Aryl sulfonic acids are prepared by electrophilic aromatic substitution of an aryl hydrogen atoms using sulfuric acid. The electrophile is the sulfur trioxide (SO3). The substituents on the ring determine the position of the new sulfonic acid group. Multiple sulfonation is usually not a problem. Because the sulfonic acid group is deactivating, addition of subsequent sulfonic acid groups is much more difficult. Unlike most electrophilic aromatic substitution reactions, sulfonation is easily reversible. Heating the aryl sulfonic acid in steam replaces the sulfonic acid group with a hydrogen atom (Section 9.7B).