Masters Degrees (Chemistry)

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Browsing Masters Degrees (Chemistry) by Subject "Accuracy"

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    Method development for the quantification of selected early rare earth elements
    (University of the Free State, 2015-01) Xaba, Sibongile Mamusa; Purcell, W.; Venter, J. A.
    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.
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    Quantification of hafnium in selected inorganic and organometallic compounds
    (University of the Free State, 2016-02) Malefo, Gontse Atlholang Adeline; Purcell, W.; Nel, J. T.; Nete, M.
    English: The aim of this study was to develop a method for the dissolution and quantification of hafnium in hafnium containing compounds (which include the metal oxide, different inorganic compounds as well as a number of organometallic complexes). Various digestion techniques such as open vessel, flux fusion and microwave acid-assisted system were evaluated. The same time different reagents which include HNO3, HCl, H2SO4 alone and in combination with salts, e.g. (NH4)2SO4 + H2SO4 were evaluated while experimental conditions such as time and temperature were varied. Different analytical techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES), infrared spectroscopy (IR), CHNS-micro analyses and X-ray crystallography were used for the quantification and characterisation of the synthesized hafnium compounds. The criteria used to select the Hf emission wavelength for ICPOES analysis was its sensitivity and the absence of spectral interferences from the acids used or the other elements present in solution. The experimental ICP-OES results obtained for the quantification of hafnium were also validated using different validation parameters which include accuracy, precision and the hypothesis test at a 95 % confidence interval to evaluate the validity of the most suitable digestion methods that were developed. Open vessels digestion of HfF4 using 98 % H2SO4 or 65 % HNO3 resulted in good hafnium recoveries which ranged from 97.8 – 99.9 % with relative standard deviation (RSD) within the range of 0.4302 – 0.4327 %. The hafnium content was also quantified in a number of newly synthesized hexafluorohafnate complexes as well as a thiocyanate complex. Hafnium recoveries ranged from 79(6) to 103(3) % for the sodium, potassium, rubidium, cesium, ammonium, methyl ammonium and tetraphenyl phosphine hexafluorohafnate complexes as well for the hafnyl thiocyanate complex. All the synthesised products were characterized with IR while the crystal structures of K2HfF6, Rb2HfF6, Cs2HfF6 and (PPh4)2HfF6.2H2O were successfully done with X-ray crystallography. Various digestion techniques such as open vessel, flux fusion and microwave acidassisted system with different mineral reagents which include HCl, 65 % HNO3 and aqua regia, 98 % H2SO4, a mixture of (NH4)2SO4 in 98 % H2SO4 and a mixture of NH4F and 98 % H2SO4 were investigated for the dissolution of hafnium oxide. Digestion by wet ashing yielded poor hafnium recoveries and ranged between 0.041(6) % and 3.82(1) %. Hafnium oxide was however successfully dissolved with flux fusion using Na2B4O7 followed by its dissolution with 98 % H2SO4. Hafnium recoveries improved from 72(4) to 100.8(7) %. Relatively poor hafnium recoveries ranging between 35(2) and 58(6) % (with time variation) were obtained using NaOH as flux and hafnium recoveries of 99(2) % were obtained using NH4HF2 as flux. Microwave acid-assisted digestion was also employed which improved the Hf recovery from 74.9(4) to 100(3) % with time and pressure as experimental variations. The method validation of the experimental results obtained for the quantification of hafnium using the hypothesis testing of at a 95 % confidence level was considered satisfactory. The experimentally obtained LOD values ranged from 0.0051 to 0.0985 and LOQs ranged between 0.051 to 0.9846 ppm in the different mineral acid used in this study. Other statistic parameters such as linearity and sensitivity were also investigated and gave satisfactory results.
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    Quantification of rhodium in series of inorganic and organometallic compounds
    (University of the Free State, 2009-11) Chiweshe, Trevor Trymore; Purcell, W.; Venter, J. A.; Mtshali, T.
    The main objective of this research was to establish an analytical method using inductively coupled plasma optical emission spectrometry (ICP-OES) to accurately quantify and optimize the percentage recovery of rhodium. Firstly a CRM was used to establish the effectiveness of the method and then rhodium metal, an inorganic sample and finally different organometallic compounds were analyzed to ensure proper recovery in different matrices. Quantitative determination of rhodium using cobalt as an internal standard yielded excellent results for the CRM, rhodium metal and RhCl3·xH2O of 99.0 % + compared to the yttrium internal standard which yielded values in the region of 140.0 %. This difference in percentage recovery was attributed to the differences in the first ionisation energy of cobalt (760.41 kJ mol-1) and yttrium (599.86 kJ mol-1) to that of rhodium (719.68 kJ mol-1). The large ionization energy difference between rhodium and yttrium made yttrium less suitable as an internal standard of rhodium analysis. Results also indicated that the rhodium recovery in RhCl3·xH2O, using the cobalt internal standard method, were shown to be influenced by the presence of easily ionized elements (EIE) such as the alkaline metals as well as unmatched acid(s) derived from sample preparation. These matrices were shown to decrease the percentage recovery of rhodium by between 2 to 14 % depending on the amount of acid or alkali metals that were added, which affected the robustness of the rhodium recovery. The experimental results for the rhodium analysis were validated for a large number of validation parameters, which included accuracy, precision, specificity, etc. to confirm whether the newly developed analytical procedure was suitable for the rhodium determination in terms of internationally required standards. The accuracy of the method in the rhodium determination of the CRM, rhodium metal and RhCl3·xH2O, were determined from the percentage recoveries of rhodium from these samples and calculated as 100.01, 99.69 and 99.79 % respectively. The percentage recovery of rhodium from the organometallic complexes was dependent on the purity of the complexes and the results were shown to vary from 81.43 to 99.97 %, with relative standard deviation (RSD) of between 0.26 and 1.87 for all the samples. The selectivity and specificity for rhodium in these samples were determined by the standard deviation of the slope (sa) and standard deviation of the intercept (sb) and was between 0.00029 – 0.00936 and 0.00102 – 0.03338 respectively. The uncertainties of the calibration curve (c) were between 0.0093 and 0.0795. The method was found to be sensitive to the acid matrices and EIE as was determined from the gradient (m) of the calibration curve which ranged from 0.2174 to 0.2933 in the determination of rhodium in RhCl3·xH2O. The rhodium limit of detection (LOD) and limit of quantitation (LOQ) were determined to be 0.0040865 and 0.040865 ppm respectively, which was feasible for measuring trace amounts of rhodium. The linearity of the calibration curve was determined from the regression coefficient (r2 and r) and ranged from 0.997 to 1.00. Statistical tests of the experimental results were calculated using the hypothesis test of the t-statistic at 95 % confidence interval to determine whether the results were acceptable as recommended by ISO 17025. The results determined from the CRM, rhodium metal, RhCl3·xH2O, [Rh(cupf)(CO)2] and [Rh(cupf)(PPh3)(Me)(I)] with t values of -1.12, -0.50, 0.00, 0.00 and -1.60 respectively, were accepted at 95 % confidence interval using the t-statistic test. The results obtained using this method was shown to be reproducible for all the experimental measurements except in the cases where the matrix effects were very complex and/or the purity of the sample under suspicion.