Catalytic spectrophotometric determination of rhodium in the rhodium-potassium periodate-ethylrhodamine B

The method of kinetic spectrophotometry for determination of trace Rh(III) had been reported by reference. We have discovered that trace the Rh(III) catalyze the decolorating oxidizine reaction of ethylrhodamine B by potassium periodate on heating in phosphoric acid and sodium chloride medium, the reaction rate on log (A0/A) values is linear over relatively range of concentration of Rh(III). Hence a new catalytic spectrophotometric method for determination of trace Rh(III) has been established. The method is simple, rapid and reproducible. The method has been used to determine trace Rh(III) in actual samples with satisfactory results. The results of condition experiment show that the maximum absorption was measured at 560 nm. The optimum conditions are 0.5-2.5 mL of 1.0 mol/L sodium chloride solution, 1.5-3.0 mL of 6.0 mol/L phosphoric acid solution, 1.4-1.8 mL of 0.01% ethylrhodamine B solution and 1.0-4.0 ml of 0.01 mol/L potassium periodate solution resulted in a maximum and constant log(A0/A) value for the system. Thus the use of 1.0, 2.0, 1.6 and 1.5 ml are recommended, respectively. The optimum temperature is 90°C. The linear relation between the log(A0/A) and concentration of Rh(III) is found in the range of 0.2-4.0 μg/25 ml and the detection limit is 2.24 × 10-10 g/ml. The relative standard deviation obtained from 11 standard solutions each containing 2 μg/25 mL Rh(III) is 2.13%. Os, Ru, Ir interferes with the spectrophotometric determination, other platinum-group metals, nonferrous and iron do not interfere. The reaction is first-order in ethylrhodamine B or rhodium, and apparent activation energy of catalytic reaction is found to be 65.21 kJ/mol. The method has been used to determine trace rhodium in some catalyst samples with the recovery of 95.6-102.6%, with relative standard diviations of 1.48-2.95%.