5,5′-Dithiobis(2-nitrobenzoic acid) is also called DTNB or Ellman′s reagent.It is a water soluble reagent, which interacts with thiol groups, and can be used conveniently at neutral pH.Thiolate anion (RS-) reacts with Ellman′s reagent (DTNB2-), to produce 2-nitro-5-thiobenzoate anion (TNB2-) and one mixed disulfide (R-S-TNB-). TNB2-has a high molar absorption coefficient, and absorbs at 412nm. This absorption is independent of pH, at pH> 7.3.