Quantification of rhodium in series of inorganic and organometallic compounds
Chiweshe, Trevor Trymore
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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.