Simultaneous determination of palladium and rhodium with2-(5-bromo-4-methyl-2-pyridylazo)-5-dimethylaminoaniline by double-wavelength overlapping spectrophotometry
HUO Yan-yan
School of Chemical Engineering, Xi’an University, Xi’an 710065,China
Abstract：A novelsimultaneous determination method of rhodium and palladium by double-wavelength overlapping spectrophotometry was established using 2-(5-bromo-4-methyl-2-pyridylazo)-5-dimethylaminoaniline (5-Br-4-CH3-PADMA) as coloring reagent. The experimental results showed thatpalladium could react with 5-Br-4-CH3-PADMA in 0.9-4.2 mol/L HClO4 medium to form a stable complex, while rhodium could react with 5-Br-4-CH3-PADMAI to form stable complex in nearly neutral medium at pH 4.2-5.0. The complex was very stable once formation and it would be not decomposed after adding strong acid. Moreover, the absorption peak red shifted and the absorbance increased. It was also found that both complexes formed from rhodiumand palladium with 5-Br-4-CH3-PADMA showed two absorption peaks with different intensities. The strong peaks were located at 605 nm and 606 nm, and the weak peaks were located at 558 nm and 563 nm, respectively. The sum of absorbance at 605 nm and 562 nm showed good linearity to the mass concentration of rhodiumand palladium, respectively. The apparent molar absorptivity of rhodium and palladium was measured by double-wavelength overlapping spectrophotometry: εRh=2.64×105L·mol-1·cm-1 and εPd =1.40 ×105 L·mol-1·cm-1. The composition of complex was n(Rh):n(5-Br-4-CH3-PADMA)=1∶2 and n(Pd)∶n(5-Br-4-CH3-PADMA)=1∶1. The proposed method was applied to thesimultaneous determination of palladium and rhodium in actualcatalyst sample. Therelative standard deviation (RSD, n=6) was 1.4% and 4.9%, respectively. The found results were consistent with those obtained by atomic absorption spectrometry.
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