Mechanism of 5-amino-2-formylbenzene sulfonic acid formation during reduction of 4,4′-dinitrostilbene-2,2′-disulfonic acid by Zero-Valent Iron

5-Amino-2-formylbenzene sulfonic acid (ABAS) as a by-product during reduction of 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNS) by Zero-Valent Iron has been detected. Experiments show that the pH and temperature are the most important factors controlling the formation of ABAS. In addition, diluted solution of DNS (<0.001 mol/L) favors the formation of ABAS other than 4,4′-diaminostilbene-2,2′-disulfonic acid (DSD). Competitive oxidative reactions in dark with the cleavage of CC double bond by Fe(OH)2+ between DNS and DSD may occur during the reduction process. In addition, we provide mechanistic explanations for the pH, temperature and initial concentration dependences of ABAS formation. Optimized conditions with pH being 5.5 and rapid cooling following incubation have been applied to yield DSD with purity of more than 99.0%.