Sciencemadness Discussion Board - Preparation of Oleum by Use of Phosphoric Acid

Preparation of Oleum by Use of Phosphoric Acid

Magpie - 24-11-2017 at 19:26

November 24, 2017

A. Introduction

H3PO4 + lots of heat ---→ HPO3 + H2O

HPO3 + H2SO4 + lots of heat --→ H3PO4 + SO3

The method used is that of CD-ROM-LAUFWERK as forwarded to this forum by garage chemist.

I performed it at 10X scale.

B. Apparatus

600 mL borosilicate beaker
250 mL borosilicate rbf
100 mL rbf
distillation glassware (tapered connections)
aluminum foil and fiberglass blankets
Meeker burner on MAPP gas
iron ring
ceramic triangle or iron screen
propane bunsen burner
heat resistant clamp for rbf
MAPP gas
propane

C. Reagents
40 mLcon sulfuric acid
135 mL 85% phosphoric acid

D. Procedure

a. Preparation of HPO3
1. Set up the iron ring w/ceramic triangle or wire gauze on a ringstand at the elevation suitable for the Meeker burner. Place 600 mL beaker on the ring.
2. Add 135 mL of phosphoric acid to the beaker.
3. Apply the strongest possible heat to the beaker with the Meeker burner on MAPP gas.
4. Intensely heat the beaker until the melt turns red or 1 ½ hrs have elapsed, whichever comes first.
5. While the HPO3 melt is still hot pour it into a 250 mL rbf (the pot).

H3PO4 to HPO3

b. Preparation of Oleum
1. Pour 40 mL of con sulfuric acid into the pot holding the molten HPO3.
2. Secure the pot to a ringstand using a heat resistant clamp.
3. Place an iron ring with wire gauze under the pot.
4. Connect a stillhead (w/glass plug), condenser, vacuum adapter, and 100 mL receiving rbf to the pot.
5. Place an aluminum foil tent around the pot up to the bottom of the stillhead.
6. Wrap a fiberglass blanket around the aluminum tent.
7. Provide the condenser with 11°C water (approximately).
8. Heat the pot strongly with the propane bunsen burner. Production rate ran from 25d/min in the beginning to half that toward the end.
9. Heat until approximaly 30 mL of oleum has collected. If everything goes smoothly this will only take ≈ ½ hr.

H2SO4 toSO3

E. Results

About 60g (30 mL) of 65-70% oleum will be in the pot. My yield was 62.4g. This is a 100% yield assuming 70% oleum and based on the H2SO4 charged.

oleum product

F. Discussion

One should be prepared to deal with problems during the distillation of oleum. Here are the problems I dealt with during today’s preparation: 1) Proper heat removal in the condenser is critical and can be a nuisance, ie, my water temperature ranged from 8°-10°C, apparently a bit too cold as considerable oleum had frozen in the condenser. This was removed by turning off the cooling water and using a heat gun. 2) Ran out of propane. Have a spare can available. 3) The rubber edge on my fiberglass blanket caught fire. Make sure it is out of the way of the bunsen flame. 4) Excessive SO3 was spewing out the tublature on the vacuum adapter at times. This is due to the condenser and/or receiver being too warm. Put the cooling water back on line. With this run everything ran smoothly when the cooling water reached 14°C toward the end. A cooler temperature may be needed in the beginning when oleum production is high.

G. Vessel Corrosion

Both the 600mL beaker and the 250mL pot become permanently corroded/etched. They are now dedicated to oleum service.

edit: All tapered glass joints (except at the receiver) are locked up. They were sealed with con H2SO4. This will keep me busy for awhile.

permanent phosphate corrosion of the pot.JPG - 124kB

permanent phosphate etch

edit: the distillation assembly broke loose after soaking in water overnight, except for the plug in the stillhead. I have it soaking in WD-40 now. If that doesn't work I will flame heat the female part.

edit2: The plug came loose after flaming the stillhead.

Questions, comments, and suggestions are welcomed.

[Edited on 25-11-2017 by Magpie]

[Edited on 25-11-2017 by Magpie]

[Edited on 25-11-2017 by Magpie]

ninhydric1 - 24-11-2017 at 19:41

For the prep of thionyl chloride I assume? Can't wait for that!

