For simultaneous determination of Co and Ni, 1 ml of sample solution containing preferably 8.0×10− 5 –8.0×10− 4 mmol total metal (as Co plus Ni) was taken; 1 ml of 1.6×10−2 M Dz followed by 1 ml of distilled water and 1 ml of concentrated NH3 solution were added to yield a 4 ml-volume mixture solution.