in the presence of the catalyst vanadium pentoxide at 450 °C and 1-2 atm
# The sulfur trioxide formed is added to sulfuric acid which gives rise to oleum (disulfuric acid)
# The oleum is then added to water to form sulfuric acid which is very concentrated. Since this process is an exothermic reaction, the reaction temperature should be as low as possible.

Purification