Magpie - 24-11-2017 at 19:53

Yes, I have to build up large stocks of oleum and SCl2 just to make 43g of thionyl chloride, ie, 142g of 65%oleum and 135.5g of SCl2. However, I will also get 47g of chlorosulfonic acid.

[Edited on 25-11-2017 by Magpie]

Vosoryx - 24-11-2017 at 21:59

You've been busy

Interesting synth. Glad you put the fact that it ate away at your glass in - that is enough to deter me from trying it with my cheap Chinese glass. How badly was it corroded? Just "Frosted" or was there actual pitting? Do you have a pic?

Tsjerk - 25-11-2017 at 00:36

Nice post! Could you elaborate at bit on how you mixed the HPO3 with the sulfuric acid? Or did they mix spontaneously?

PHILOU Zrealone - 25-11-2017 at 03:06

Nice work and presentation as usual Magpie

In principle the same procedure could be use to make valuable N2O5...wich has almost the same range of boiling point as SO3 and this is much lower than the BP of any H3PO4 or anhydride of it...
Eventually in the case of heat sensitivity cold trap distillation under partial vaccuum may be used...

HPO3 is a kind of acid anhydride (-O-P(=O)(-OH)-)n (usually n=3 and cyclic)...this molecular skeleton is found into polyphosphates.
==> reaction could be called trans-anhydridification

Magpie - 25-11-2017 at 03:20

Quote: Originally posted by Vosoryx

You've been busy

It is frosting. I can get better pictures for you soon. What you see in my pictures is what it looks like from the outside.

I wouldn't worry about your glass as long as it is borosilicate.

Magpie - 25-11-2017 at 03:22

Quote: Originally posted by Tsjerk

Nice post! Could you elaborate at bit on how you mixed the HPO3 with the sulfuric acid? Or did they mix spontaneously?

Thank you.

I just poured it into the HPO3 very hot melt.

Magpie - 25-11-2017 at 03:29

Thanks PHILOU

Quote: Originally posted by PHILOU

HPO3 is a kind of acid anhydride...

Yes, definitely an anhydride: H3PO4 ---> HPO3 + H2O

Could 6HPO3 + 6NaOH ----> (NaPO3)6 + 6H2O

This would be sodium hexametaphosphate, aka Calgon

[Edited on 25-11-2017 by Magpie]

[Edited on 25-11-2017 by Magpie]

aga - 25-11-2017 at 04:46

Great work Magpie !

Brave too - i certainly haven't got the solid steel balls required to make oleum

Magpie - 25-11-2017 at 05:53

aga: no balls needed, only a good fume hood.

Magpie - 25-11-2017 at 05:57

Quote: Originally posted by Magpie

Quote: Originally posted by Vosoryx

You've been busy

It is frosting. I can get better pictures for you soon. What you see in my pictures is what it looks like from the outside.

I wouldn't worry about your glass as long as it is borosilicate.

Here's the pictures:

BromicAcid - 25-11-2017 at 06:37

Nice to see this in action, I remember the flurry of activity on the forum when people first started dehydrating phosphoric acid realizing the possibilities and then it died off just as quickly.

S.C. Wack - 25-11-2017 at 07:12

Is Krytox a no, or did it just seem unwise? Pardon my dickness but I think if many people tried this, the glass would break on several. IMHO you could be a little lucky the beaker didn't crack and it would be crazy to use it for anything ever again; the RBF is disposable too.

Magpie - 25-11-2017 at 08:37

Quote: Originally posted by S.C. Wack

Is Krytox a no, or did it just seem unwise? Pardon my dickness but I think if many people tried this, the glass would break on several. IMHO you could be a little lucky the beaker didn't crack and it would be crazy to use it for anything ever again; the RBF is disposable too.

I badly wanted to use Krytox but assumed the SO3 would just eat it alive, carbonizing it, and seizing the joints worse. I assume that my present joint seizings are due to carbon contaminants in the Rooto acid I used for joint grease.

