Selective determination of holmium in rare earth mixtures by second-derivative spectrophotometry with benzoylindan-1,3-dione and cetylpyridinium chloride

Abstract

The absorption spectra of 4f electron transitions of the holmium complex with benzoylindan-1,3-dione and cetylpyridinium chloride were studied by conventional and derivative spectrophotometry. The molar absorptivity of the complex at the maximum absorption band was about 22 times greater (at 450 nm) than that in the absence of the complexing agent. The use of second-derivative spectra both eliminates the interference from other rare earths and improves the sensitivity for holmium. The calibration graph showed that the complex obeys Beer's law up to 25 µg cm^–3 of holmium. The corresponding value of Sandell's sensitivity is 0.36 µg cm^–2. The RSD evaluated from 11 independent determinations of 10 µg of holmium is 0.96%. The detection limit, obtained from the sensitivity of the calibration graph and for 3s_b(s_b= standard deviation of a blank without holmium, n= 11), was found to be 0.43 mg dm^–3 of holmium. The quantification limit, obtained for 10s_b, was 1.43 µg cm^–3 of holmium. A method for the direct determination of holmium in rare earth mixtures with good selectivity and accuracy is described.