Quantitative Recovery of Bis(1-nitoroso-2-naphtolato) Cobalt Complex as Precipitates for Gravimetric Analysis

The addition of cobalt to steels alters their properties, such as decrease in temperature of martensitic dislocation and degradation of hardenability. Quantitative recoveries of the precipitated bis(1-nitoroso-2-naphtolato) cobalt complex are essential to ensure the accuracy of the gravimetric analysis of cobalt in steel using 1-nitroso-2-naphthol as a precipitant. Factors affecting quantitative recoveries of the bis(1-nitoroso-2-naphtolato) cobalt complex as precipitates were investigated, including a concentration of 1-nitroso-2-naphthol, an aging time, and coexisting ions. We found that optimal 1-nitroso-2-naphthol concentrations for quantitative precipitation of the complex were regulated by initial amounts of cobalt in steel samples to be determined. Above the optimal concentrations, the recoveries of the complex decreased with increasing the 1-nitroso-2-naphthol concentrations. This decrease in the recoveries is rationalized by the formation of higher-order complexes of divalent cobalt and the rate-determining step to release the ligands from the oxidized trivalent cobalt complexes. Under the optimal concentrations of 1-nitroso-2-naphthol, the achievement of the quantitative recoveries of the precipitates required a much longer aging time than that specified in the standard method. The coexisting zinc ions after the separation of the iron matrix did not interfere with the quantitative recoveries of the cobalt complex. Quantitative recoveries of cobalt in a certified standard steel sample were yielded during the precipitation and filtration steps under the optimum conditions specified here.