A sulfonic acid  refers to a member of the class of organosulfur compounds with the general formula R−S(=O)₂−OH, where R is an organic alkyl or aryl group and the S(=O)₂(OH) group a sulfonyl hydroxide. As a substituent, it is known as a sulfo group. Sulfonic acid, sulfonic also spelled sulphonic, any of a class of organic acids containing sulfur and having the general formula RSO₃,H in which R is an organic combining group. The sulfonic acids are among the most important of the organosulfur compouds ; the free acids are widely used as catalysts in organic syntheses, while the salts and other derivatives form the basis of the manufacture of detergents, water-soluble dyes and catalysts, sulfonamide pharmaceuticals, and ion-exchange resins.