DTNB is known as Ellman's reagent. It is used for the colorimetric determination of thiol groups in biological samples.It is fairly soluble in water. Colorless DTNB is converted to yellow 5-Mercapto-2-nitrobenzoic acid in the presence of thiol compounds. For example, DTNB in combination with S-acetylthiocholine iodide as the substrate is widely used to assay choline esterase activity.