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 o f 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 soluti on 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 volum e with water. An aliquot of this solution was taken into 10 cm(3) of a volu metric 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) wer e 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 buffe r (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 we re available for the determination of cobalt. The HPLC method was applied t o iron and steel certified reference materials. The cobalt contents in the CRMs determined by this method agreed with the guaranteed ones.