Gas-chromatographic determination of ultramicro amounts of selenium in pure sulphuric acid

Ultramicro amounts of selenium in pure sulphuric acid are converted into selenous acid with a bromine-bromide redox buffer solution. The selenous acid reacts quantitatively with 4-nitro-o-phenyl-enediamine to form 5-nitropiaselenol which can be extracted into toluene. The extract is very sensitive to electron-capture detection in gas chromatography, and the sensitivity is higher than that of 5-chloro-or 4, 5-dichloro-piaselenol. The calibration curve (peak heights) is linear up to 0.15 mug of selenium in 1 ml of toluene. Pure sulphuric acid, commercially available, contains 10(-6)$ 10(-5) % selenium.