely new to me in this context. There are glass shops in big-enough chemistry departments/divisions, but I hadn't heard of specialty ceramics other than those used for the military, and those tend not to be on site.

Do we have any idea which clays, or anything else about this?





I have checked ancient reports and I found that they use stone retorts. I saw the photos but have no idea how they were made. But it was before they found that lead and some other heavy metal phostphates have lower decomposition temperature. Also, usage of alumin(i)um or some other metals instead of carbon can lower the temperature and reaction time as well.
The problem with glass that even if it will not break during decomposition it will definitely break on cooling. So, ceramic is better but still I see it only as disposable retorts. So you make a clay with consistence of plasticine, make a retort putting your ingredients inside, then heat it. First clay hardens than reaction happens. Then you dispose the retort.
As a candidates I would select clays based on phosphates. Let say, aluminium phosphate (poly/meta). With raising of temperature too high it would be converted to Al2O3, the goal is to find composition which will keep the form at the required temperature range.

@clearly_not_atara: looks interesting, but probably too complex if the final goal is P4 and not phosphite.

[Edited on 31-3-2025 by teodor]