A colourless crystalline aromatic compound, C6H4(OH)2; m.p. 111°C. It is made by the fusion of benzenedisulphonic acid with sodium hydroxide and used mainly in the dyestuffs industry. On fusing with phthalic anhydride it forms fluorescein dyes. It is also used to make cold-setting adhesives (with methanal), plasticizers, and resins.