A continuous flow process for sulfonating 1,2.diaminobenzene comprises introducing a sulfonation mixture into a microreactor inlet of a continuous flow microreactor to produce a flow of the sulfonation mixture through the continuous flow microreactor. The sulfonation mixture comprises 1,2 aminobenzene dissolved in a molar excess of sulfuric acid. The continuous flow microreactor comprises one or more individual fluidic modules each having various features with respect to channel width and thermal management. The process further comprises maintaining a reaction temperature of from about 150.230 deg C. in at least a portion of the individual fluidic modules while the sulfonation mixture flows from the microreactor inlet to the microreactor outlet. Thereupon, the sulfonation mixture is received from the microreactor outlet. Finally, a sulfonated reaction product is precipitated out of the sulfonation mixture received from the microreactor outlet. The sulfonated reaction product is 3,4.diaminosulfonic acid, free of doubly sulfonated impurities.