Mono- and Binuclear Complexes in a Centrifuge-Less Cloud Point Extraction System for the Spectrophotometric Determination of Zinc(II)

The hydrophobic reagent 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR) was investigated as part of a cloud point extraction (CPE) system for the spectrophotometric determination of Zn(II). In the system, complexes with different stoichiometries, including 1:1 and 2:2 (Zn:HTAR), are formed. Their ground-state equilibrium geometries were optimized at the B3LYP/6-31G level of theory. The obtained structures were then used to calculate vertical excitation energies in order to generate theoretical UV/Vis absorption spectra. The comparison between theoretical and experimental spectra demonstrated that, under optimal conditions, a binuclear complex containing oxygen bridging atoms is the dominant species. The absorbance was found to be linearly dependent on the concentration of Zn(II) within the range of 15.7 to 209 ng mL–1 (R2 = 0.9996). The fraction extracted (%E), logarithm of the conditional extraction constant (log Kex), and molar absorption coefficient (ε) at λmax = 553 nm were calculated to be 98.3%, 15.9, and 4.47 105 L mol–1 cm–1, respectively. The method developed is characterized by simplicity, convenience, profitability, sensitivity, and ecological friendliness. It has been successfully applied to the analysis of pharmaceutical and industrial samples.