Stoichiometry of superconducting YBa2Cu3Oy. Determination of Cu(III)/Cu(II) ratio and oxygen content

The electrical properties of the high T_c superconductor YBa₂Cu₃O_y depend on its oxygen content. The oxygen content is indirectly determined by iodimetric measurement of the oxidation state of copper. A combination of two titrations, with and without addition of KI, prior to dissolving the sample permits the determination of the two species Cu(II) and Cu(III). A simplified automatic titration with potentiometric detection of the end-point is described. The method is suitable for rapid and reliable determinations of the Cu(III)/Cu(II) ratio and total copper content and for controlling the stoichiometry of the compound.Samples of the superconductor were analysed and the stoichiometric coefficient y for oxygen was determined with excellent results. Typically, y=6.811 ± 0.0063 (s.d.) (n=5). A comparison of the total copper content (found by direct analytical determination) with the copper concentration calculated from the stoichiometric formula gives an evaluation of possible deterioration of the sample.