It is prepared by azo coupling of the bis(diazonium) derivative of benzidine with naphthionic acid. A variety of benzidine derivatives are used in dyes and polymers. This intermediate undergoes the benzidine rearrangement to afford 3,3'-dichlorobenzidine. Upon treatment with the bis(diazonium) derivative of benzidine, 1-aminonaphthalene-4-sulfonic acid gives Congo red. Benzidine is prepared in a two step process from nitrobenzene. It is positive in the benzidine test and weakly positive in methyl red test. An enzyme in blood causes the oxidation of benzidine to a distinctively blue-coloured derivative. Benzidine has been linked to bladder and pancreatic cancer. In the past, benzidine was used to test for blood.