ABSTRACT
Compounds containing two or three such groups are sometimes referred to in the histochemical literature as tetrazonium and hexazonium derivatives respectively; see below for further comments on the nomenclature of diazonium salts. Diazonium salts are reactive electrophiles, and hence are unstable in aqueous solution, especially at extremes of pH. Occasionally only the amine is commercially available, the diazonium salt being prepared in the laboratory immediately prior to use. Depending on stereochemistry and patterns of substitution, reaction rates of diazonium salts with coupling agents vary markedly; as has been briefly reviewed by Zollinger. Azo dye reaction products arising with these reagents are of sufficiently high affinity to resist extraction into dehydrating and clearing solvents. Fast blue B is also used for the indirect staining of tissue substrates, for instance of carbonyls following their reaction with naphthoic acid hydrazide. To obtain highly substantive final stains, a copper complex of the azo dye reaction product is sometimes produced, by treatment with copper sulfate.
