Visual Detection of Selenium(IV) Using a Gallium(III) Complex Retained in a Support Filter

A sensing membrane for selenium(IV) was fabricated by retaining tris(2,4-pentanedionato) gallium(III) (Ga(acac)(3)) in a glass-fiber filter. From a sample solution containing selenium(IV), gaseous hydrogen selenide was generated by reducing-vaporization and was passed through the membrane while turning the color to reddish pink. The color difference (Delta E*(ab)) before and after the reaction was measured by a reflection spectrometer. The Delta E*(ab) value increased with the increase of the selenium(IV) concentration. From the reflective spectra of reacted membranes; gallium(III) selenide (Ga2Se3) was suggested to be formed. Most of the coexisting cations showed no significant interference to the reaction, besides a large amount of arsenic(V) and antimony(III) suppressed the color change. Since adding iron(II) salt resulted in enhancing the color difference, 10 mg dm(-3) of iron(II) was used as an accelerating additive. In order to investigate the influence of ligands in gallium(III) complexes, several gallium(III) compounds were retained in a filter, and examined as sensing materials. The complexes of beta-diketones and dithiocarbamates had appropriate sensing activity, while inorganic salts, like gallium(III) sulfate or nitrate, showed no color change. This suggests that ligands having larger pKa values are effective for the reaction. The complexes of monobasic, dibasic and tribasic carboxylic acids showed decreased color differences in this order, and those of multidentate ligands, e.g., EDTA, exhibited no color change. These results showed excessively stable gallium(III) complexes have no reactivity to hydrogen selenide. Visual detection of 0.01 mg dm(-3) selenium(IV) was achieved in the presence of 10 mg dm(-3) iron(II) in a 50 cm(3) sample solution.