Aspects of Substoichiometry:The Rare Earths

Abstract

Substoichiometry has been studied in relation to the analytical chemistry of the rare earth metals. Substoichiometric analysis by solvent extraction is difficult with these metals due to the lack of strong, readily extractable chelates. There are, however, clear advantages in having new methods of trace analysis among current developments in the chemistry of this group. In seeking a solution to this problem, it has been possible to investigate more fully some of the underlying principles of substoichiometry as applied to isotope dilution analysis, and to a lesser extent activation analysis. No specific reagents are available for rare earth substoichiometry, but complexones such as EDTA can be used after preliminary separation of interferences. Use of this reagent involves substoichiometric reaction in aqueous solution, and the separation step requires the removal of unreacted metal into the second phase. The reagent system rare earth, EDTA, thenoyltrifluoroacetone was studied. A theory was developed and verified from which optimum conditions could be calculated under which substoichiometric separation of the EDTA complex was possible. Model analyses were conducted using standard known solutions of terbium and ytterbiun. Salicylic acid was studied as an extractant for rare earth metals, to decide whether such ion association systems offer useful alternatives in substoichiometry. The higher concentrations required introduce complications to the theory cited above. Despite this, and the lower degree of accuracy attainable in the experimental results, this type of extractant can be considered in an extension of the theory and its behaviour predicted in the same way. Ion exchange resins have been examined as an alternative to solvent extraction for removing free metal from the substoichiometric complex. The results confirmed that substoichiometry is possible under these conditions. A detailed study of the complex equilibria emphasised the importance of kinetics, and the need for more strict control of solution parameters.