Chemister
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Pyrosulfate salts for drying sulfuric acid?
Why aren't sodium pyrosulfate and other pyrosulfate salts used as dehydrating agents / desiccants? It seems like it should be relatively easy to make
sodium pyrosulfate by heating sodium bisulfate to ~300C - 400C.
It hydrolyzes back to sodium bisulfate, and given how aggressively its corresponding acid (disulfuric acid aka oleum) absorbs water I would doubt that
this reaction is significantly reversible, meaning that it should be good at dehydrating things.
The main application I can think of for this would be removing excess water from sulfuric acid. As far as I understand, it isn't too difficult to
concentrate sulfuric acid to ~70-80% by boiling off water, but concentrating it past that is quite difficult and dangerous. Instead of trying to
distill it to 98%, it seems like it should be possible to just add some sodium pyrosulfate to remove the rest of the water forming sodium bisulfate as
a byproduct.
I couldn't find any info on the solubility of sodium bisulfate in concentrated sulfuric acid, but if it is soluble, that could be problematic as this
method would lead to significant sodium contamination. That said, for many reactions which need conc sulfuric acid (like Fischer esterification)
sodium contamination would be probably be completely fine, so this isn't necessarily a huge problem. Additionally, it might be possible to use sodium
pyrosulfate in place of sulfuric acid as a dehydrating agent in some reactions.
Another thing that I am curious about is whether or not sodium pyrosulfate reacts with alcohols. If not, it might be possible to use it to dry
alcohols. I found some references[1][2] saying that sodium bisulfate decomposes to sulfuric acid and sodium sulfate in alcohols, so if that is true it
could be a problem. I also wonder if that could be a method to make sulfuric acid (add sodium bisulfate to alcohol, filter insoluble sodium sulfate,
boil to remove alcohol).
Important note: If you decided to experiment with pyrosulfates/sulfuric acid + alcohols be very careful as it might be possible for sulfate esters to
form which can be extremely toxic/carcinogenic. As such, it probably isn't a great idea to experiment with that if you don't really know what you are
doing.
Tl;dr
It seems like sodium pyrosulfate might be an underrated chemical that can be used to make concentrated sulfuric acid / be used in place of
concentrated sulfuric acid in some reactions.
By the way, I found this video making sodium pyrosulfate: https://www.youtube.com/watch?v=9IlwSUDjLWI
[1]https://www.atamanchemicals.com/sodium-bisulfate_u26081/
[2]https://www.chembk.com/en/chem/Sodium%20bisulfate
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bariumbromate
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if you can heat it that hot just go all the way to sulfur trioxide and SLOWLY mix with water to get pure h2so4
you should only need 420C to get there, not much more than 400c
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Chemister
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True, but sulfur trioxide seems like it massively sucks to work with, and requires a whole lot more equipment to handle than what is needed for making
sodium pyrosulfate. This method would avoid SO3 or oleum which would make it much safer and more accessible.
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teodor
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According to my lab experience making SO3 is simpler than pyrosulfate in more than 50% concentration - it requires prolonged heating because the water
is going off very slowly. Also, I doubt you can use it for drying H2SO4. I would start looking on the behaviour of the system Pyrosulfate - H2SO4 -
H2O. If you unable to find this I can search for you.
But theoretically the question is very interesting one. Interaction of condensed salts and sulfuric acid. Not necessarily pyrosulphate. My current
experiments are with condensed phosphates and I am checking behaviour toward H2SO4 with some salts I get. The whole area of Condensed salts worth
investigation. I am very interested to read your experiments with pyrosulfates.
Making SO3 from sulfamic acid is not dangerous because the rate of decomposition is slow and you have time to adsorb vapours. I am more comfortable
with this process than with oleum operations.But making SO3 is more question of practice, and condensed salts experimentation is also about getting
both practice and knowledge. So please try and inform.
[Edited on 20-8-2025 by teodor]
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clearly_not_atara
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| Quote: | | Another thing that I am curious about is whether or not sodium pyrosulfate reacts with alcohols. If not, it might be possible to use it to dry
alcohols. |
I am pretty sure that it does -- the salts NaSO4Me and NaSO4Et are well-known and easy to produce.
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Varungh
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Just go all the way to oleum
Labcoatz did it using a quartz flask and a generic collection flask and a generic condenser column
If you can go thet high, just push it to SO3.
This can be mixed with cold H2SO4 to concentrate it
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teodor
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OP,
1. Please look at https://www.sciencemadness.org/talk/viewthread.php?tid=63569
2. Do some basic math:
100g 96% H2SO4 = 4g H2O
SO3 (17.8g) + H2O (4g) = H2SO4
Na2S2O7 (49.3g) = Na2SO4 + SO3 (17.8g)
So, 50g of PURE Na2S2O7 (not heated NaHSO4) per 100g (53 ml) 96% H2SO4 to get concentratio close to 100% (excluding air vapours).
As I said heating NaHSO4 in a typical home lab setup can give approx 50% Na2S2O7 / 50% NaHSO4. So 100g per 50 ml only to get rid of 4% of water.
