4-(2-Pyridylazo)resorcinol (PAR), with CAS number 60-10-6, is an azo dye compound derived from resorcinol and 2-aminopyridine. It is a dark red to brownish-red crystalline powder, soluble in water and common organic solvents like ethanol and acetone. Its molecular structure features a pyridylazo group conjugated to a resorcinol moiety, which is responsible for its strong chelating properties and color changes upon metal binding. This compound is primarily used as a highly sensitive metallochromic indicator in analytical chemistry. It forms intensely colored complexes with a wide range of metal ions, including zinc, cadmium, lead, cobalt, nickel, copper, and rare earth elements. These complexes exhibit distinct color shifts, enabling the visual or spectrophotometric detection and quantitative determination of these metals in various samples, such as water, alloys, and biological fluids. PAR is commonly employed in complexometric titrations and as a reagent in spectrophotometric assays. Beyond its role as an indicator, PAR serves as a versatile chelating agent in laboratory research. It is used to study metal-protein interactions, in the separation of metal ions via chromatography, and as a staining agent in histochemistry. Due to its toxicity and potential environmental impact, appropriate safety precautions, including the use of personal protective equipment, are required during handling.