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以试剂2-(5-溴-4-甲基-2-吡啶偶氮)-5-二甲氨基苯胺(5-Br-4-CH3-PADMA)为显色剂,建立了双波长叠加分光光度法同时测定铑和钯的新方法.结果表明:钯与5-Br-4-CH3-PADMA在0.9~4.2 mol/L 高氯酸介质中,形成稳定络合物;而铑与5-Br-4-CH3-PADMA在pH值为4.2~5.0的近中性介质中形成稳定络合物,络合物一旦形成则很稳定,向其中加入强酸酸化,该配合物不仅不分解,反而吸收峰红移,吸光度增大.研究还发现,铑、钯与5-Br-4-CH3-PADMA形成的络合物,均呈现两个强弱不等的吸收峰,强峰分别位于605 nm和606 nm,弱峰分别位于558 nm和563 nm.在605 nm和562 nm处,其各自的强弱峰对应的吸光度之和与溶液中铑、钯的质量浓度具有良好的线性关系.铑、钯质量浓度分别在0~0.55 μg/mL 和0~1.04 μg/mL范围内符合比尔定律;利用双波长叠加的分光光度法测得铑、钯的表观摩尔吸光系数分别为εRh=2.64×105 L·mol-1·cm-1和εPd= 1.40 ×105 L·mol-1·cm-1,铑络合物的组成为n(Rh)∶n(5-Br-4-CH3-PADMA) =1∶2,钯络合物的组成为n(Pd)∶n(5-Br-4-CH3-PADMA)=1∶1.方法用于实际样品催化剂中铑和钯的同时测定,结果的相对标准偏差(RSD,n=6)分别为1.4%和4.9%,测定值与原子吸收光谱法测定值相一致.

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=2.64×105 L·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.