Sulfanilic acid (4-aminobenzenesulfonic acid) is an organic compound with the formula H<sub>3</sub>NC<sub>6</sub>H<sub>4</sub>SO<sub>3</sub>. It is an off-white solid. It is a zwitterion, which explains its high melting point compared to the related compounds aniline and benzenesulfonic acid. It is a common building block in organic chemistry.
Sulfanilic acid can be produced by sulfonation of aniline with concentrated sulfuric acid. This proceeds via phenylsulfamic acid; a zwitterion with a N-S bond. Eugen Bamberger originally proposed a mechanism involving a series of intramolecular rearrangements, with phenylsulfamic acid forming orthanilic acid, which rearranged to sulfanilic acid on heating. Subsequent radiosulphur studies showed that the process is intermolecular, with the phenylsulfamic acid desulfating to generate sulfur trioxide, which then reacts with aniline at the para position in manner similar to a Bamberger rearrangement.
As the compound readily forms diazo compounds, it is used to make dyes and sulfa drugs. This property is also used for the quantitative analysis of nitrate and nitrite ions by diazonium coupling reaction with N-(1-Naphthyl)ethylenediamine, resulting in an azo dye, and the concentration of nitrate or nitrite ions were deduced from the color intensity of the resulting red solution by colorimetry.
It is also used as a standard in combustion analysis and in the Pauly reaction.
Reflecting its wide use, sulfanilic acid is found in the leachates of landfills. It is produced by reduction of some azo dyes.