1,4-phenylenediamine-2.5-disulfonic is obtained from p-phenylenediamine and 25% oleum at 140 ℃ or by heating p-phenylenediamine sulfate and 98% sodium hydrogen sulfate in a mixture of 1,2,4-trichlorobenzene and 1-chloronaphthalene to the boiling point of the solvent mixture (215 ℃), removing the solid roduct, dissolving it in water, neutralizing it with sodium hydroxide solution to pH 7.5 – 8, and clarifying the solution with kieselguhr. The product contains 5% of monosulfonic acid, but only a small amount of the isomeric 2,6-disulfonic acid; yield 90%
