Merck, D8130-25G, 5,5′-Dithiobis(2-nitrobenzoic acid)

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.