First Application of 3,5-Dinitrocatechol and Benzalkonium Chloride in Cloud Point Extraction: Spectrophotometric Determination of Vanadium(V)

The ability of 3,5-dinitrocatechol (3,5-DNC) to form anionic species with vanadium(V) in a mildly acidic medium was exploited to develop a spectrophotometric method for vanadium determination. The method is grounded on the formation of an ion-association complex with benzalkonium chloride (BAC), which is efficiently extracted into a surfactant-rich phase formed in a mixed micellar medium consisting of Triton X-114 and BAC. The stoichiometry of the extracted complex was established as (BA+)[VO(3,5-DNC)2], and the formula of its anionic component, [VO(3,5-DNC)2]−, was verified by quantum-chemical calculations at the B3LYP/6-311G level of theory and by comparison of the experimental absorption spectrum with simulated theoretical spectra. The results indicate that the thermodynamically more stable anti isomer is preferentially formed and extracted. The developed novel centrifuge-less cloud point extraction–spectrophotometric method exhibits an absorption maximum at 426 nm, an apparent molar absorptivity of 1.6 × 106 L mol−1 cm−1, a limit of detection of 1.7 ng mL−1, and a linear range of 5.7–29.1 ng mL−1. The method was successfully applied to the determination of vanadium in spent catalysts and vanadium-based dietary supplements. Its analytical performance and sustainability were evaluated using modern assessment tools, including RGBfast.