Ellman's reagent is usedfor the modification of free thiols in proteins. It rapidly forms a disulfide bond with the thiol and releases a thiolate ion which is colored. The maximal absorbance of this thiolate is at 412 nm. It's presence can be plotted against a standard curve to determine the total amount of free thiols in proteins.
Figure 1: Reaction of Ellman's Reagent with sulfhydryls