When sulphanilic acid is added (in the picture its sulphonamide is shown instead), the nitrites form a diazonium salt. When the azo dye agent (N-alpha-naphthyl-ethylenediamine) is added a pink colour develops. This diamine is used in place of the simpler and cheaper alpha-naphthylamine because this latter is a potent carcinogen and moreover the diamine forms a more polar and hence a much more soluble dye in acidic aqueous medium