Dickness has its place. This is the second time I've used these vessels for making oleum. Although I can't remove this scale I don't think it has compromised the strength of the glass. Chemglass (apologies to Dr Bob) has especially thick walls. I will use these same vessels for my next run of oleum.

Vosoryx - 25-11-2017 at 08:51

Thanks for including the pics.

Magpie - 25-11-2017 at 10:47

I reread the specifications on Krytox. I think this badass lubricant can handle the job and I will be using it next time.

[Edited on 25-11-2017 by Magpie]

nitro-genes - 28-11-2017 at 11:43

Nice write up!

How likely is cracking glassware during dehydration of the phosphoric acid? In another thread I read something about copper being passivated, maybe adding a small amount of a copper salt can passivate something like stainless enough to also do the dehydration (US2653177)? How soluble is the solid HPO3 in the sulfuric acid, can you also add the solid HPO3 (it crumbles very easily) instead of pouring the hot melt directly into sulfuric?

Magpie - 28-11-2017 at 11:59

I have fired the same vessels several times now with no hint of cracking.

I don't thikn it matters how you mix them - you are going to heat the shit out of them anyway & they will mix.

nitro-genes - 28-11-2017 at 12:11

Thanks for the reply, had one cracking during distillation of sulfuric, not a pleasant thing (my cheapskate glassware maybe).

Maybe not appropriate to post here, but from the the patent mentioned above:

"It has been found that the attack by phosphoric acid on high chrome-nickel steels containing molybdenum is markedly decreased at acid concentrations above about 100% calculated as orthophosphoric acid...
From the figure it is seen that as the acid concentration rises above about 95% and especially when it is above 100%, the rate of corrosive attack decreased very rapidly as the acid concentration is increased"

So it seems that it is especially the partially dehydrated phosphoric is corrosive, maybe starting from diammoniumphosphate heating in stainless would also work?

Bert - 30-1-2018 at 08:07

Quote: Originally posted by Magpie

I reread the specifications on Krytox. I think this badass lubricant can handle the job and I will be using it next time.

[Edited on 25-11-2017 by Magpie]

Did you try the Krytox? If so, which variety, what were the results-

Krytox LVP grease carbonized in the joints on a member during a sulfuric acid distillation, did he just choose an incompatible variety? There are a lot of greases and oils available in that product line-

https://m.ebay.com/sch/i.html?_from=R40&_nkw=krytox&...

Magpie - 30-1-2018 at 13:03

I have not tried Krytox in this application yet.

Here's whats on the lable: Krytox GPL205
0.5 oz/D10170248

Waffles SS - 5-2-2018 at 06:14

That is great Magpie,

If you sure that you made pure HPO3 then it should be very interesting to test reaction of ACOH with it according to GB299342 patent

[Edited on 5-2-2018 by Waffles SS]

Bert - 5-2-2018 at 06:25

A gallon of 85% Phosphoric is sitting in our cleaning supplies. Used to clean up Aluminum pontoons on barges we shoot fireworks off of. Wonder if I have a "disposable" grade piece or two of glass, but good enough not to fail before done doing this??

What is the best manufacturer of cheaper Oriental borosilicate labware these days? Had some Bomex, it was like paper.

[Edited on 5-2-2018 by Bert]

JJay - 5-2-2018 at 06:35

The best is probably Laboy or Synthware. I've had very few problems with GG-17 flasks (Nanshin has a pretty good selection).

Bert - 11-2-2018 at 14:09

After watching the oleum from Sodium bisulfate thermal process work done by Hennig Brandt with his nifty quartz tube retort, and this work with phosphoric acid in borosilicate glass- I had a couple of thoughts

If you were needing a bit more than these smallish quantities but did not care to invest in larger vessels? (Apparently the pyrex is a CONSUMABLE, replacements could get expensive, plus the increased joy of having a LARGE one fail at operating temperature...)

The 85% phosphoric acid (H3PO4) used to make HPO3 is not consumed, but takes up the water from the strong sulfuric acid and is mostly re converted to H3PO4. If a vessel suffiently robust and non reactive were used, could it not merely be disconnected from the condenser and oleum collection vessel, the heating continued and increased to the point of driving off the water again and regenerating the HPO3?

