The effect of pH on the extraction of microgram levels of hafnium as the nitrosophenylhydroxylamine complex with various non-aqueous solvents

Abstract

The separation and determination of microgram levels of hafnium has historically been a cumbersome, difficult procedure. The nitrosophenyl-hydroxylamine (cupferron) complex of hafnium can be extracted from an aqueous solution with an immiscible non-aqueous solvent. The pH at which the extraction occurs and the solvent used are both contributing factors which determine the percent of hafnium cupferrate extracted from the aqueous phase. In this study, the hafnium remaining in the aqueous phase, following an organic decomposition, is determined by x-ray fluorescence following precipitation and filtration of hafnium cupferrate on a membrane filter. Other methods of hafnium detection were found to be unsuitable for this application. Graphite furnace atomic absorption was inadequate due to insufficient hafnium atomization, residual hafnium in the graphite tube, and high detection limits. Those factors lead to poor reproducibility of the standard curve. Another x-ray fluorescence technique in which hafnium was collected on a graphite slurry, mixed with bakelite, and pressed into a wafer for x-ray fluorescence determination, proved to be inferior to the membrane filtration technique because of lower total counts of hafnium for the same levels of hafnium and difficulties in reproducing data. The extraction curves of pH versus percent extraction of hafnium cupferrate depend on the hafnium cupferrate equilibria and their relationship with the various characteristics of the solvents over the pH range being researched. The extraction curves obtained exhibited two areas of high extractability in highly acidic solutions and slightly alkaline solutions. The curves also contained two areas of lower extractability in weakly acidic solutions and strongly alkaline solutions. Characteristics of the solvents studied that were found to exert major influences on the extraction of the hafnium cupferrate complex include polarity, the dielectric constant, and functional groups present. The use of chloroform as the extraction solvent with an aqueous-phase pH of less than 1.0, provides the most selective, quantitative extraction of hafnium. Depending on the particular application, this proper combination of pH and solvent can provide the desired conditions for either analysis, or removal of hafnium in the organic extract or the aqueous portion after the extraction.