Quantification of hafnium in selected inorganic and organometallic compounds

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.