Determination of cobalt by ion-pair liquid chromatography with 2-nitroso-1-naphthol-4-sulfonic acid

Cobalt was spectrophotometrically determined as an anionic metal complex of 2-nitroso-1-naphthol-4-sulfonic acid (nitroso-NW acid) on a reversed-phase high-performance liquid chromatograph. An ODS column(4.6 mm i.d.×100 mm) was used, and the mobile phase consisted of 1.4×10(-2) M tetrabutylammonium bromide and phosphate buffer (pH 8,5×10(-3) M) in a mixture of 52 vol% methanol and 48 vol% distilled water. To 7.5ml of sample solution, 1 ml of citrate buffer solution(2 M, pH 5.4) and 1 ml of nitroso-NW acid(4×10(-3) M) were added. After 10 min, 0.5 ml of 2×10(-3) M EDTA was further added, and the resultant solution was diluted to 10 ml with distilled water. One hundred μl of the solution was injected on to the column. Detection was made on a UV detector at 368 nm. The calibration curve was linear in the range of (125)×10(-7) M of cobalt. By the proposed method, cobalt contents in four commercial nickel salts, Ni(NO(3))(2) · 6H(2)O, NiSO(4) (NH(4))(2) SO(4)·6H(2)O, NiSO(4)·6H(2)O, and NiCl(2)·6H(2)O, were determined to be (0.0030.06)% with relative standard deviation of 1.3% (seven determinations of the nitrate salt)