Efficiency enhancement of the spectrophotometric estimation of cobalt in waters and pharmaceutical preparations using dispersive liquid-liquid microextraction and microcells with long optical paths.

In this paper, dispersive liquid-liquid microextraction (DLLME), long optical path microcells, and a selective chromogenic reagent were employed to improve the analytical efficiency of cobalt determination by spectrophotometry. The methodology proposed in the present study is based upon the microextraction of a cobalt(II) complex with 1-[4-[(2-hydroxynaphthalen-1-yl)methylideneamino] phenyl]ethanone (HNE) by DLLME and measurement of the absorbance of the sedimented phase using a microcell with an optical path length of 50 mm (Microcell-50). DLLME was performed using a binary mixture containing 900 μL of methanol as a dispersing solvent and 400 μL of CHCl3 (extraction solvent) at pH 6-8 adjusted by a mixture of HCl and NaOH. The electronic spectrum of the dark brown complex recorded in the sedimented phase using Microcell-50 shows a well-defined peak at λmax 324 ± 3 nm with a molar absorptivity of 1.08 × 106 M-1 cm-1. Cobalt was monitored at a detection limit (LOD) of 0.08 μg L-1 and in the linear concentration range of 0.45-10 μg L-1, while the limit of quantitation (LOQ), relative standard deviation (RSD), and the enhancement factor (EF) were 0.264, 1.6 μgL-1, and 223, respectively. Our method was evaluated by determining cobalt in certified reference materials and experimental samples, and the results were compared with ICP-MS measurements. Moreover, the chemical structure of the [Co(C38H28O2N)2] complex was suggested through using different characterization techniques such as Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), thermal analysis, and powder X-ray diffraction.