Nigrosin

In staining dyes, nigrosin (CI 50415, Solvent black 5) is a mixture of black synthetic dyes made by heating a mixture of nitrobenzene, aniline, and hydrochloric acid in the presence of copper or iron.

Its main industrial uses are as a colorant for lacquers and varnishes and in marker pen inks.

[2] Aside from its main use in lacquers, small amounts of nigrosin are used for negative staining of bacteria.

An advantage of using this method, rather than regular positive stains like methylene blue or carbol fuchsin, is that prior fixation by heat or alcohol is not needed, so the organisms are seen in more lifelike shapes.

[5] Nigrosin has minor application in industrial evaporation as a light-absorbing water-soluble pigment.