Unique Anion-exchange Properties of 3,3'-Diaminobenzidine Resulting in High Selectivity for Rhodium(III) over Palladium(II) and Platinum(IV) in a Concentrated Hydrochloric Acid Solution.

The effective recovery of Rh(III) from mixtures also containing Pd(II) and Pt(IV) is one of the most difficult tasks in platinum group metal refining. Adding 3,3'-diaminobenzidine (DAB) to 7 and 10 M HCl aqueous solutions containing Rh(III), Pd(II), and Pt(IV) chlorido species affords the effective separation of Rh(III) from Pd(II) and Pt(IV) through a process where Rh(III) becomes sequestered into solid phases composed of DAB. The stoichiometry and inner coordination sphere of the metal in Rh-DAB complexes were determined by estimating the Rh(III), H+, and Cl- concentrations in the solid phase and X-ray absorption fine structure measurements to clarify the mechanism of DAB selectivity for Rh(III). These results indicate that the Rh-DAB reaction in a concentrated HCl solution occurs in two steps: (1) the precipitation of DAB trihydrochloride salts, where DAB's amino groups are protonated and (2) anion exchange of the trihydrochloride salts for chloride ions with [RhCl6]3-, which is the predominant species in a concentrated HCl solution. By contrast, ion-pair complexes with [PdCl4]2- and [PtCl6]2- were not observed in DAB phases. The significantly lower affinity of the DAB trihydro cation for [PtCl6]2- and [PdCl4]2- than for [RhCl6]3- in 7 and 10 M HCl solutions accounts for the effective separation of Rh(III) from Pd(II) and Pt(IV).