H2SO4SO3

H2SO4SO3 is commonly referred to as oleum, a solution of sulfur trioxide in sulfuric acid. The notation reflects that oleum is not a discrete molecule but a mixture in which varying amounts of SO3 are dissolved in H2SO4. Commercial oleum is described by the percentage of SO3 by weight or by the molar ratio of SO3 to H2SO4, and it can range from relatively dilute solutions to highly concentrated, fuming grades.

Preparation and composition

Oleum is produced by absorbing sulfur trioxide gas into concentrated sulfuric acid under strictly anhydrous conditions. The composition varies, with grades typically described as containing from roughly 20% to about 70% SO3 by weight; higher contents are termed fuming sulfuric acid. The precise composition determines reactivity and compatibility with other reagents.

Properties and handling

Oleum is a highly reactive, dense, and viscous liquid. It is extremely hygroscopic and releases heat when it comes into contact with water, converting SO3 into additional sulfuric acid. It is corrosive to skin and eyes and can react violently with moisture or certain organic materials. Safe handling requires dry equipment, appropriate ventilation, and storage in corrosion-resistant containers, typically made of glass or protected materials compatible with strong acids.

Applications

In industry and synthesis, oleum serves as a sulfonating and sulfating agent, enabling introduction of sulfonic groups or formation of sulfate esters in organic substrates. It is also used to generate sulfuric acid of higher strength via absorption of SO3 into existing acid, and in processes that require a controlled supply of SO3 in situ rather than as a pure gas.

Safety notes

Oleum reacts vigorously with water and moisture; accidental contact can release heat and sulfuric acid mist. Proper protective equipment, dry handling practices, and containment measures are essential in any use or storage.