3. Also look https://www.sciencemadness.org/talk/viewthread.php?tid=63946...
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Chemister
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Good point. That should have been the first thing that I checked. For some reason I was expecting that it would absorb much more water, but I suppose
the fact that it only absorbs 1 mole of water per mole of pyrosulfate is pretty abysmal.
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teodor
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No, Chemister, the first thing to check there was such thread before 
Also, if you say that "pyrosulfate is relatively easy to meake" it is good to provide some reference to your claim. For example, according to
Attachment: Pyro-Sulfate Ion In Solution.pdf (148kB)
This file has been downloaded 83 times
the pyrosulfate is relatively easy to make by mixing sulfate and SO3 or concentrated sulfuric acid.
Which is quite contrary to the method of drying H2SO4 with pyrosulfate, do you agree? So, the third step is to show that you are aware about the
reverse reaction and make some hypothesis why you think it can work in reverse.
But anyway your thread inspired me to check one more method of creating pyrosulfate according to https://doi.org/10.1134/S1070427216010043
I expected it should work better than heating KHSO4 alone. This uses the general approach of using ammonium salt for anion condensation. For example
Na(NH4)HPO4 -> NaPO3 + NH3 + H2O
is a very satisfactory method of making Graham's salt.
As you see 2 K(NH4) SO4 = K2S2O7 + 2NH3 + H2O
doesn't work as expected. I did experiment with K2SO4/(NH4)2SO4 mixture, at some point it fuses together, but the reaction proceed too slow at 400C,
also only fraction of the salts is converted to pyrosulfate according to the paper.
May be try the potassium/ammonium bisulfate mix? But at the current stage based on the experiments of getting K2S2O7/SO3 I performed so far I still
think that making SO3 is a bit easier than pyrosulfate.
(I think the best practical method published here is decomposition of persulfate as in the Garage Chemist's publication of making SO3)
P.S. Also theorizing any way of dehydrating H2SO4 it is good always to start with 4 basic equations describing the concentrated H2SO4 system:
2 H2SO4 <-> H3SO4+ + HSO4-
2 H2SO4 <-> H3O+ + HS2O7-
H2S2O7 + H2SO4 <-> H2SO4+ + HS2O7-
(actually those 2 are result of the most important self-dissociation reaction
2 H2SO4 <-> H2O + H2S2O7
which is not independent so usually is excluded from the system)
H2O + H2SO4 -> H3O+ + HSO4-
The most important here that the equilibrium constant of the last equation is 1, so there is actually no water in concentrated sulfuric acid, there
are hydronium ions only. That's explain many things about its usage in organic chemistry as a dehydrating agent. But also that's explain why it is not
possible to dehydrate sulfuric acid with many very strong dehydrating agent which probably can work with water molecules only, for example,
chlorosulfonic acid doesn't react with concentrated sulfuric acid (at least at normal temperature).
To look how it can work with pyrosulfate I would start looking at the methods of making pyrosulfate from sulfate and concentrated acid to get the idea
how addition of pyrosulfate can shift those basic ionic equations from there.
[Edited on 21-8-2025 by teodor]
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teodor
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As was described in this thread: https://sciencemadness.org/talk/viewthread.php?tid=5495
H2SO4 is requred as a catalist to make this reaction possible:
Na2S2O7 -> SO3 + Na2SO4
There was no mechanism of its action proposed, but I think when we are talking about drying H2SO4 by pyrosulfate and getting
SO3 with H2SO4 the same mechanism should work, the difference is only in the composition of the reaction mixture - more acid or more pyrosulfate.
I will try to make some speculations how it works.
I suppose that there is no qualitative change in the reaction between H2SO4 and Na2S2O7 if there is some small amount of water present
(based on the presence of the self-dissociation reaction of H2SO4 described in the previous post: 2 H2SO4 <-> H2O + H2S2O7 ).
So I will describe only 100% H2SO4 case to reduce the number of basic H2SO4 equations from 4 to 3, but I suppose that 96% case could be derived from
this
basic mechanism.
I didn't find the mechanism of Na2S2O7 solution in H2SO4 but by analogy with
K2SO4 + H2SO4 = 2K+ + 2HSO4- (Gillespie, Robinson "The sulfuric acid solvent system")
I suppose we have
Na2S2O7 + H2SO4 -> 2Na+ + HS2O7- + HSO4-
Let's again write 4-1 (without water) basic equations here:
1) 2 H2SO4 <-> H3SO4+ + HSO4-
2) 2 H2SO4 <-> H3O+ + HS2O7-
3) H2S2O7 + H2SO4 <-> H2SO4+ + HS2O7-
Because we are talking about heating, we need also
4) H2S2O7 -> SO3 + H2SO4
And because SO3 can escape by the force of the heat, equilibrium 3) will be shifted left.
But increasing of HS2O7- ions as the result of pyrosulfate dissolution will require increasing
of H2SO4+ to make shifting 3) to the left possible. This demand will shift 1) to right increasing concentration
of HSO4-.
