Hydroquinone → p-Dihydroxy benzene disulfonic acid

190 g of p-dihydroxy benzene monosulfonic acid, 100 ml of dioxane, and 100 ml of perchloroethylene are mixed in a flask provided with a reflux condenser. The flask is heated in an oil bath to the reflux temperature (in the oil bath at 140° C.). It is refluxed for half an hour, and then is left to cool a little to set the coolant for distillation. In the flask, two layers are formed: a colourless lower layer, and a coloured upper layer in which the acid is dissolved. The flask is heated again and 100 ml of the solvent mixture are distilled. In the mixture collected, two layers form, the upper, moderate, is aqueous. To the flask provided with a reflux condenser, 98 g of sulfuric acid containing 20 % of SO3 are added, and it is heated again, to a lower temperature, so that the reflux is barely noticeable (bath temperature: 130° C.), for 4 hours. The liquids accompanying the acid are removed by distilling at reduced pressure, the residue is dissolved in a little water, 2 g of active charcoal are added, and it is filtered. The solution is concentrated, cooled, and is precipitated by adding five volumes of acetone. The precipitate is filtered, and 189 g of p-dihydroxy benzene disulfonic acid, with a melting point of 166°-169° C. are obtained.