A stopped flow injection - Chemical relaxation - Catalytic spectrophotometric procedure for the determination of cobalt

A concentration-jump chemical relaxation catalytic procedure by using stopped flow injection is described with detection by spectrophotometry. The photoreduction of the Fe(III)-bipyridyl complex to Fe(II)-bipyridyl is pre-equilibrated at dark for at least 12 h. This solution is afterwards delivered by a peristaltic pump and mixed up with a stream of Co2+ solution as a catalyst. The mixture is confluenced down-stream with a flow of Fe(III) solution from a second pump, the concentration of which is about 30% of the pre-equilibrated Fe(III)-bipyridyl solution and acts as the perturbation (concentration-jump). The chemical relaxation process was monitored under thermodynamically non-equilibrium state, and the characteristic parameter of the relaxation process, relaxation time tau, is derived. The calibration graph between 1/tau and cobalt concentration is linear within 0.1-2.5 mug mL(-1). The detection limit (LOD) and precision (RSD) are 9 ng mL(-1) and 1.6%, respectively, both have been improved as compared to 12 ng mL(-1) and 3.6% with a former reported manually operated procedure. The present procedure has been applied to the determination of cobalt in a certified reference material and a soil sample.