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Author: Subject: Preparation of Oleum by Use of Phosphoric Acid
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[*] posted on 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?

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[*] posted on 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.




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[*] posted on 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.
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[*] posted on 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?



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[*] posted on 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...




[Edited on 04-20-1969 by clearly_not_atara]
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