A highly sensitive chelator for metal staining, bromopyridylazo-diethylaminophenol.

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol has been found to be capable of staining very small amounts of metals, particularly copper, iron, lead, and zinc, in tissue. The differentiation of a target metal from interfering metals is achieved by using masking agents, polyphosphates, and potassium cyanide.