Preconcentration of trace cobalt with the ion pair of 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol and tetraphenylborate onto microcrystalline naphthalene or column method and its determination by derivative spectrophotometry.

Cobalt-2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP)-tetraphenylborate ion associated complex is quantitatively adsorbed on microcrystalline naphthalene in the pH range 3.5-9.5 from a fairly large volume of the aqueous samples (preconcentration factor ~30). After filtration, the solid mass consisting of the cobalt complex and naphthalene was dissolved with 5 ml of dimethylformamide (DMF) and the metal determined by first-derivative spectrophotometry. The cobalt-5-Br-PADAP complex can alternatively be quantitatively retained on ammonium tetraphenylborate-naphthalene adsorbent filled in a column (preconcentration factor 120) in the same pH range and determined similarly. The detection limit is 30 ppb (signal-to-noise ratio=2) and the calibration curve is linear over 0.3-8.0 mug of cobalt in 5 ml of the final DMF solution. Eight replicate determinations of 1.0 mug of cobalt gave a mean peak height of 0.208 (at 611.5 nm) with a relative standard deviation of 1.2%. The sensitivity of the method is 1.04 (dA/dnm) ml mug(-1) found from the slope of the calibration curve. The interference of a large number of anions and cations on the determination of cobalt has been studied and the optimized conditions developed were utilized for its trace determination in various standard alloys and biological samples.