Multivariate optimization of cloud point extraction for the simultaneous spectrophotometric determination of cobalt and nickel in water samples

In this paper a new and simple cloud point extraction (CPE) method has been developed for the preconcentration and simultaneous spectrophotometric determination of binary mixtures of cobalt and nickel in water samples, without prior separation steps. This method is based on the mean centering of ratio spectra. 1-nitroso-2-naphthol was used as the chelating agent and an anionic surfactant, sodium dodecyl sulfate (SDS), was used as extracting agent at room temperature. Central composite design (CCD) based response surface method (RSM) was applied for the optimization of main experimental parameters such as pH of solutions, concentration of the chelating agent, surfactant and salt. Under the optimum conditions the analytical characteristics of the method such as limit of detection, linear range, relative standard deviation (R. S. D.) and relative standard error (R. S. E.) were calculated. Calibration graphs were linear in the range of 5-300 and 10-320 ng mL(-1) with detection limits of 0.73 and 0.85 ng mL(-1) for Co(II) and Ni(II), respectively. The limit of quantification was 2.43 and 2.83 ng mL(-1) for Co(II) and Ni(II), respectively. The interference effect of some anions and cations was also tested. The method was successfully applied to the simultaneous determination of Co(II) and Ni(II) in water samples.