Method development for the quantification of selected early rare earth elements

English: The aim of this study was to develop an analytical procedure to accurately quantify the early REE (La, Ce and Nd) in pure REE metal, in inorganic compounds as well as in a mixture containing all three elements and finally in organometallic complexes. The study involved the use of different inorganic acids such as H2SO4, HNO3 and HCI as dissolution reagents and determination of their influence on the La, Ce and Nd recoveries. Both bench top and microwave dissolution were used in this study. Different analytical techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES), CHNS-micro analyser and infrared spectroscopy (IR) were used to characterise and quantify the REE in the different samples. Validation parameters such as accuracy, precision, linearity, sensitivity, etc., using ICP-OES analysis were also evaluated (Chapter 4) to determine the suitability of the digestion and quantifying methods for REE analyses. Analytical lines (wavelengths) were carefully selected to minimize or prevent any spectral overlap with the other rare earth elements investigated in this study. Secondly, the acid matrix was strictly matched throughout the analytical process to ensure accurate analytical measurements and well controlled experimental conditions. Bench top dissolution with 98 % H2SO4, 65 % HNO3 or 32 % HCI yielded excellent La and Nd recoveries from the pure REE metal, inorganic compounds as well as in a mixture containing all three elements, ranging from 97.9(5) to 100.6(1) %. Ce recovery in the pure metal only ranged from 81.37(9) to 95.30(8) %. Microwave assisted acid digestion was employed to improve the recovery of Ce metal. The Ce recoveries improved to 96.5 and 99.6 % after microwave digestion. The efficiency of the acids in dissolving and recovering Ce in the pure metal sample was in the order HCI < HNO3 < H2SO4. Excellent metal recoveries for the synthesised organometallic complexes ranging from 98.0(2) to 103.6(4) % for acac, imda and nta were obtained for the different organometallic complexes. ([Ln(dap)(NO3)3] Ln = La, Ce) yielded metal recoveries ranging from 88.1(5) to 95.5(3) % using bench top dissolution. The metal recoveries of TPPO complexes ranged between 98.45(5) and 99.4(2) % after microwave digestion. The method validation done in Section 5.8 was considered satisfactory. All calibration curves showed good linearity with excellent r2 values which range between 0.9997 to 1 and fairly constant slopes. The elemental LODs ranged between 0.0030 - 0.0188 ppm and LOQ between 0.0289 - 0.1881 ppm in the different acid matrices used in this study. It can be concluded that the La, Ce and Nd analyses in pure REE metal, inorganic compounds and organometallic complexes were successful and the results met most of the guidelines within acceptable criteria as set out in ISO 17025. The successful metal recoveries in acac, imda, nta and TPPO complexes were also due to the fact that these complexes have been crystallographically characterised and their chemical structures are well known, whereas the metal-dap complexes has not yet been fully characterised. Characterisation of synthesised organometallic complexes and ligands using IR also played a huge role in illustrating the possible coordination of the metal and ligands. The IR spectra were analysed by simple comparisons of the stretching frequencies between the unreacted samples and the reaction products. The shifting to low or high wavenumbers and disappearance or appearance of peaks was used as a measure for the possible formation of a new product.