Simultaneous determination of rhodium and palladium with 5-(-5-Iodo-2-pyridylazo)-2,4-diaminotoluene by double-wavelength overlapping spectrophotometry

A novel method for the simultaneous determination of rhodium and palladium with 5-(-5-Iodo-2-pyridylazo)-2,4-diaminotoluene(5-I-PADAT) by double-wavelength overlapping spectrophotometry was developed.The experimental results show that in the medium of 0.6 ~ 1.8 mol/L HCIO4,Pd(II) reacts with 5-I-PADAT to form a stable complex,but Rh(III) can't react in such a strong acidic medium.Rh reacts with 5-I-PADAT in the pH range of 4.0~5.2 to form a red complex which can be transferred into a deep-colored stable violet-blue species by adding mineral acid.The experimental results also show that both rhodium and palladium complexes exhibit two absorption peaks.The strong peaks are at 580 nm and 584 nm,and the weak peaks are at 538 nm and 547 nm,respectively.Their sum for each element(Rh or Pd) showed good linearity with their concentration,respectively.By using the difference of acidity of reaction and the characteristic mentioned above,the entitled method was established.The apparent molar absorptivities of the rhodium and palladium complexes are 2.76×105L ·mol-1·cm-1 and 1.10×105 L·mol-1·cm-1,respectively.Beer's law was obeyed in the range of 0~0.5 μg/mL for Rh and 0~1.0 μg/mL for Pd.The method is simple and rapid,with high sensitivity and good selectivity.It has been applied to the determination of rhodium and palladium in catalyst samples with satisfactory results.