sulfonic acid (səlfŏn´Ĭk), organic compound containing the functional groupRSO₂OH, which consists of a sulfur atom, S, bonded to a carbon atom that may be part of a large aliphatic or aromatic hydrocarbon, R, and also bonded to three oxygen atoms, O, one of which has a hydrogen atom, H, attached to it. The hydrogen atom makes the compound acidic, much as the hydrogen of a carboxylic acid (see carboxyl group) makes it acidic (see acids and bases). However, while carboxylic acids are weak (with dissociation constants of about 10^-5), sulfonic acids are considered strong acids (with dissociation constants of about 10^-2). Because sulfonic acids are so acidic, they generally exist as their salts and thus tend to be quite soluble in water. Sulfonic acid groups are often introduced into organic molecules such as dyes to stabilize them for use in aqueous dye baths. Sulfonic acid groups also improve the washfastness of wool and silk dyes by enabling the dye to bind more tightly to the fabric. The most important use of sulfonic acid salts (sulfonates) is in the detergent industry. Sodium salts of long-chain aliphatic or aromatic sulfonic acids are used as detergents. Unlike ordinary soaps, which contain carboxylic acid salts, soaps containing sulfonates do not form a scum in hard water because the calcium and magnesium ions present in the hard water do not form insoluble precipitates with sulfonates as they do with carboxylates. Some sulfonic acid derivatives, e.g., the sulfa drugs, are important as antibiotics.