Extraction and spectrophotometric determination of cobalt with dithizone

Cobalt(II) in acetate-tartrate buffer (pH 6.0-7.3) is extracted quantitatively as cobalt(III) dithizonate with excess of dithizone in CCl₄. The molar absorptivity in the CCl₄ phase is 4.6 × 10⁴ 1.mole^-1.cm^-1 at the absorption maximum 550 nm. The calibration graph is linear for 1-10 μg of cobalt in 10 ml of CCl₄ when excess of dithizone is removed by back-extraction with 0.01M aqueous ammonia. Most interferences can be overcome by (a) initial extraction with dithizone at pH 1.3, (b) selective back-extraction into hydrochloric acid (pH 1 to 2), (c) oxidation of iron and tin to iron(III) and tin(IV) and addition of fluoride to complex the former, and (d) selective reaction of nickel dithizonate with 1,10-phenanthroline in the CCl₄ phase followed by back-extraction of nickel into 0.1M acid. The method has been applied to determination of cobalt in a copper-nickel-zinc alloy and a nimonic alloy.