Method for precisely analyzing the stoichiometry of NaxCoO₂-type superconductor material

• Published in: Analytical and bioanalytical chemistry Vol. 383; no. 7-8; pp. 1075 - 1081
• Main Authors: Acker, Jörg, Krabbes, Gernot
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg : Springer-Verlag 01-12-2005
• Subjects: Easily ionized element; ICP-OES; Internal standard; precision; Superconductor; titration
• ISSN: 1618-2642; 1618-2650
• DOI: 10.1007/s00216-005-0041-6

An analytical procedure for precisely determining the stoichiometry of NaxCoO₂-type superconductor material is presented. Sodium and cobalt contents, ranging between 3.5 and 11 mg L-¹ and 18 and 32 mg L-¹, respectively, were measured simultaneously using CID-ICP-OES. Sodium was found to significantly lower the emission intensity of cobalt, so the addition of 6.4 g L-¹ of the ionization buffer LiCl was required to compensate for this effect. The recoveries and precisions of the measurements were significantly increased by internal standardization using yttrium: Co(II) emission intensities at 230.786 nm, 237.862 nm, and 238.346 nm can be corrected using Y ion emission intensities, as can the atomic emissions of Co at 345.351 nm and Na at 589.592 nm. The cobalt contents of three real superconductor samples were independently verified by complexometric titration using EDTA. The valence state of cobalt was determined with a relative uncertainty of ~0.5% by redox titration using sodium oxalate as reductive agent and Ce(SO₄)₂ solution. The final stoichiometries of the superconductor samples can be calculated using the Na and Co contents, and the Co valence state. Conclusions about the quality of the prepared samples in terms of phase purity and presence of side products are drawn.