Trace determination of rare earths by adsorption voltammetry at a carbon paste electrode

A novel adsorption voltammetry has been described for the determination of trace rare earths in the presence of alizarin complexon (ALC) at a carbon paste electrode (CPE). A sensitive adsorptive oxidation peak was found for the complexes of middle and heavy rare earths(III) with ALC in a supporting electrolyte of 0.12 M HAc + NaAc and 0.03 M potassium biphthalate (pH 5.0) for linear-scanning from −0.2 to 0.8 V at 100 mV s−1. The sensitivity of the heavy rare earths (Dy, Ho, Er, Tm, Yb, and Lu) is higher than that of other rare earths. Under the optimized procedure, a detection limit of 5.0 × 10−10 M holmium(III) (S/N=3) was realized for a 120 s preconcentration and the linear current–concentration relationship was observed up to 2.0 × 10−7 M. The relative standard deviation is 3.8% for 20 successive determinations of 4.0 × 10−8 M Ho(III) on the same electrode surface. The electrode processes of the adsorbed complex were investigated. The method developed was applied for determination of the sum of rare earths (except La, Ce, Pr, Nd, and Sc) in rare earth nodular cast iron samples with satisfactory results.