Studies On The Adsorption Voltammetry Of The Complexes Of Rare Earths (â…¢) At Carbon Paste Electrodes

Carbon paste electrodes (CPEs) are characteristics of low background currents and flat measure baseline over a range of the large positive potentials in aqueous solution. The paper investigates for the first time oxidation peaks of the complexes of rare earths at a CPE by adsorptive stripping voltammetry, and presents several sensitive procedures for quantifying various rare earth elements based on the adsorptive oxidation peak. A new approach is provided for determination of non-electroactive metals such as rare earths, zirconium, thorium etc.The paper consists of three chapters.The first chapter deals with the adsorption voltammetry for the determination of middle and heavy rare earths in the presence of alizarin complexon (ALC) at a CPE. In a supporting electrolyte of 0.12 mol/L HAc-NaAc and 0.03 mol/L potassium biphthalate (pH5.0), the complexes of middle and heavy rare earths(III) with ALC yield a sensitive adsorptive oxidation peak when linear-scanning at 100mV/s from -0.2 to 0.8 V. The second-order derivative peak currents are proportional to the concentrations of middle and heavy rare earths(III). The sensitivity of the complexes of heavy rare earths(III) (Dy, Ho, Er, Tm, Yb, Lu) is higher than that of middle rare earths(III) (Sm, Eu, Gd, Tb)and Y. For heavy rare earths the detection limit attains 5.0×10-10 mol/L (S/N=3) for a 120 s preconcentration, the linear range is 1.0×10-9 -2.0×10-7 mol/L. For middle rare earths and Y the linear range is 3.0×10-9-3.0×10-7mol/L, and the detection limit is 2.0×10-9 mol/L(S/N=3) for a 120 s preconcentration. The relative standard deviation is 3.8 % for 20 successive determinations of 4.0×10-8 mol/L Ho3+ on the same electrode surface. The electrode processes of the adsorbed complex are investigated. The developed method has been applied to determination of the sum of rare earths (except La, Ce, Pr, Nd, Sc) in the samples of rare earth nodular graphite cast iron with satisfactory results.The second chapter presents a new procedure for determination of trace cerium based on the adsorption voltammetry of the Ce3+- A1C complex at a CPE. The procedure is convenient to determine alone cerium in the presence of other rare earths because there is a 100 mV difference between the peak potentials of Ce3+- A1C and other rare earth(III)-ALC complexes in a supporting electrolyte of 0.08 mol/L HAc-NaAc and 0.012 mol/L potassium biphthalate(pH 4.7) when linear-scanning at 100 mV/s from -0.2 V to 0.8 V. The second-order derivative peak currents are directly proportional to Ce3+ concentration over a range of 6.0×10-9-3.0×10-7mol/L. The detection limit is as low as 3.0×10-9mol/L (S/N=3) for 120 s preconcentration times. The RSD of 3.5% is obtained for 15 determinations of Ce3+at 4.0×10-8mol/L level on a same CPE surface. The proposed method has been applied to determination of cerium in the samples of rare earth nodular graphite cast iron.The final chapter is devoted to electrochemical behavior of the scandium(III)-ALC complex at a CPE. A sensitive adsorptive oxidation peak of the scandium(III)-ALC complex is obtained at the peak potential of 0.66 V in 0.08 mol/L HAc-NaAc- 0.02 mol/L potassium biphthalate(pH4.3) when linear-scanning from -0.2 V to 0.8 V at 150 mV/s. The concentrations of Sc(III) are directly proportional to the second-order derivative peak currents over a range of 2.0×10-9-6.0×10-7 mol/L. The detection limit for scandium is as low as 1.0×10-9 mol/L (S/N=3) for 120 s accumulation time. The relative standard deviation is 3.7% for 15successive determinations on the same electrode surface at the 4.0×10-8 mol/L level. The electrode processes of the adsorbed complex are proposed. The method has been applied successfully to the determination of scandium in ore samples.From the above, adsorptive oxidation behavior of various rare earths (III)-ALC at a CPE is very different within the positive potentials. The difference can be used for determination of middle and heavy rare earths, cerium and scandium, respect...