4-Diazobenzenesulfonic acid - moistened with water (H2O~50%)

4-Diazobenzenesulfonic acid is a reagent used for the determination of phenols, amines and, bilirubin. It also has extensive usage as a protein-modifying reagent. This is because it reacts with a wide variety of functional groups in proteins including: tyrosine, histidine, amino, and thiol residues. It is known to react with proteins located on the exterior of erythrocytes; additionally, it has been used to study chloroplast and mitochondrial membranes.