FE(III)/FE(II) SEPARATION BY SOLVENT-EXTRACTION FOR THE DETERMINATION OF OXYGEN STOICHIOMETRY IN YTTRIUM BARIUM COPPER-OXIDE SUPERCONDUCTOR MATERIALS

Oxygen stoichiometry is a critical parameter defining the Tc of cuprate superconductors (e.g. YBa2Cu3O7). On dissolution excess or deficiency of oxygen can be converted into shifts of the Fe(II)/Fe(III) concentration ratio of an aqueous solution. For small samples a solvent extraction technique for the separation of Fe(III) from Fe(II) was developed, to make use of the superior sensitivity of atomic spectrometry (AAS, ICP-AES). The system n-benzoylphenylhydroxylamine (BPHA)/CHCl3 was chosen because it is relatively inactive as a redox partner. Despite the catalytic effect of Cu(II) on the oxidation of Fe(II), oxidation blanks can be kept down at negligible levels. Less than 0.55% of residual Fe(II) is converted to Fe(III) during the extraction procedure (argon atmosphere). In the presence of air, oxidation levels are still practical (≤3%). Extraction is from 0.3 mol/l HBr providing excellent recovery of Fe(III) (e.g. 98.8%). All Fe(II), Y, Ba (including BaSO4 precipitate) and 99.4% of the Cu remain in the aqueous phase. Fe(III) is rapidly back-extracted into an aqueous phase by 6 mol/l HCl for dilution and aspiration into the flame or ICP. Recovery of Fe(III) after the two extraction steps is still 98.3%. © 1990 Springer-Verlag.