Determination of average valence of thallium in superconductors having high critical temperature by capillary electrophoresis

Abstract

The determination of the average valence of thallium in a high-Tc superconductor was investigated by capillary electrophoresis. The amount of sample required was decreased from 5-7 mg to 0.2 mg by a procedure developed in this investigation. As the thallium(I) aqua ion, itself, absorbed in the UV domain, the ion was measured directly at 215 nm after migration as a cation at 25 kV. The shorter migration time enhanced the precision. Therefore, the applied voltage was selected to be 25 kV at a migration time of 3 min. The sample solution was injected in the capillary by the hydrodynamic method. A 10 nl portior of sample was introduced based on the siphon principle by injection for 900 s. A calibration curve, which was linear at 0-10 ppm, was obtained under the optimal conditions. On the other hand, thallium(III) was detected at 242 nm as an anion in the presence of a large amount of chloride ions. The applied voltage on the thallium(III) was -30 kV using a migration solution (pH buffer solution) adjusted at pH 2.25. Then, the migration time was 7 min. The calibration curve for thallium (III) obtained from the peak area was linear at a concentration below 250 ppm. A simultaneous composition analysis of the thallium system superconductor was performed by an indirect detection method after the addition of imidazole into the migration solution. Ca2+, Ba2+, and Cu2+ were separated at 12 min and determined at 210 nm. The compositions of two kinds of sample examined were 1:2:2:3 (Tl:Ba:Ca:Cu) and 1:2:4:5. The average valences (2.97 and 2.95) of thallium in superconductors could be calculated according to the following equation: n = 3 {2[Tl+/Total Tl]}. The results agreed well with those obtained by iodometry as a conventional method. The total sample amount required in this method was 0.2 mg for Tl(I) and Tl(III), which was dissolved in 0.5 ml of 0.01 M HCl. © 2006 The Japan Society for Analytical Chemistry.