Solid phase pre-concentration of cobalt(II) using pyridine–thiocyanate reagents: X-ray crystal structure of the extracted ternary cobalt(II) complex

Abstract

Mixture of pyridine-2,6-dimethanol (PDM) and thiocyanate (SCN −) (1:1, mole ratio) immobilized on silica served as a very efficient sorbent for selective retention of Co(II) from other associated metal ions at trace level. The maximum sorption capacity for Co(II) was found to be 0.203 mmol g −1 at pH 9.0. Sorbed Co(II) was completely eluted by 3.5 mL of 3 mol L −1 HCl and measured using flame atomic absorption spectrometer (FAAS). The structure of the extracted Co(II) complex was confirmed by single crystal X-ray structure and Fourier transform infrared (FTIR) spectroscopy. Thermo gravimetric analysis (TGA) of the chelated Co(II) complex revealed its stability at the optimum extraction temperature (55 °C). The method was reproducible with a relative standard deviation (RSD) of 0.6% ( N = 10) with three sigma detection limit (N = 10) of 0.6 μg g −1. A pre-concentration factor, 94 was achieved. Interferences from Mn 2+ and Cu 2+ ions were eliminated by prior oxidation of Mn 2+ by KIO 4 to MnO 4 − and masking of Cu 2+ with NH 4SCN, respectively. A plausible mechanism for the selective extraction of Co(II) was attributed to the formation of a first order water insoluble inner–metallic complex as confirmed by the single crystal X-ray structure analysis. The developed method has been tested for trace level separation and estimation of cobalt in some certified reference materials. Analyses of some biological and environmental samples were performed.