2) doesn't matter here comparing with the case without pyrosulfate where 2) is the only source of H2S2O7- ions.
Without pyrosulfate by the same process of heating 2) will be shifted right (becaus of HS2O7- demand)
increasing H3O+ concentration and we have
H2O + H2SO4 -> H3O+ + HSO4-
which is not possible to shift left because it actually has the equilibrium constant equal to 1 as I mentioned in the previous post.
So, yes, I think when we are talking about drying of H2SO4 by pyrosulfate and using H2SO4 as
a catalyst of pyrosulfate decomposition we are always taling about the reaction described here.
P.S. The source of 4 basic equation is the article Gillespie, Robinson "The sulfuric acid solvent system" in "Advances of Inorganic Chemistry and
Radiochemistry, vol. 1" or the book Audrieth & Kleinberg "Non-aqueous solvents"
_Update_
II. The same mechanism of H2SO4 decomposition in the presence of pyrosulfate probably prevents completion of the reaction
2 NaHSO4 -> Na2S2O7 + H2O
This was mentioned in the thread I cited at the beginning of the message, in the sources of article I attached to the previous message and also
according to my experiments with KHSO4 only 45% of the salt is decomposed. In the case of LiHSO4 there is no possible separate the step of dehydration
and decomposition at all (Gmelin).
Probably in the process of thermal decomposition of bisulfate there is a second route
2 NaHSO4 -> Na2SO4 + H2SO4
and any H2SO4 formed becomes a catalist of convertion Na2S2O7 back to bisulfate, so the reaction goes up to some equilibrium. To check this hypothesis
it is enough to find a way of removing any H2SO4 which is formed here.
III. It is still interesting how pyrosulfate is formed from bisulfate in H2SO4 with concentration more than 100% (oleum). The sources are in the
article I attached to the previous message. By the way, I didn't find proof that it can happen in concentrated H2SO4 according to the article, 2 from
3 references cited there are invalid and 1 I am unable to find. So, my hypothesis is that pyrosulfate has a lower solubility than sulfate for some
particular cations, that way dissolving sulfate or bisulfate in oleum with heating and cooling makes this conversion possible.
[Edited on 23-8-2025 by teodor]
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teodor
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I had a feelling that something is wrong with the H2SO4 catalyst. And indeed, if one would check the proposed equations he obviously can see that one
mole of H2SO4 is consuming per mole of SO3.
So, if we would accept the mechanism of reaction as dissolution of K2S2O7 and then decomposition of H2SO4, the overal equation will be
Na2S2O7 + H2SO4 -> 2NaHSO4 + SO3
and not
Na2S2O7 -> Na2SO4 + SO3
This is quite natural, because in regard to the solvent Na2SO4 to H2SO4 is the same as Na2O to H2O (see the references I cited yesterday).
Can you imagine that in some thermal process we don't stop on NaOH stage but apply heat further to dehydrate it to oxide?
The same would be in the Na2SO4 / H2SO4 system, the effective SO3 generation is probably only to the point of consuming all free H2SO4 and
forming bisulfate. So, I suppose
H2SO4 is not a catalist, but a limiting reagent
If one would reread Garage Chemist's report and would calculated the yield based on the amount of sulfuric acid used he/she will see
that it looks likely so.
I would like to check it experimentally, so my assumption that the yield of SO3 is proportional to H2SO4 part in the mixture.
[Edited on 24-8-2025 by teodor]
It is worth to mention that dry Na2S2O7 can exist in 2 forms: ionic and molecular depending on the method of preparation. That one which is prepared
by combining Na2SO4 and SO3 is a molecular compound (Gmelin). The behaviour in regard to heating could be different. But if we use H2SO4 solution of
puyrosulfate I think the difference is doesn't exist anymore.
[Edited on 24-8-2025 by teodor]
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teodor
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I didn't make experiment to check wether H2SO4 is limiting reagent yet (due to some delay in the lab preparation), but today I have discovered that
NurdRage already did such experiment
https://www.youtube.com/watch?v=wB2zzm8VP9Y
So, yes, it looks like the hypothesis that the pyrosulfate decomposition is actually
Na2S2O7 + H2SO4 -> 2NaHSO4 + SO3
looks trustworthy.
One thing to check : if we accept the theory of decomposition I wrote in the previous messages (and as the consequence of this that the equation with
water doesn't matter when all water is ionised) that the yield should not be dependent on H2SO4 concentration after some point until stoichiometry
(pyrosulfate to H2SO4) is right. The thing is that SO3 generation _decreases_ H2SO4 residual concentration, so this generation is possible up to the
point when ionic capacity for H3O+ will be over and the equilibrium
H2SO4 <-> H3O+ + HS2O7-
will start to consume pyrosulfate ions not for SO3 but for H2SO4 generation .
This last step is actually H2SO4 dehydration by pyrosulfate.
[Edited on 1-9-2025 by teodor]
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