Analytical Sciences

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

Tomoya SUZUKI,* Takeshi OGATA,* Mikiya TANAKA,* Tohru KOBAYASHI,** Hideaki SHIWAKU,** Tsuyoshi YAITA,** and Hirokazu NARITA*

*Environmental Management Research Institute, National Institute of Advanced Industrial Science and Technology (AIST) 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
**Materials Sciences Research Center, Japan Atomic Energy Agency (JAEA), 1-1-1 Koto, Sayo, Hyogo 679-5148, Japan

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).

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