Extraction System for the Spectrophotometric Determination of Tungsten(VI) with 4-Nitrocatechol and Benzalkonium Chloride

Abstract

A novel chromogenic system for the liquid-liquid extraction and determination of trace amounts of tungsten(VI) was investigated. The system comprises 4-nitrocatechol (4NC) as a chromogenic reagent, sulfuric acid as a complexing medium, and benzalkonium chloride (BAC) as a source of bulky cations (BA⁺), which readily form chloroform-extractable ion-association complexes. The impact of foreign ions and reagents was studied, and the optimal conditions for the sensitive, selective, and inexpensive determination of tungsten(VI) were identified. The limit of detection, linear working range, and molar absorptivity at l_max (422 nm) were determined to be 31 ng cm⁻³, 0.1–4.4 µg cm⁻³, and 5.49 × 10⁴ dm³ mol⁻¹ cm⁻¹, respectively. The composition of the extracted complex was 1:2:2 (W:4NC:BA). Two potential structures of its anionic component, [WO₂(4NC)₂]^2–, were discussed based on optimizations at the B3LYP/CEP-4G theoretical level and comparison between theoretical and experimental spectra.