Determination of selenium sulfide using 1,3-dibromo-5,5-dimethylhydantoin (DBH). Analytical methods of pharmacopeias with DBH in respect to environmental and economical concern Part 11.
USP 2000 (The United States Pharmacopeia, Rockville, MD, 24th ed., 2000, pp. 1514-1515) and PH. EUR. 1997 (European Pharmacopoeia, 3rd ed., Council of Europe, Strasbourg, 1997, p. 1459) dissolve selenium sulfide by boiling with fuming nitric acid for about 1 h. After cooling to room temperature and dilution with water nitrous acid and nitrogen oxides are removed with urea also by boiling before the iodometric titration is performed. This method can be importantly simplified and improved, when using 1,3-dibromo-5,5-dimethylhydantoin (DBH) in glacial acetic acid in presence of solid potassium bromide. Selenium sulfide is dissolved during 5-10 min at room temperature. The excess of DBH and bromine can be removed with 5-sulfosalicylic acid also without boiling. The point of the indicator change for the iodometric titration is improved in comparison to the method of the pharmacopeias, because the colloidal solution of selenium mostly coagulates. Also the sulfur content of selenium sulfide can be determined with DBH.
The elimination of selenium(IV) from aqueous solution by precipitation with sodium sulfide.
In this study, the removal/precipitation of selenium with sodium sulfide from initially weakly acidic sulfate solutions containing 300 mg/L of selenium(IV) at 23 °C was studied. The results showed that, below a pH of approximately 7.0, the precipitation reaction was complete at a sulfide to selenium ratio above 1.8 and less than 11 with less than 0.005 mg/L of soluble selenium remaining in solution. When the pH rose between 7.0 and 9.5 the precipitation of selenium was incomplete. Above pH 9.5 the solution turned dark red but no precipitation was apparent. The precipitation reaction started as soon as the sodium sulfide was added in the selenium-bearing solution and was completed in less than 10 min. The orange "selenium sulfide" precipitates, characterized using X-ray diffraction, scanning electron microscopy and chemical analysis, were crystalline in the form of aggregated dense particles with their sulfur/selenium molar ratio varying from 1.7 to 2.3. The precipitate was deduced to be a Se-S solid solution consisting of ring molecules of the following Se(n)S(8-n) formula, where n = 2.5-3. Long term leachability tests (>2 month equilibration) under ambient conditions at pH 7 showed the produced precipitate to be essentially insoluble (<0.005 mg/L).
HRTEM study of orthorhombic zirconia in MgO-PSZ
High-resolution transmission electron microscopy (HRTEM) was used to study the phase of orthorhombic ZrO2 formed in magnesia partially stabilized zirconia (MgO-PSZ) during HRTEM specimen preparation. Based on the three reported crystal structures of orthorhombic ZrO2, with the space groups Pbcm, Pbc21 and Pbca, here it is shown that orthorhombic ZrO2 formed in MgO-PSZ has the Pbcm structure.