Determination of cobalt in iron and steel by reversed-phase HPLC using 2-(5-bromo-2-pyridylazo)-5-[N-n-propyl-N-(3-sulfopropyl)amino]phenol chelate as a precolumn derivatizing reagent

The determination of trace amounts of cobalt in iron and steel by reversed-phase HPLC was studied, The iron and steel were decomposed with a mixture of hydrochloric acid and nitric acid. After hydrochloric acid was added into the decomposition solution, the iron matrix in the solution was removed by extraction with 10 cm(3) of 4-methyl-2-pentanone. The decomposition solution was then fumed by sulfuric acid and dried. The residue was dissolved in 5 cm(3) of dilute nitric acid, followed by diluting to an appropriate volume with water. An aliquot of this solution was taken into 10 cm(3) of a volumetric flask and 1.0 cm(3) of a 2-(5-bromo-2-pyridylazo)-5-[N-n-propyl-N-( 3-sulfopropyl)amino]phenol solution (15 x 10(-3) mol dm(-3)); then, 1.0 cm(3) of tris (hydroxymethyl) aminsmrthane buffer (2.0 mol dm(-3), pH 8.0) were added. After the solution was diluted to 10 cm(3), 100 mm(3) was injected into the liquid chromatogph. The eluent used was aqueous-methanol (40 : 60 w/w) containing 1.25 x 10(-3) mol kg(-1) of EDTA, 5.0 x 10(-3) mol kg(-1) of tetrabuthyammonium bromide, and 1.0 x 10(-2) mol kg(-1) of acetate buffer (pH 4.0). The eluant was monitored at 589 nm. The calibration graphs were linear over the range from 1 x 10(-9) mol dm(-3) to 1 x 10(-7) mol dm(-3). The detection limit, defined as 3 sigma {3-times the standard deviation of the blank value (n = 5)}, was 4.4 x 10(-10) mol dm(-3). Both the standard addition method and the calibration curve method based on the peak heigh were available for the determination of cobalt. The HPLC method was applied to iron and steel certified reference materials. The cobalt contents in the CRMs determined by this method agreed with the guaranteed ones.