Structural and spectrophotometric characterization of 2-[4-(dimethylamino)styryl]-1-ethylquinolinium iodide as a reagent for sequential injection determination of tungsten.

Structure, spectrophotometric and protolytic properties of the styryl dye 2-[4-(dimethylamino)styryl]-1-ethylquinolinium iodide (R) as well as its complex with tungsten were studied. The selective protonation of dimethylamino group was confirmed by density functional theory investigation through the computation of Fukui function, NPA partial atomic charges, and NICS(0) aromaticity indexes. The TD-DFT study explains the experimental change of color by excluding the dimethylamino group from HOMO orbital upon protonation. The acid dissociation constant, the optimum wavelength and the molar absorptivity of R were found to be: 3.02, 501nm and 4.0×104 Lmol-1 cm-1 , respectively. The protolytic properties of the reagent were found to change significantly in the presence of tungsten(VI). Analysis of bond critical points between the anions and Quinaldine Red cation gives the selectivity raw HWO4 - >MoO4 - >H2 VO4 - >ReO4 - >ClO4 - , that perfectly match with the experimental data. Based on this observation, a non-extractive sequential-injection spectrophotometric method for the determination of tungsten was developed. The absorbance of the colored extracts obeys Beer's law up to 55.2mgL-1 of W at 520nm wavelength. The limit of detection calculated from a blank test (n=10) based on 3s was 0.96mgL-1 . The developed method was applied for the determination of tungsten in model samples.