Sulfanilic acid (4-aminobenzenesulfonic acid) is a grey crystalline solid which exists as a zwitterion and has an unusually high melting point. As the compound readily forms diazo compounds, it is used to make dyes and sulpha 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.
Sulfanilic acid can be produced by sulfonation of aniline：
Sulfanilic acid has four primary end uses. It is used as an intermediate in the production of yellow food dye, specific pharmaceutical applications such as sulfodrugs, optical brighteners for white paper and as a concrete additive.