For single Co determination, 1 ml of sample solution containing preferably 2.0 × 10 − 5 – 4.0 × 10−4 mmol (1–24 mg) Co was taken; 1 ml of 8.0 × 10 − 3 M Dz solution (20-fold of max- imum amount of Co) followed by 1 ml of distilled water, and 1 ml of concentrated NH3 solution were added to yield a final volume of 4 ml.