One could then do a cyclic process, adding more 98% H2SO4, distilling SO3, disconnect collection equipment, re constitute to HPO3, add more H2SO4, re connect to condensor/collector, distill the next increment of SO3 and so on, ad infinitum? Never needing to remove the H3PO4 from the aparatus until you had made yourself howevee much oleum you required?

Would quartz glass or Vycor be sufficiently resistant to heat, acid mixtures and SO3 for cycling in this fashion?

Magpie - 11-2-2018 at 14:20

Yes, the recycling of H3PO4 is possible, and was mentioned by garage chemist.

I have never used vycor but have used quartz at high temperature to make oleum.

There is an Amish glassblower in Pennsylvania who has fabricated quartz retorts for me at a very reasonable price. Just send him a sketch of what you want.

byko3y - 11-2-2018 at 20:29

Carbon, graphite and gold are the only common material capable of withstanding hot phosphoric/metaphosphoric acid. Platinum and silver won't work for that btw.
Quartz can hold the hot phosphoric acid up to 250 C without noticable destruction, while borosilicate is usable only below 200 C.
To make metaphosphoric acid you need either vacuum and 300 C, or 350-400 C at atmospheric pressure. But at these temperatures carbon, graphite, gold, and platinum all will be attacked by sulfur trioxide.
There's a similar problem with controlled nuclear fusion.

fusso - 17-4-2019 at 22:19

Quote: Originally posted by byko3y

Carbon, graphite and gold are the only common material capable of withstanding hot phosphoric/metaphosphoric acid. Platinum and silver won't work for that btw.
Quartz can hold the hot phosphoric acid up to 250 C without noticable destruction, while borosilicate is usable only below 200 C.
To make metaphosphoric acid you need either vacuum and 300 C, or 350-400 C at atmospheric pressure. But at these temperatures carbon, graphite, gold, and platinum all will be attacked by sulfur trioxide.

Please explain how the acids attack the materials. What are the equations for the rxns?

clearly_not_atara - 19-4-2019 at 12:35

Silicon carbide is apparently used for some of these kinds of extremely corrosive solutions.

https://www.gab-neumann.com/Corrosion-resistance-of-sintered...

Less than one micrometer per year in 85% H3PO4 or 98% H2SO4 at 100 C is a good outlook. Boron carbide also has similar properties; this book reports that H2SO4 + H3PO4 + CrO3 attacks it, but maybe in the absence of chromates it would be more resistant:

https://books.google.com/books?id=px_yCAAAQBAJ&pg=PA80&a...

MrDoctor - 2-2-2025 at 20:05

one thing im not clear on; at what actual temperature does the metaphosphoric acid dehydrate the sulfuric acid? and to what degree?
My understanding is that if not fully consumed, the hydrated phosphoric acids should still be a solid yes? in which case, is there a scenario wherin you can mix your acids, heat to a degree that wont eat borosilicate, or quartz if that is more resistant and wont suffer any corrosion, to produce merely 100% water-free sulfuric acid, without distilling it, and then just accept that wholesale, decanting/filtering, with some phosphoric acid contamination as well as the salts of, at this point most likely copper or iron.

Also, i had this really stupid idea, im not sure if it would work, i just thought, if metaphosphoric acid is a rather glass-like substance, what if one were to melt it in a crucible and coat it in the acid? at high temperatures where the acid is solid enough to hold its shape, could equally hot sulfuric acid be dripped into the crucible, causing a instant reaction releasing SO3 without having sulfuric acid exist in the liquid phase, meaning that the materials for the assembly might not need to resist liquid sulfuric acid.
such a reaction might be pushed forward additionally by performing this all under vacuum, which i believe wasnt previously an option since the temps are too high to safely apply hard vacuum in borosilicate, and quartz is attacked too easily to be worthwhile either.
batch yields would be terribly low, also im not sure how you address the hydrated phosphoric acid either, but this does allow for moderate temperature and hard vacuum with, theoretically no damage to any glassware or exceptionally high temperatures either for that matter.