KINETIC DETERMINATION OF COBALT AND NICKEL BY FLOW-INJECTION SPECTROPHOTOMETRY

Reactor replacement is proposed as means of realizing different sample residence times in flow-injection analysis. As an application, an improved spectrophotometric determination of cobalt and nickel is proposed. The method exploits differences in reaction rates between cobalt and nickel citrate complexes and 4-(2-pyridylazo)resorcinol (PAR). The influence of pH, reagent concentrations, temperature, ionic strength, flow-rates and masking agent addition were investigated. Selectivity was enhanced by adding EDTA plus pyrophosphate and exploiting kinetic discrimination. The system is very stable and allows the determination of cobalt and nickel up to 5.00 mg l-1 with a sampling frequency of 40 h-1. For tool steel samples characterized by low nickel/cobalt concentration ratios, the system can be simplified, allowing kinetic masking of nickel and single cobalt determination. The reagent consumption is 65.3 mug of PAR per determination, and precise results (R.S.D. < 1%) in agreement with inductively coupled plasma atomic emission spectrometry are obtained at a sampling frequency of 100 h-1.