Sulfur trioxide

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Sulfur trioxide
Chemical formula SO3
OTP appearance clear liquid 
Molar Mass(g/mol) 80.066 
Melting Point(°C) 16.9 
Boiling Point(°C) 45 
Density(g/cc) 1.92
NFPA 704



Natural occurrence

  • Volcanic vents produce sulfur di- and tri- oxides.


  • Toxic. Inhalation or ingestion: poision. Skin exposure: irritant
  • Potentially explosive reaction with water



It's worth noting that even with enormous effort to keep materials, components, and the environment dry, it is likely that any production of sulfur trioxide will actually produce sulfuric acid instead.

From sulfur dioxide

Sulfur dioxide will oxidize (over long periods of time) to sulfur trioxide

2 SO2 + O2 2 SO3

Since this oxidation is very slow at normal temperatures and pressures, the process is usually accelerated in some way:

  • As the "elemental sulfur" method above, but mix in an equal mass of saltpeter before burning. The nitrogen oxides act as coreactants, promoting sulfur dioxide to sulfur trioxide rapidly.
    SO2 + NO2 SO3 + NO
    2 NO + O2 2 NO2 (recycled)

Sulfur dioxide and dry air are combined and blown over catalysts to promote the oxidation process. Simple iron oxide will work, as will platinum.

  • Pass sulfur dioxide and oxygen or air over a platinum or vanadium pentoxide (V2O5) catalyst at 420-600°C
    2 SO2 + O2
    420-600°C; 1.2-1.4atm}
    2 SO3

From sodium bisulfate

N.B. Does not work with other bisulfates, which emit sulfur dioxide
  1. Heat sodium bisulfate slowly to 315°C, decomposing it to sodium pyrosulfate
  2. Calcine at this temperature for some time until no more steam evolves
    2 NaHSO4
    Na2S2O7 + H2O(v)
  3. Calcine sodium pyrosulfate at 460°C
    Na2SO4 + SO3(g)
  4. Retain the distillate. It is sulfur trioxide.
  5. The residue is primarily sodium sulfate.

From iron (II) sulfate

Iron (II) sulfate (FeSO4•7H2O) thermally decomposes decomposes in several stages releasing water, sulfur dioxide and sulfur trioxide:

FeSO4•H2O(s) + 6 H2O(v)
FeSO4(s) + H2O(v)
2 FeSO4
Fe2O3(s) + SO2(g) + SO3(g)

The third step is possibly composed of two steps, to and from iron (III) sulfate

6 FeSO4 2 Fe2O3 + Fe2(SO4)3 + 3 SO2
Fe2(SO4)3 Fe2O3 + 3 SO3
  1. Gather 278ubm of iron (II) sulfate heptahydrate (natural OTP hydration) in an alembic
  2. Gather 9ubm of water in the receiver
  3. Heat retort (without connecting receiver) to 100°C until no more steam emerges
    This will eliminate all but one of the structural water, and any liquid water
  4. Connect the receiver, insuring contact between the materials exiting the alembic and the water in the receiver
  5. Heat the alembic to red heat
    NB: the water in the alembic should be as close to 9ubm as possible at all times. Do not let it evaporate below that level.
  6. When no more sulfur fumes emerge from the alembic, take it off the heat
    Check: The contents of the alembic should be finely powdered hematite or magnetite.
    Check: The contents of the receiver should be sulfuric acid, possibly with some dissolved sulfur dioxide.

sulfuric acid

Combine sulfuric acid and phosphorus pentoxide

2 H2SO4(l) + 2 P2O5(s) H4P4O12(s) + 2 SO3(g)

See also