Catalytic determination of cobalt by a concentrationjump chemical relaxation method

A catalytic procedure incorporated with a concentration-jump chemical relaxation method was proposed, based on detection by UV-VIS spectrophotometry. The chemical relaxation process of Fe(III)-bipyridyl complex reduced to Fe(II)-bipyridyl by photoreduction in the presence of cobalt(II) as a catalyst had been studied. The system was first pre-equilibrated, and afterwards a perturbation (concentration-jump) of Fe(III), which is about 30% of the pre-equilibrium concentration was introduced. The system was allowed to relax to a new state of equilibrium, and the relaxation process was recorded. The calibration graph of relaxation time (1/tau) and cobalt concentration is linear for 0-2.5µgml-1. The 3sigma detection limit is 12ngml-1, and the relative standard deviation for 1µgml-1 level of Co2+ is 3.6% (n=4). The procedure had been validated by the determination of cobalt in a certified reference material and a soil sample, with coefficients of variances of 1.3 and 2.9%, respectively. A recovery test is also performed for the soil sample by spiking, and the recovery of 98% was obtained.