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

Abstract

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.