Sulfur trioxide, chemical formula SO3, is a highly corrosive and easily melted white solid at room temperature. It is the acid anhydride of sulfuric acid.
Sulfur trioxide is an extremely strong dehydrating agent that causes immediate and highly exothermic charring of virtually any organic material it comes into contact with. Sulfur trioxide reacts with water in the air to form a dense fog of concentrated sulfuric acid.
- SO3 + SCl2 → SOCl2 + SO2
Sulfur trioxide is a volatile liquid that fumes in contact with open air. It has a relative narrow liquid range, melting at 16.9 °C and boiling at 45 °C.
SO3 is available for purchase only to professional chemists due to its extreme hazards. It is commonly sold as a solution in sulfuric acid known as oleum.
SO3 can be made in low yield through the pyrolosys of sodium persulfate, first forming ozone and then SOSO3. A catalytic amount of 100% sulfuric acid is required.
Nearly the same reaction takes place when sodium bisulfate is very strongly heated, first evolving water and then SO3.
In industry sulfur trioxide is produced via the contact process. Purified sulfur dioxide is mixed with dry air and injected through a bed of V2O5 catalyst heated to temperatures between 400-600 °C (the optimum temperature is 450 °C), at 1-2 atm. The resulting hot sulfur trioxide is recirculated and passed through more layers of catalyst to increase the yield. The gaseous SO3 is cooled by passing it through a heat exchanger and can be collected if required. The condensed trioxide is dissolved in concentrated H2SO4 in the absorption tower to form oleum.
- SO3 reacts with SCl2 to form thionyl chloride
- SO3 reacts (violently) with water to form sulfuric acid, with dilute sulfuric acid to form concentrated sulfuric acid, and with the concentrated acid to form oleum.
- A mixture of SO3 and sulfuric acid is effective at sulfonating aromatic compounds, forming useful reagents like benzenedisulfonic acid and toluensulfonic acid.
Sulfur trioxide is highly corrosive and will fume in open air, resulting in a highly dangerous and corrosive sulfuric acid mist. Since this mist is incredibly hazardous, standard protection clothing is insufficient, as the acid mist will attack most organic fibers as well as wreck most air filtration systems and may condense on the cloth. A showering installation may be required in the worst case scenario.
Sulfur trioxide reacts violently with plain water 1.
Sulfur trioxide is best stored as a solution in sulfuric acid, also known as disulfuric acid.
Pure sulfur trioxide should only be stored in sealed ampoules, as it will corrode most metal and plastic containers and their lids. Teflon containers, sealed with PTFE tape can also be used, but are very expensive. Lastly, SO3 containers can also be kept in a glovebox, to limit the hazard in the event of spill.
Due to its relative narrow liquid range, between 16.9 °C 45 °C, it's mandatory to keep sulfur trioxide away from any source of heat, although if kept in a place too cold it will freeze.
SO3 should be carefully neutralized. It reacts explosively with water and most bases, so dilution must be performed very slowly. The sulfate salt can then be washed down the sink. A safer way involves slowly adding sulfur trioxide over dry calcium carbonate powder, which results in anhydrous calcium sulphate and carbon dioxide.