Formazan

They have a variety of colors from dark blue to deep red to orange, depending on the original tetrazolium salt used as the substrate for the reaction.

Hydrazones, which are electron-rich compounds, react with diazonium salts either at a nitrogen or a carbon atom to produce formazans.

Diazonium salts couple to the amine nitrogen in the hydrazone with displacement of a hydrogen to give the intermediate, which then rearranges to the formazan.

Under basic conditions, ethyl nitrate reacts at the methylene position to yield 3-methyl-1,5-diphenylformazan, which can also be obtained from the reaction of phenylazoethane with isoamyl nitrite.

Additionally, formazans can be obtained by the decomposition of substituted tetrazolium salts either photochemically or under the influence of ascorbic acid in an alkaline medium.