Preparation of Oleum by Use of Phosphoric Acid

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 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]

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

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?

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

Quote: Originally posted by Vosoryx

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?

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?

Quote: Originally posted by PHILOU

HPO3 is a kind of acid anhydride...

Great work Magpie !

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

Quote: Originally posted by Magpie

Quote: Originally posted by Vosoryx   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? 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:

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.

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?

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]

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.