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

Abstract

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).